Spectroelectrochemistry of Substituted Anilines
von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz
genehmigte Dissertation zur Erlangung des akademichen Grades
doctor rerum naturalium
(Dr. rer. nat.)
Vorgelegt von
M. Sc. Abdel Aziz Qasem Mohammad Jbarah
geboren am 27.11.1973 in Kuwait
eingereicht am 14.06.2006
Gutachter: Prof. Dr. Rudolf Holze
Prof. Dr. Klaus Banert
Prof. Dr. Waldfried Plieth
Tag der Verteidigung: 07.11.2006
Bibliographische Beschreibung und Referat
Bibliographische Beschreibung und Referat
A. A. Jbarah
Spektroelektrochemie substituierter Aniline
Die Elektrochemie und die Spektroelektrochemie von Nitroanilinen (ortho-, meta- und para- Isomere) und deren entsprechenden Diaminoverbindungen (ortho-, meta- und para-Phenylendiamin)
wurden an zwei verschiedenen Elektroden (Platin und Gold) und in zwei Elektrolytlösungen (saure
und neutrale Perchloratlösung) untersucht. Die erhaltenen Messergebnisse wurden als Referenz für
die spektroelektrochemische Untersuchung von Polyvinylaminen mit o- oder pNitroanilinsubstituenten verwendet. Es wurden außerdem spektroelektrochemische Untersuchungen mit anderen Polyvinylaminen, die das Wurster Kationradikal oder Stilbene als Substituenten
enthalten, durchgeführt.
Die oxidative und reduktive Elektrochemie von drei Nitroanilinisomeren wurde in neutraler (0.1 M
KClO4) und saurer (0.1 M HClO4) wässriger Elektrolytlösung mit zyklischer Voltammetrie und
oberflächenverstärkter Ramanspektroskopie (Surface Enhanced Raman Spectroscopy SERS) untersucht. Die zyklischen Voltammogramme, die mit einer Goldelektrode in saurer Elektrolytlösung
für o- und p-Nitroanilin aufgezeichnet wurden, zeigten die Bildung von o- und p-Phenylendiamin
beim Potenzialdurchlauf in kathodischer Richtung. In neutraler Elektrolytlösung ist die Situation
anders und die Endprodukte der elektrochemischen Reduktion dieser Isomere sind o- und pAmino-N-phenylhydroxylamin. Aus den zyklischen Voltammogrammen, die mit Gold- und Platinelektroden bei anodischem Potenzialdurchlauf für diese Isomere in saurer und neutraler Elektrolytlösung aufgezeichnet wurden, erhält man folgende Reihenfolge für die Lage der Oxidationspotentiale m-Nitroanilin > p-Nitroanilin > o-Nitroanilin. Eine Sauerstoff-Gold-AdsorbatStreckschwingung wurde zwischen 400 und 430 cm-1 in den SER-Spektren der drei isomeren Nitroaniline in beiden Elektrolytlösungen bei positiven Elektrodenpotenzialen beobachtet. Das SERSExperiment zeigte auch eine senkrechte Orientierung der adsorbierten Nitroaniline zur Oberfläche
der Goldelektrode. Für die isomeren Phenylendiamine wurde in beiden Elektrolytlösungen und mit
beiden Elektroden im anodischen Durchlauf das gleiche Verhalten beobachtet. Das beim EinElektronenübergang erhaltene Oxidationsprodukt (Radikalkation) reagiert im Fall von o- und mPhenylendiamin über eine C-N-Kopplung mit einem weiteren Radikal zum Dimer (1.Schritt der
Elektropolymerisation). p-Phenylendiamin wird nach dem ECE-Mechanismus (E = Elektronentransfer, C = chemische Reaktion) oxidiert, wobei die Ladungsübertragung in zwei Schritten erfolgt, gekoppelt mit Säure-Base-Reaktionen, was zur Bildung des Diimin führt. Aus den SERSMessungen kann man schlussfolgern, dass m- und p-Phenylendiamin waagerecht zur Metalloberfläche über den Benzenring und die Stickstoffatome adsorbiert sind. Die Adsorption von oPhenylendiamin erfolgt über die Stickstoffatome und mit schräger Orientierung zur Metalloberfläche.
Die zyklischen Voltammogramme, die mit einer Goldelektrode in saurer und neutraler Elektrolytlösung von den Polyvinylaminen mit Nitroanilinsubstituenten aufgenommen wurden, zeigen dasselbe Verhalten wie Nitroanilinmonomere beim Potenzialdurchlauf in kathodischer Richtung. Die
für diese Polymere im anodischen Durchlauf erhaltenen Zyklovoltammogramme unterscheiden
sich von denen für die Monomere. Die Zahl der Adsorptionsplätze und die Adsorptionsstärke der
Polyvinylamine verändern sich in Abhängigkeit vom Elektrodenpotential, vom Prozentsatz und der
Art des aromatischen Substituenten am Polymerrückgrat und vom pH-Wert der Lösung.
Stichwörter: Spektroelektrochemie, Elektrochemie, Nitroanilin, Phenylenediamin, SERS, Polyvinylamin, Zyklische Voltammetrie, ECE
2
Abstract
Abstract
A. A. Jbarah
Spectroelectrochemistry Of Substituted Anilines
The electrochemistry and spectroelectrochemistry of nitroanilines (ortho, meta, and
para isomers) and their respective amino compounds (ortho-, meta- and paraphenylenediamines) have been investigated at two different electrodes (platinum and gold)
and in two different electrolyte solutions (acidic and neutral perchlorate). The results of
these investigations were used as a reference for the spectroelectrochemistry of polyvinylamines containing o- or p-nitroaniline substituents. Spectroelectrochemical investigations
of polyvinylamine containing Wurster radical cation or stilbene as a substituent were also
carried out.
The oxidative and reductive electrochemistry of the three isomeric nitroanilines has
been studied in neutral (0.1 M KClO4) and acidic (0.1 M HClO4) aqueous electrolyte solutions with cyclic voltammetry and Surface Enhanced Raman Spectroscopy (SERS). The
cyclic voltammograms recorded with a gold electrode in acidic electrolyte solution showed
formation of o- and p-phenylenediamine in the negative going potential scan for o- and pnitroaniline respectively. In neutral electrolyte solution the situation is different and the final products of electrochemical reduction of these isomers are o- and p-amino-Nphenylhydroxylamine. The order of increasing electrochemical oxidation potential is mnitroaniline > p-nitroaniline > o-nitroaniline as observed from cyclic voltammograms recorded with a gold and platinum electrodes and in the positive going potentials scan for
these isomers in acidic and neutral electrolyte solutions. An oxygen-gold adsorbate stretching mode was detected between 400 to 430 cm-1 in SER-spectra of the three isomeric nitroanilines in both electrolyte solutions at positive electrode potentials. The SERS experiments showed also a perpendicular orientation of adsorbed nitroanilines on a gold electrode with respect to the metal surface.
General trends are observed in the anodic scans of isomeric phenylenediamines at both
electrodes and in both electrolyte solutions. The one-electron electrochemical oxidation
product (radical cation) in case of o- and m-phenylenediamine go into fast C-N coupling
3
Abstract
between radicals to form dimers (the first step of electropolymerization). The pphenylenediamine is oxidized according to an ECE mechanism (E = electron transfer reaction, C = chemical reaction), which involved two charge transfer steps coupled with acidbase reactions to form diimine. As we deduced from SERS measurements, m- and pphenylenediamine adsorbed in flat orientation with respect to the metal surface via benzene
ring and nitrogen atoms, respectively. o-Phenylenediamine adsorption is taking place via
nitrogen atoms and with tilted orientation with respect to the metal surface.
The cyclic voltammograms recorded with a gold electrode in acidic and neutral electrolyte solutions of polyvinylamines containing o- or p-nitroaniline substituents exhibit the
same features like nitroaniline monomers in the negative going potentials scan. The result
observed in the anodic scan for these polymers are different from those observed for monomers. Adsorption site and strength of the polyvinylamine polymer varies according to the
applied electrode potential, percentage and type of the aromatic substituent at the polymer
backbone, and the pH of the medium.
Keywords: Spectroelectrochemistry, Electrochemistry, Nitroanilines, Phenylenediamines, SERS, Polyvinylamine. Cyclic voltammetry, ECE
4
Zeitraum, Ort der Durchführung
Die vorliegende Arbeit wurde in der Zeit von Oktober 2002 bis Mai 2005 unter Leitung
von Prof. Dr. Rudolf Holze am Lehrstuhl für Physikalische Chemie/Elektrochemie der
Technischen Universität Chemnitz durchgeführt.
5
Acknowledgement
Acknowledgement
Firstly I would like to thank my supervisor Prof. Dr. Rudolf Holze for giving me the
opportunity to carry out this work and for his constant encouragement, support and constructive discussion, which enriched my knowledge and skilled my experience.
I would like also to thank Prof. Dr. Stefan Spange and Isabelle Roth for supporting the
polymer samples and constructive discussion.
I am indebted to Dr. Alexander Auer for helpful discussions in quantum chemical calculations.
Deep thanks to Susanne Vogel, for continuous help and encouragement to complete my
Ph. D.
I would like to thank the rest of the electrochemistry group, past and present, for their
cooperation and much friendship.
I am grateful to Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft for funding.
Finally, I deeply appreciate Chemnitz University of Technology that embraced me as
one of its students.
6
Dedication
Dedication
To The Soul of My Mother
To My Father, Brothers and Sisters
To Whom I Love
Abdel Aziz
7
- Table of Contents Bibliographische Beschreibung und Referat
2
Abstract
3
Zeitraum, Ort der Durchführung
5
Acknowledgment
6
Dedication
7
Table of contents
8
List of abbreviations
11
Chapter 1
13
1
Introduction
13
1.1
Nitroanilines
13
1.2
Polyvinylamine
14
1.3
Polyelectrolytes: definition, importance and properties
16
1.4
Cyclic voltammetry
19
1.5
SERS background and history
19
1.6
Computational chemistry
22
1.6.1
Density functional theory
22
1.6.2
Computational electrochemistry
23
1.6.3
Heat of formation
23
1.7
Aim and scope of this study
24
Chapter 2
26
2
Experimental
26
2.1
26
Cyclic voltammetry
8
Table of Contents
2.2
UV-Vis spectroscopy
26
2.3
Raman spectroscopy
26
2.4
Infrared spectroscopy
27
2.5
Materials
27
2.6
Computational aspect
27
Chapter 3
29
3
Electrochemistry of nitroanilines, phenylenediamines and polyvinylamines
29
3.1
Nitroaniline reduction
29
3.2
Nitroaniline oxidation
37
3.3
Phenylenediamine oxidation
41
3.4
Polyvinylamine reduction
47
3.5
Polyvinylamine oxidation
57
Chapter 4
67
4
67
UV-Visible spectroscopy
Chapter 5
72
5
Adsorption of nitroanilines and phenylenediamines on gold
72
5.1
p-Nitroaniline adsorption on gold
72
5.2
o-Nitroaniline adsorption on gold
76
5.3
m-Nitroaniline adsorption on gold
79
5.4
p-Phenylenediamine adsorption on gold
83
5.5
o-Phenylenediamine adsorption on gold
86
5.6
m-Phenylenediamine adsorption on gold
89
Chapter 6
92
6
92
Vibrational spectroscopy of solid polyvinylamines
9
Table of Contents
Chapter 7
101
7
Polyvinylamines adsorption on gold
101
7.1
Polyvinylamine adsorption on gold
101
7.2
p-Nitroaniline-substituted polyvinylamine adsorption on gold
106
7.3
o-Nitroaniline-substituted polyvinylamine adsorption on gold
119
7.4
Wurster-substituted polyvinylamine adsorption on gold
134
7.5
Stilbene-substituted polyvinylamine adsorption on gold
140
Summary
143
Future work
147
References
148
Appendixes
160
List of publications
166
Selbständigkeitserklärung
167
Curriculum Vitae
168
10
List of Abbreviations
List of Abbreviations
CV
Cyclic voltammogram
CVs
Cyclic voltammograms
ECE
Electron transfer, chemical reaction, electron transfer
ESR
Electron spin resonance
UV-Vis
Ultraviolet-Visible
SERS
Surface enhanced Raman spectroscopy
SER-spectra
Surface enhanced Raman spectra
SMSERS
Single molecule surface enhanced Raman spectroscopy
MORC
Metal oxidation-reduction cycle
EF
Enhancement factor
p-NA
para-nitroaniline
o-NA
ortho-nitroaniline
m-NA
meta-nitroaniline
p-PDA
para-phenylenediamine
o-PDA
ortho-phenylenediamine
m-PDA
meta-phenylenediamine
PVAm
Polyvinylamine
PVAms
Polyvinylamines
MRS
Modulation reflectance spectroscopy
DFT
Density functional theory
AM1
Austin Model 1
B3LYP
Becke’s three-parameter exchange functional, in combination with
the Lee-Yang-Parr correlation function
IR
Infrared
FTIR
Fourier transform infrared
Calc. vib. freq.
Calculated vibrational frequency
R. int.
Raman intensity
Ea
Eanodic
Ec
Ecathodic
dE/dt
Scan Rate
SCE
Saturated calomel electrode
ESCE
Potential vs. the saturated calomel electrode
11
List of Abbreviations
δ
In-plane deformation mode
γ
Out-of-plane deformation mode
γas
Twisting mode
γs
Wagging mode
ν
Stretching mode
νs
Symmetric stretching mode
νas
Asymmetric stretching mode
βs
Scissoring mode
βas
Rocking mode
λ
Wavelength
#
Number
a.u.
Arbitrary unit
n.a.
Not assigned
Fig.
Figure
12
- Chapter 1 -
1 Introduction
During recent years much attention has been paid to electrochemical investigations of
substituted benzenes and substituted anilines by both electrochemical and spectroelectrochemical techniques. Part of these studies is dedicated to electropolymerization of aniline
and substituted aniline [1-5]. The mechanism of electropolymerization was investigated
and the polymer was characterized in these studies using in situ electron spin resonance
(ESR), in situ UV-Vis, ex situ FTIR and in situ Raman spectroscopic techniques. Polyaniline has also been studied extensively as a special member of the conducting polymer family because of its stability and technological applications [6-9]. In other studies the reduction behavior of isomeric nitroanilines at pH > 7 in the presence of camphor and aprotic
solvents was discussed [10]; radicals derived from nitrobenzene [11] and 2,4dinitrobenzene [12] were investigated by cyclic voltammetry and other methods. Several
nitrobenzene derivatives were polarographically studied in order to explain the polarographic behavior of phenylhydroxylamines [13].
1.1 Nitroanilines
p-Nitroaniline as a particularly prominent member of the family of isomeric nitroanilines has attracted attention because of the specific effects of an electron withdrawing nitrogroup and an electron donating amino-group being in para position at an aromatic ring system [14-16]. This results in low-energy electronic transitions with charge migration within
the molecule [17] and greater nonlinear susceptibility [18-20] making this molecule interesting as material for nonlinear optics [21-25]. Its use as an end-group in thiol-based selfassembled monolayers has been reported [26]. Pre-resonance and resonance Raman spectra
for nitroanilines have been investigated [27,28]. Electro-optic properties and photoreduction of nitroanilines have been studied [29,30]. Several papers have been published focusing on properties and applications of substituted nitroanilines [31-33].
13
Chapter 1
Phenylenediamines, which are of considerable practical interest because of their use in
various technological applications like biosensors [34-36], can be prepared by chemical
and electrochemical routes from the corresponding nitroanilines. Consequently, all nitroanilines have attracted interest.
The adsorption of p-nitroaniline on a platinum electrode has been studied with modulation reflectance spectroscopy [37] (MRS, a variation of electroreflectance spectroscopy,
see also [38]) and the adsorption of monosubstituted benzenes at a gold electrode has been
studied using surface-enhanced Raman spectroscopy (SERS) [39,40]. SERS with these
molecules has attracted great attention in a variety of research fields, for example surface
science, analytical chemistry [41-44] and nanotechnology (see section 1.5).
We selected o-, m- and p-nitroaniline (o-NA, m-NA, p-NA) as suitable molecular systems for a comparative study of electrosorption behavior, because they have at least three
different anchoring sites (the nitro, the amino function and the aromatic ring system). In
addition these results shall serve as a point of reference for studies of the spectroelectrochemistry of nitroaniline-substituted polyvinylamines [45] (see section 1.2). There is currently considerable interest in the design of new organic “push-pull” chromophores that
possess good thermal and photochemical stability and that can be incorporated into polymers either as dopants or as part of the polymer backbone or side chain [46-48].
1.2 Polyvinylamine
Polyvinylamine (PVAm) is a cationic polyelectrolyte with simple structure. It has an
exceptional potential reactivity because of the presence of primary amino-groups along the
hydrocarbon main chain. Furthermore, an improved control of the ionic properties, such as
charge density and acid-base strength, is possible by simple protonation, because the
charge carriers, commonly ─ NH 3+ X − groups, are located directly along the polymer back-
bone. The completely hydrolized product, PVAm, is an important polyelectrolyte, it possesses the highest known charge density along the polymer backbone. The acid-base behavior of PVAm has been studied extensively by means of potentiometric titration [49-51].
PVAm is widely used in catalysis [52,53], chelation [54], biomedical research [55], liquid
14
Chapter 1
chromatography [56,57] and paper-making [58,59]. Several aromatic pendant groups can
be used to modify the electronic properties of non-conjugated conducting polymers [6065]. Polymers with the aromatic pendant group find applications in photoconducting and
electronic devices.
In this study PVAm and PVAms with different nitroaniline substituents and different
molecular mass are investigated electrochemically at gold and platinum surfaces. We report also the adsorption behavior of these polymers at a gold surface as evidenced with surface-enhanced Raman spectroscopy. The main subject in this dissertation is the study of
the effect of several factors on the electrochemical and adsorption behavior of these polymers. These factors included percentage and type of nitroaniline substituent at PVAm
backbone, molecular weight of the polymer, applied electrode potential and the supporting
electrolyte (0.1 M HClO4 and 0.1 M KClO4). The studied PVAms are listed in Table.1 and
illustrated in Fig. 1. The obtained results are compared with the spectroelectrochemical results of nitroaniline isomers, which are also reported in this study. Further interpretations
are based on the reported physical properties and adsorption behavior of polyelectrolytes
and PVAm in literature, which are summarized in this text.
Table 1.
Percentage of nitroaniline stbstituent and molecular weight of PVAms.
Mole % of nitroaniline
substituent
Molecular weight
g/mole
0
60000
p-nitroaniline-substituted polyvinylamine (PVAm1)
2-3
60000
p-nitroaniline-substituted polyvinylamine (PVAm2)
11.3
15000
p-nitroaniline-substituted polyvinylamine (PVAm3)
8.7
15000
o-nitroaniline-substituted polyvinylamine (PVAm4)
1
60000
o-nitroaniline-substituted polyvinylamine (PVAm5)
25
15000
o-nitroaniline-substituted polyvinylamine (PVAm6)
14.5
15000
o-nitroaniline-substituted polyvinylamine (PVAm7)
12.7
400000
o-nitroaniline-substituted polyvinylamine (PVAm8)
25.5
400000
Not available
15000
Not available
15000
Not available
15000
Polymer
Polyvinylamine (PVAm)
4-amino-N,N-dimethyl-3-nitroaniline- substituted polyvinylamine
(radical cation form, PVAm9)
2-nitro-p-phenylenediamine-substituted polyvinylamine
(radical cation form, PVAm10)
stilbene-substituted polyvinylamine (PVAm11)
15
Chapter 1
PVAm
PVAm4, PVAm5, PVAm6
PVAm7, PVAm8
PVAm1, PVAm2, PVAm3
X
Y
X
NH2
NH2
Y
X
NH
NH2
NH
NO2
NO2
PVAm9
PVAm10
NH2
Y
X
Y
X
PVAm11
NH2
NH
NH
NH2
NH
NO2
NO2
NO2
N
N
H3C
Y
X
CH3
H
H
O2N
Fig. 1. Molecular structure of PVAms.
1.3 Polyelectrolytes: definition, importance and properties
Polyelectrolytes are polymers (long-chain molecules) with ionizable side-groups (of either anionic or cationic character). In water or any other ionizing solvent, they dissociate
into a “macroion” (meaning polyelectrolyte backbone with the charges along the chain)
and many mobile “counterions” (Fig. 2).
16
Chapter 1
counterion
ionizable group
Fig. 2. General structure of polyelectrolyte in an ionizing solvent.
In addition to their essential functions in human physiology and cellular mechanisms in
the form of proteins, polypeptides and nucleic acids, polyelectrolytes have found a number
of important applications in major fields of science and material engineering. Their applications in chemistry are mainly centered at the interface of polymer, materials, colloids,
surface and analytical chemistry. Polyelectrolytes have found many uses in a number of
technical applications. These include paper-making [66], waste water treatment [67], inkjet printing [68], modifying polymer light emitting diodes (PLED), hybrid liquid crystal
(LC) display devices [69] and the food industry [70]. In all of these applications the adsorption behavior of polyelectrolytes is important.
Several papers have been published focusing on the adsorption behavior of polyelectrolytes. These included experimental studies of adsorption to minerals [71,72], adsorption on
solid substrate (mica) [73] and adsorption to cellulose [66]. Several theoretical works have
also been published during the last few years including extension of the Scheutjens-Fleer
[74,75] model by van der Schee and Lyklema [76] and by Evers et al. [70,77] as well as
mean-field calculations and Monte Carlo simulations [78-85]. Some surface force studies
related to adsorption of cationic polyelectrolytes have also been presented [81,86-90], as
well as a few on the adsorption of anionic polyelectrolytes [91-93]. Moreover, Dhoot et al.
[94] have reported surface force measurements in systems containing both cationic and
anionic polyelectrolytes.
17
Chapter 1
When a polyelectrolyte adsorbs on an oppositely charged surface, it is often found that
the adsorbed amount compensates or slightly overcompensates the surface charge and that
the adsorption increases with increasing salt concentration [95-97]. At low electrolyte concentration the adsorbed layers are thin and the adsorbed amount hardly depends on molecular weight. This indicates a flat conformation of the polyelectrolyte and these trends refer to highly charged polyelectrolytes. Recent theories on polyelectrolyte adsorption [7698] are in agreement with these experimental findings. The somewhat older treatment of
Hesselink [99] predicts very thick adsorbed layers but finds the same effect of increasing
salt concentration. The usual argument is that for highly charged polyelectrolytes the repulsion between the segments dominates the adsorption behavior. At high salt concentration the repulsion is screened; hence the polyelectrolytes behave more like uncharged
polymers. They can adopt conformations with loops and tails and the adsorbed amount increases. Naturally, a fully screened polyelectrolyte can only adsorb if there is an attractive
interaction between segments and surface, which is not electrostatic in nature.
However, sometimes the adsorption of polyelectrolytes is found to decrease with increasing salt concentration [71,100-105] or is not affected at all [100]. In most of these
cases the polyelectrolytes have a low charge [71,100,103,104] but highly charged polyelectrolytes can also show this behavior [101,102,105]. Theoretically, decreasing adsorption,
or even complete desorption, with increasing salt concentration is considered by Hesselink,
Wiegel and Muthukumar [99,106,107]. This effect is expected if the attraction between
polyelectrolyte and surface is mainly electrostatic in nature, since salt screens not only the
segment-segment repulsion but also the segment-surface attraction. Thus, in polyelectrolyte adsorption, added salt has two antagonistic effects and it depends on the balance between electrostatic and nonelectrostatic attraction whether or not increasing the salt concentration leads to an increase or a decrease in adsorption. If this force balance is changed,
for instance by increasing the segment charge, the influence of the salt concentration can
be reversed. Durand et al. [100] found such a reversal for cationic polyacrylamides (copolymers of acrylamide and an acrylate with a quaternary ammonium-group) adsorbing on
montmorillonite. With a cationic monomer content (Ƭ) of 1% the adsorbed amount decreased with increasing salt concentration, for Ƭ = 5 % there was no salt effect whereas for
cationic polyacrylamides with Ƭ = 13 % and Ƭ = 30 % the adsorbed amount increased with
increasing salt concentration.
18
Chapter 1
1.4 Cyclic voltammetry
Yet within the more academic tradition, electrochemistry has been seen as a large and
important area of physical chemistry. Several researchers make electrochemical measurements on chemical systems for a variety of reasons. They may be interested in obtaining
thermodynamic data about reactions. They may want to generate an unstable intermediate
such as a radical ion and study its rate of decay or its spectroscopic properties. The goal
might be the analysis of a solution for trace amounts of metal ions or organic species. In
these examples electrochemical methods are employed as tools in the study of chemical
systems.
One of the most widely used electrochemical techniques, where the potential varying
linearly in time is the controlled variable and the resulting current is the measured variable,
is usually known as cyclic voltammetry [38]. The reduction and oxidation potentials of a
molecule can be derived from cyclic voltammetry, and this may enable calculation of
thermodynamically significant values of E0, if the electrode process is reversible. When the
overall electrode process is not reversible other phenomena, in particular chemical reaction, e.g. polymerization or dehydrogenation, may occur in addition to the charge-transfer
process. In many investigations of molecular adsorption preceding the charge-transfer and
the subsequent charge transfer itself electrochemical methods (e.g. cyclic voltammetry, polarography, chronoamperometry) are used together with spectroscopic methods to obtain
more information about the adsorbate orientation relative to the electrode surface [37-39].
1.5 SERS background and history
The electrochemical and spectroscopic signals can be acquired simultaneously in one
spectroelectrochemical experiment. The spectroscopic techniques involved in spectroelectrochemistry include UV-Vis absorption spectra [108-110], fluorescence spectra [111], infrared spectra [112,113], Raman spectra [114, 115] and electron spin resonance spectra
[116]. One of the most active field in spectroelectrochemistry is surface enhanced Raman
spectroscopy (SERS).
19
Chapter 1
In 1974 Fleischmann et al. [117] have observed a significant increase in the intensity of
Raman bands for pyridine at a roughened silver electrode. The rather unexpected increase
was explained by the increased surface area of the roughened electrode, which leads to increase in the number of adsorbed molecules. In 1977 Jeanmaire and Van Duyne [118] were
the first to evaluate the intensity of the SERS effect. They estimated the enhancement factor for pyridine Raman scattering on silver to be as much as 105-106. They confirmed the
experiments by Fleischmann et al. but they also found that the enormously strong Raman
signal measured from pyridine on the rough silver electrode must be caused by a true enhancement of the Raman scattering efficiency itself, and that it cannot be explained only
by an increase in the number of Raman scattering molecules. Independently, other investigators confirmed this conclusion also [119-120].
This surface-enhanced Raman scattering is strongest on silver, but is observable on
gold and copper as well. Surface-enhanced Raman scattering (SERS) arises from two different contributions, which are mainly discussed [121].
The first one uses the electromagnetic (EM) model, which is related to the optical
properties of free-electron-like metals [122-125]. These properties include the surface
shape and size (for example, the existence of small irregularly shaped metal particles, regularly aligned nano-size particles, grating surfaces, colloidal metal particles or fractal aggregates). Irradiation of a microscopically rough metal surface gives rise to local plasma
modes. The electric field at such a surface becomes very large if the incident photon energy is in resonance with a normal mode of the conduction electrons in the metal. This
leads to enhanced Raman scattering from those molecules which are close to the metal surface.
The second approach uses the charge transfer (CT) theory, which is based on the concept of "active sites" at the metal surface [126,127]. A specific interaction between the
metal and the molecules occupying these sites induces enhanced Raman scattering. The nature of this interaction is controversial. Some investigators have suggested that chargetransfer processes are involved between the metal and the molecule via the active sites,
which are supposed to be metal adatoms or clusters. This model implies that Raman enhancement is restricted to molecules immediately adjacent to the metal while the electro-
20
Chapter 1
magnetic approach predicts an enhancement also for molecules at a distance of more than
10 Å from the surface.
Molecules with lone-pair electrons or π-clouds show the strongest SERS. The effect
was first discovered with pyridine. Other aromatic nitrogen, oxygen and sulfur containing
compounds, such as aromatic amines, phenols, thiols, carboxylic acids and aromatic nitro
compounds, are strongly SERS active (see previous works as example [39,40,128-132]).
The intensity of the surface plasmon resonance is dependent on many factors including
the wavelength of the incident light and the morphology of the metal surface. The wavelength should match the plasma wavelength of the metal. The wavelengths of incident light
used by researchers for silver are 488 nm (blue line) and 514.5 nm (green line) and in the
case of copper or gold the red line (647.1 nm) [133-140]. The best morphology for surface
plasmon resonance excitation is small (<100 nm) particles or an atomically rough surface.
Surface-enhanced Raman scattering (SERS) spectroscopy is an important technique
that can display intrinsic interfacial sensitivity and selectivity. The Raman signals from
single molecules can show an increase by 14 or 15 orders of magnitude in single molecule
SERS (SMSERS) cases [141,142]. This effect enables SERS spectroscopy to be one of the
most sensitive ultra trace analytical method. SERS is the first vibrational spectroscopy applicable to in situ metal-solution interfaces [143] and has widespread applications to electrochemical interfaces [144-146]. For example, when the potential of an electrode is modulated between two values, SERS can be used to monitor the surface species at the two
modulated potentials. SERS has also been successfully applied to the characterization of
molecular species formed during electrochemical reduction processes [147-149]. Furthermore, several applications of SERS have been written for self-assembled monolayer
[150,151], chemical detection [152], biochemistry [153-155] and single molecule detection
[142,156].
The most challenging characteristic of SERS spectroscopy is that the surface enhancement factor (EF) is extremely sensitive to the sizes, shapes and orientations of nanostructured SERS-active surfaces. Traditional SERS active surfaces are typically metal oxidation-reduction cycle roughened electrodes (MORC electrodes) or metal colloids. The
MORC electrodes are generally produced in electrolyte solutions by applying an oxidizing
21
Chapter 1
potential to dissolve the metal and a reducing potential to redeposit the active metal. The
reproducibility of these nanostructured metal surfaces, however, is limited by using electrochemical oxidation-reduction cycles. The colloid solutions are principally synthesized
by reducing metal salt solutions. In colloid, the metal particles size and the aggregate size
are extremely influenced by the initial chemical concentrations, temperature, pH and rate
of mixing [157]. The nanostructured morphology of these traditional SERS-active surfaces
has a variety of sizes, shapes and orientations [158,159].
1.6 Computational chemistry
Computational chemistry is simply the application of chemical, mathematical and
computing skills to the solution of chemical problems. It uses computers to generate information such as properties of molecules or simulated experimental results. One of the
most known method used to calculate vibrational frequencies is density functional theory
(DFT) method.
1.6.1 Density functional theory
The density functional theories have been developed after the Hohenberg-Kohn theorem appeared in 1964 [160]. DFT calculations as a post self-consistent field method have
been used extensively in literature [161-164] as well as in this study to calculate structures
of molecules and their infrared absorption and Raman scattering intensities. DFT methods
allow us to calculate the structures, electrostatic properties and Raman scattering intensities
of molecules and to assign their Raman bands. This wide interest in the vibrational frequencies calculated by the DFT method is due to its excellent accuracy as MP2 method
[165]. However, DFT calculations are substantially less demanding computationally than
MP2 calculations. The existence of different density functionals led to many reports, which
compare vibrational frequencies [165-169] and other molecular properties [170,171] obtained by these functionals. Most of these publications conclude that the B3LYP functional
is the preferred method for vibrational analysis.
22
Chapter 1
1.6.2 Computational electrochemistry
In electrochemistry solvent and surface effects will definitely play an important role in
electrochemical processes proceeding in a condensed phase. They cannot be calculated
with sufficient accuracy with any computational method. Therefore, semiempirical as well
as ab initio methods assume the molecules under study to be in the gas phase. They are not
expected to support the results of cyclic voltammetry in every detail, but nevertheless a
correlation of computational and electrochemical data should be possible. In previous work
semiempirical molecular orbital theory and DFT are used to compute one-electron oxidation potentials for aniline and a set of 21 mono- and disubstituted anilines in aqueous solution [172]. In this study, a good correlation is found between oxidation potential and the
energy of highest occupied molecular orbital for neutral aniline in aqueous solution.
1.6.3 Heat of formation
Heat of formation and molecular geometry are important properties for any reaction to
take place chemically or electrochemically. The fact that these properties might be difficult
to estimate by experiments clearly demonstrates the importance of predictive computational approaches. Various theoretical methods have been selected to calculate the molecular geometry and heat of formation of a molecule. Among them, the semiempirical and ab
initio methods are the most commonly used. Although the ab initio approaches can calcu-
late the heat of formation of a molecule, they can only be applied to ‘small to medium size’
systems [173,174]. Furthermore, in order to obtain precise ab initio results, more complicated electron correlation methods must be considered. Hence, this method requires huge
amounts of calculation time. Recently, the ab initio and density functional theory (DFT)
methods using isodesmic reactions to calculate the heat of formation for the studied molecule have been reported extensively [175-179]. However, it is difficult to find the heats of
formation for some reactants or products of the isodesmic reaction of substituted aniline
compounds. Semiempirical methods are especially designed to obtain heat of formation of
chemical systems [180-182]. In spite of their drawbacks, semiempirical methods are useful
for our system in addition of their low computational cost.
23
Chapter 1
1.7 Aim and scope of this study
In this study, cyclic voltammograms (CVs) were recorded for o-NA, m-NA, and p-NA
and their respective amino compounds o-, m-, and p-phenylenediamine (o-PDA, m-PDA,
p-PDA) in neutral (KClO4) and acidic (HClO4) aqueous electrolyte solutions at gold and
platinum electrodes. The CVs of phenylenediamines were recorded to aid interpretation of
the reduction process of nitroanilines in terms of products and intermediates. The electrolyte was chosen because of the rather weak adsorption of the perchlorate anion compared
with most organic molecules. The adsorption behavior of several anions including ClO −4 at
silver and gold electrodes has been reported [183]. An early polarographic study suggested
that ortho- and para-substituted aromatic nitro compounds were reduced to the corresponding amines via a six-electron process [184], whereas more recently o-NA and m-NA
were reduced to the respective PDA in acidic solution at Ti/ceramic TiO2 electrodes [185].
Similar results were reported for m-NA and p-NA reduced at a copper electrode [186] and
for m-NA in the presence of the Ti3+/Ti4+ redox system [187]. The electrochemical data of
these monomers are useful references for the electrochemical data obtained from cyclic
voltammetry of PVAms, which were recorded in the same electrolyte solutions and at the
same electrodes.
SERS data of nitroanilines, phenylenediamines and PVAms adsorbed on gold electrode
in neutral and acidic perchlorate electrolyte solutions at different electrode potentials are
reported; additional information obtained from UV-Vis spectroscopy, normal Raman spectra of solid monomers (nitroanilines, phenylenediamines) and normal infrared spectra of
solid PVAms as far as necessary in order to interpret the SER-spectra are included. To find
direct evidence for the interaction of nitrogen or oxygen with the gold surface, we carefully
investigated the SER-spectra in the low-frequency region (100-500 cm-1), where the band
due to the Au-N or Au-O stretching could be observed.
The aim of using quantum chemical calculation in this study is to perform semiemprical calculation in the gas phase by AM1 (Austin Model 1) method [188] to calculate
heats of formation and heats of reaction of some reactants, intermediates and products involved in electrochemical reactions. In addition, the molecular structures of nitroaniline
and phenylenediamine isomers were optimized using the density functional of B3LYP and
24
Chapter 1
the basis set of 6-31G(d). Polarizabilities, vibrational frequencies and Raman intensities for
the optimized structure were calculated using B3LYP/6-31G(d). In order to compare intuitively the calculated frequencies and intensities with the observed ones, Raman spectra
were simulated from the calculated vibrational frequencies and Raman intensities of these
monomers. The ability and accuracy of the B3LYP method in this field are summarized in
subsection 1.6.1. Also the electron spin density of intermediates (radicals), which might be
formed during electrochemical process, was calculated using DFT (B3LYP/6-31G(d))
method. DFT calculations have been used to predict outcomes of electropolymerization reactions by determining the spin density distribution of monomeric radical cations and
hence the coupling positions in the resulting dimers or polymers [189]. The theoretical data
of radicals (for example electron spin density) provide us with useful details of the stability
or reactivity of these intermediates. Such details can be used in interpretation of the electrochemical reactions at electrode surfaces.
25
- Chapter 2 -
2 Experimental
2.1 Cyclic voltammetry
Cyclic voltammograms were recorded with gold and platinum sheet working electrodes
using neutral (0.1 M KClO4, pH = 4.90, unbuffered) and acidic (0.1 M HClO4, pH = 1.2)
aqueous solutions as supporting electrolyte and a custom built potentiostat interfaced with
a standard PC via an ADDA-converter card operated with custom-developed software. A
saturated calomel electrode and a gold or platinum sheet were used as reference and
counter electrodes, respectively, in H-cell compartments separated by glass frits. All potentials are quoted relative to the saturated calomel electrode (ESCE).
2.2 UV-Vis spectroscopy
UV-Vis spectra of solutions were recorded with 1 and 10 mm cuvets on a ShimadzuUV 2101PC spectrometer.
2.3 Raman spectroscopy
Raman spectra were recorded on a ISA T64000 spectrometer connected to a Spectraview 2D CCD detection system. Normal Raman spectra of solid nitroanilines (capillary
sampling technique) and SER-spectra were recorded using 488 nm and 647.1 nm exciting
laser light provided by Coherent Innova 70 Series ion lasers, the laser power was measured
at the laser head with a Coherent 200 power meter.
Roughening of the gold electrode (polycrystalline 99.99 %, polished down to 0.3 µm
Al2O3) used to confer SERS activity, was performed in a separate cell with an aqueous solution of 0.1 M KCl by cycling the electrode potential between ESCE = - 800 mV and ESCE
= 1650 mV for approximately 10 minutes [190].
26
Chapter 2
2.4 Infrared spectroscopy
The IR spectra were recorded with a Perkin Elmer FT-IR 1000 spectrometer. The
measured samples were prepared using KBr disk technique.
2.5 Materials
Electrolyte solutions were prepared from 18 M Ohm water (Seralpur Pro 90 c), o-, m-,
and p-nitroaniline (Fluka AG), perchloric acid (Acros, p.A.) and potassium perchlorate
(Merck, G.R.) were used as received. Recrystallization of m-phenylenediamine (Aldrich,
99+%), o-phenylenediamine, and p-phenylenediamine (Merck, p.A.) from diethyl ether
was performed under nitrogen atmosphere. The concentration of the supporting electrolytes was 0.1 M. Saturated solutions of the nitroanilines (approximately 4 mM p-NA, 8
mM o-NA, 6.5 mM m-NA) were used in all electrochemical and SERS experiments. For
recording CVs and SER-spectra of phenylenediamines 2.7 mM solution was used.
The polymer samples were supplied by the polymer chemistry group, Chemnitz University of Technology / Chemnitz, Germany. The chemical structure details are shown in
the introduction (see section 1.2). In case of polymer solutions we used 0.05 g of polymer
per 100 mL of acidic electrolyte (0.1 M HClO4) in all measurements. In 0.1 M KClO4 the
PVAm6 and PVAm8 have a very poor solubility. Therefore, we could not perform any
measurements for these polymers in this electrolyte solution. The concentration of other
polymers (PVAm4, PVAm5, PVAm7, PVAm9 PVAm10 PVAm11) used in all measurements in neutral electrolyte (0.1 M KClO4) is 0.05 g of polymer per 100 mL of electrolyte.
All solutions were freshly prepared, purged with nitrogen (99.999%) and all the measurements were performed at room temperature (20 ˚C).
2.6 Computational aspects
Calculation of the vibrational spectra (Raman) of the single molecule in gas phase was
conducted using the Gaussian 03 software package [191]. The 6-31G(d) basis set and
27
Chapter 2
Becke’s three-parameter exchange functional, in combination with the Lee-Yang-Parr correlation function (B3LYP), were used. The calculated values of frequencies were scaled
down uniformly by a factor of 0.9614, as reported by Scott and Radom [192]. The calculated spectra were plotted by means of GaussSum 0.8 visualization software package assuming the full width at half maximum of each peak is 7 cm-1. B3LYP/6-31G(d) level
theories are also used in the calculation of electron spin densities of radical cations.
The Gaussian 03 software package is also used to calculate the heats of formation by
semiempirical method for single molecules in the gas phase. The molecular geometries of
investigated molecules are fully optimized at the AM1 (Austin Model 1) level of theory
[188] without any sort of geometrical restrictions. In all calculations, the neutral species
were treated as a conventional closed-shell system, whereas the radical cation was regarded as an open-shell system.
28
- Chapter 3 -
3 Electrochemistry of nitroanilines, phenylenediamines and polyvinylamines
3.1 Nitroaniline reduction
Cyclic voltammograms recorded with a gold electrode in a 0.1 M HClO4 electrolyte solution saturated with p-NA scanned toward negative potentials started at ESCE = + 700 mV
show a wave at Ec,SCE = - 126 mV, which is caused by reduction of the p-nitro-group (Fig.
3a). A redox wave with Ea,SCE = + 536 mV and Ec,SCE = + 436 mV is observed in the second and subsequent cycles.
1
(a) p-NA
I / mA
0
-1
st
1 Cycle
nd
2 Cycle
-2
-3
-0.5
0.0
0.5
ESCE / V
6
(b) p-PDA
I / mA
4
2
0
-2
-4
0.0
0.2
0.4
0.6
ESCE / V
Fig. 3. Cyclic voltammograms obtained with a gold electrode in a solution of (a) 0.1 M
HClO4 saturated with p-NA (scanned toward negative potentials) and (b) 2.7 mM
concentration of p-PDA (scanned toward positive potentials); dE/dt = 0.1 V s-1,
room temperature, nitrogen purged.
29
Chapter 3
Assignment of this redox wave to a reversible oxidation of the amino-group of p-NA,
as mentioned in previous work [193], is unlikely; the oxidation potential of p-NA aminogroup was observed at much higher electrode potentials as discussed below. Actually the
redox wave observed in the second cycle of the reduction of p-NA corresponds to the
reversible oxidation of amino-groups of the p-PDA. This conclusion is based on
comparison of this redox wave after p-NA reduction with the redox wave obtained when
scanning toward positive potentials with p-PDA (Fig. 3b). Formation of p-PDA by
electrochemical reduction of p-NA has been discussed in details in previous work in which
reverse-pulse polarography was used [194], and the six-electron reduction process was
described in terms of the following ECE (electron transfer, chemical reaction, electron
transfer) mechanism (Scheme 1) [195-199]. The first reaction intermediate, the radical
anion (not shown in Scheme 1), has frequently been reported based on electron spin
resonance spectroscopy [200,201].
NHOH
NO2
NH
+ 2e-, + 2H+
+ 4e-, + 4H+
- H2O
NH2
NH2
- H2O
NH2
NH
NH2
Scheme 1. ECE mechanism of the reduction of p-NA via six-electron reduction process.
According to this ECE mechanism, o-PDA was also observed as a product of the electrochemical reduction of o-NA under the same conditions as used for p-NA. The cathodic
wave observed with o-NA at Ec,SCE = - 342 mV corresponds to an anodic one at Ea,SCE = +
546 mV in the return scan and the second cycle (Fig. 4a); no reduction wave is observed in
this case. The same shape and position of this anodic wave is obtained with o-PDA when
scanning toward positive potentials (Fig. 4b). Obviously the electrooxidation products
formed from the electrochemically generated o-PDA when scanning toward positive potentials are consumed very rapidly by a non-electrochemical reaction; according to results discussed below electropolymerization may occur.
30
Chapter 3
(a) o-NA
I / mA
0
st
1 Cycle
nd
2 Cycle
-5
-10
-0.5
0.0
ESCE / V
0.5
1.0
(b) o-PDA
I / mA
4
2
0
0.0
0.2
0.4
0.6
ESCE / V
Fig. 4. Cyclic voltammograms obtained with a gold electrode in a solution of (a) 0.1 M
HClO4 saturated with o-NA (scanned toward negative potentials) and (b) 2.7 mM
concentration of o-PDA (scanned toward positive potentials); dE/dt = 0.1 V s-1,
room temperature, nitrogen purged.
In contrast with our observations with both o-NA and p-NA, obviously no m-PDA as a
product of electrochemical reduction of m-NA is formed when scanning toward negative
potentials limited only by hydrogen evolution (Fig. 5a) on the basis of a comparison with
the behavior of m-PDA (Fig. 5b). However, some reduction of product of m-NA causing a
redox wave with Ea,SCE = + 351 mV and Ec,SCE = + 222 mV observed in the backward scan
and subsequent potential cycles must have been formed. This redox wave (Fig. 5a) is
significantly different from the poorly defined anodic wave observed at Ea,SCE = + 870 mV
for electrooxidation of m-PDA when scanning toward positive potentials (Fig. 5b). Early
polarographic studies of nitroanilines in absolute ethanol and in ethanol-water and
tetrahydrofuran-water mixtures [202-204] suggested, that o-NA and p-NA underwent sixelectron reduction processes whereas m-NA gave four-electron waves only at low pH
[205]. In later studies of reduction of the isomers complicated ECE-mechanisms have been
reported [10] and the substantial effect of the electrolyte solution, including pH, has been
pointed out [206,207]. Furthermore, for o-NA and p-NA the possibility of formation of
31
Chapter 3
quinone-like species would facilitate reduction to the respective amine whereas for m-NA
the reduction might yield intermediates subject to subsequent rearrangement and condensation. In scans toward negative potentials of solutions saturated with nitroanilines in acidic
electrolyte the CVs recorded with a platinum electrode showed only hydrogen evolution.
5
(a) m-NA
I / mA
0
st
1 Cycle
nd
2 Cycle
-5
-10
-15
-0.3
0.0
4
ESCE / V
0.3
0.6
I / mA
(b) m-PDA
2
0
0.0
0.3
0.6
0.9
1.2
ESCE / V
Fig. 5. Cyclic voltammograms obtained with a gold electrode in a solution of (a) 0.1 M
HClO4 saturated with m-NA (scanned toward negative potentials) and (b) 2.7 mM
concentration of m-PDA (scanned toward positive potentials); dE/dt = 0.1 V s-1,
room temperature, nitrogen purged.
In neutral unbuffered electrolyte (0.1 M KClO4) results for all the nitroanilines, at both
gold and platinum electrodes, were different from those obtained in scans toward negative
potentials in acidic electrolyte solutions. The cathodic waves observed for nitroaniline
reduction (p-NA at Ec,SCE = - 821 mV (Fig. 6a), o-NA at Ec,SCE = - 766 mV (Fig. 6c), mNA at Ec,SCE = - 764 mV (Fig. 6e)) at gold electrode are associated with two poorly defined
anodic waves and one cathodic wave for p-NA (Ea1,SCE = - 100 mV, Ea2,SCE = - 10 mV,
Ec,SCE = - 173 mV) and o-NA (Ea1,SCE = + 30 mV, Ea2,SCE = + 195 mV, Ec,SCE = - 88 mV),
and a redox wave for m-NA (Ea,SCE = - 288 mV, Ec,SCE = - 358 mV).
32
Chapter 3
5
(b) p-PDA
I / mA
I / mA
0
(a) p-NA
st
1 Cycle
nd
2 Cycle
-10
-1.0
-0.5
0.0
0
0.0
0.5
(c) o-NA
4
I / mA
I / mA
0
st
1 Cycle
nd
2 Cycle
-10
-1.0
-0.5
0.0
0.5
5
0.4
0.6
ESCE / V
(f) m-PDA
I / mA
I / mA
st
1 Cycle
nd
2 Cycle
0.0
0.2
(d) o-PDA
0.0
(e) m-NA
-0.5
0.6
0
0
-1.0
0.4
2
ESCE / V
-10
0.2
ESCE / V
ESCE / V
0
0.5
0.0
0.5
1.0
ESCE / V
ESCE / V
Fig. 6. Cyclic voltammograms obtained with a gold electrode in a solution of 0.1 M
KClO4 saturated with nitroanilines (scanned toward negative potentials starting at
ESCE = 0.3 V) and 2.7 mM phenylenediamines (scanned toward positive potentials
starting at ESCE = 0 V); dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
Previous investigations of p-NA with normal and reverse-pulse polarography at high
pH suggested three electroactive products of p-NA reduction [194]. The authors suggested
that the two anodic waves reflect the oxidation of p-amino-N-phenylhydroxylamine and pPDA and the cathodic wave corresponds to reduction of p-nitrosoaniline, which is formed
during a homogeneous reaction as follows:
NHOH
NH
+
NH
NO
NH2
+
NH2
NH2
Scheme 2. Formation of p-nitrosoaniline from homogeneous reaction.
33
NH2
Chapter 3
Our results imply the formation of p-amino-N-phenylhydroxylamine, which is oxidized
when scanning toward positive potentials, and p-nitrosoaniline, which is subsequently reduced. Formation of p-PDA is unlikely because its oxidation was observed at higher potentials only (see CVs recorded under the same condition (Fig. 6b)). Assignment of the poorly
resolved double peak in the anodic scan to two different reduction intermediates or further
products is currently impossible. Stočesová, on the other hand, proved that in acidic medium nitrobenzene is reduced to aniline through a phenylhydroxylamine intermediate
whereas in alkaline medium (pH ≥ 5) phenylhydroxylamine is the final product and no
further reduction occurs [208].
m-NA is reduced more easily (at less negative potentials) than both o-NA and p-NA.
This reflects the position effect of the substituent. Substituents increasing the electron density at the nitro-group make the reduction more difficult, as is observed for o-NA and pNA, whereas the opposite is true for m-NA in which the nitro-group is in a position which
reduces the electron density at the nitro-group, as discussed elsewhere in detail [202]. The
same order of reduction potentials was also observed in CVs recorded with a platinum
electrode in the neutral electrolyte solution (p-NA at Ec,SCE = -923 mV, o-NA at Ec,SCE = 882 mV, m-NA at Ec,SCE = -875 mV, Fig 7).
I / mA
0
m-NA
o-NA
p-NA
-5
-10
-0.8
-0.4
0.0
0.4
ESCE / V
Fig. 7. Cyclic voltammograms obtained from a platinum electrode in 0.1 M KClO4 saturated with nitroanilines, scanned toward negative potentials starting at ESCE = 0.3
V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
34
Chapter 3
In neutral electrolyte, o-NA is more easily reduced than p-NA (Fig. 6 and 7). This
behavior is due to the effect of hydrogen bonding involving the nitro and amino-groups in
o-NA. The tendency to decrease the resonance energy of the conjugated system involving
the nitro-group and o-amino-group through electrostatic attraction between induced terminal charges tends to make the nitro-group more easily reducible. In general, the strength of
an unsymmetrical hydrogen bond A─H····B is increased by increasing the resultant
positive charge of A and the negative charge of B. This was exactly reported by Runner
[202] and can be shown for o-NA as follows.
O
O
N
N
O
O
H
H
N
N
H
H
In acidic electrolyte, p-NA is more easily reduced than o-NA (Fig. 3a and 4a). This
situation of o-NA and p-NA is quite different compared to the neutral solution. This behavior, as mentioned above, is due to reduction of these isomers to amino-Nphenylhydroxylamine and to phenylenediamine in neutral and acidic electrolytes respectively.
For an improved understanding, we studied the electrochemical reduction processes
(see ECE mechanism above) of o-NA and p-NA with the semiempirical method AM1 (see
Experimental Section). From computational investigations heats of reactions and heats of
formation, which describe the electrochemical reduction processes of o-NA and p-NA,
have been obtained, they are illustrated in Fig. 8. The negative heats of reaction (HR1, HR4)
are consistent with the exothermicity of the formation of amino-N-phenylhydroxylamine in
the gas phase. From this point of view, more negative calculated heats of reaction should
correlate with less negative electrochemically determined values (easier reduction). According to the calculated heats of reaction (HR4 < HR1), o-NA is reduced to o-amino-Nphenylhydroxylamine easier than p-NA and this is what we observed in the case of electrochemical reduction of these isomers in neutral electrolyte solution. Since phenylenediamine is the reduction product of these two isomers in acidic electrolyte, we shall compare
35
Chapter 3
the reduction potentials obtained experimentally with the calculated total heat of reaction
involved in the ECE mechanism (HR, Tot1 in case of p-NA, HR, Tot2 in case of o-NA) or with
the heats of reaction including electrochemical processes only ((HR1 + HR3) in case of pNA, (HR4 + HR6) in case of o-NA). In both cases the calculated heats of reaction show that
p-NA is reduced to p-phenylenediamine easier than o-NA (HR1 + HR3 < HR4 + HR6 or HR,
Tot1
(Exothermic) < HR, Tot2 (Endothermic)).
NO2
HR1
-6.759
kcal/mol
NHOH
HR2
58.749
kcal/mol
NH2
NH2
NH
HF
14.624
kcal/mol
HF
21.383
kcal/mol
HR4
-9.969
kcal/mol
NHOH
NH2
NH2
HF
19.663
kcal/mol
HF
73.373
kcal/mol
HR5
60.369
kcal/mol
NH
NH2
HR6
- 49.265
kcal/mol
NH2
NH
+
-
+ 4e , + 4H
+ 2e , + 2H
NH2
+
HR, Tot2
1.135
kcal/mol
-H2O
-H2O
HF
20.566
kcal/mol
HR, Tot1
-1.720
kcal/mol
-H2O
-H2O
-
NH2
+ 2e-, + 2H+
+ 4e-, + 4H+
NO2
HR3
-53.710
kcal/mol
NH
HF
10.597
kcal/mol
HF
70.966
kcal/mol
HF
21.701
kcal/mol
Fig. 8. Calculated heat of reaction in each step and heat of formation for each reactant
and product included in the ECE mechanism of electrochemical reduction of oNA and p-NA isomers according to semiempirical method AM1.
The heats of reaction HR1, HR4, HR1 + HR3 and HR4 + HR6 are correlated with reduction
potential of p-NA in neutral electrolyte, reduction potential of o-NA in neutral electrolyte,
reduction potential of p-NA in acidic electrolyte and reduction potential of o-NA in acidic
electrolyte, respectively (Fig. 9). The perfect correlation coefficient implies a perfect
agreement between theoretical and experimental data.
36
Chapter 3
0
2
R = 0.9403
-200
E SCE / mV
-400
-600
-800
-1000
-70
-60
-50
-40
-30
-20
-10
0
-1
Heat of reaction H R / kcal mol
Fig. 9. Correlation of the calculated heats of reaction (♦ for HR4 + HR6, ◊ for HR1 + HR3,
∆ and □ for HR1, ▲ and ■ for HR4) with the electrochemical reduction of o-NA
(■at gold in neutral electrolyte, ♦ at gold in acidic electrolyte, ▲ at platinum in
neutral electrolyte) and p-NA (□ at gold in neutral electrolyte, ◊ at gold in acidic
electrolyte, ∆ at platinum in neutral electrolyte).
3.2 Nitroaniline oxidation
Special behavior of m-NA was also observed in CVs recorded with a gold electrode
when scanning toward positive potentials, starting at ESCE = 0 mV, in acidic electrolyte solution. Oxidation of the amino-group causes an anodic wave at Ea,SCE = + 1045 mV, substantially lower than the anodic waves obtained with p-NA at Ea,SCE = + 1123 mV and
with o-NA at Ea,SCE = + 1150 mV (Fig. 10a). The same order of oxidation waves of the nitroanilines with approximately the same positions was observed when using a platinum
electrode (Fig. 10b); this is in perfect agreement with the order previously observed with a
platinum electrode in an acetonitrile-based nonaqueous electrolyte solution [209].
37
Chapter 3
(a) gold electrode
m-Nitroaniline
o-Nitroaniline
p-Nitroaniline
I / mA
4
2
0
0.0
0.5
1.0
1.5
ESCE / V
I / mA
3
2
(b) platinum electrode
m-Nitroaniline
o-Nitroaniline
p-Nitroaniline
1
0
0.0
0.5
1.0
1.5
ESCE / V
Fig. 10. Cyclic voltammograms obtained with gold and platinum electrodes in a solution
of 0.1 M HClO4 saturated with nitroanilines, scanned toward positive potentials,
dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
In a scan toward positive potentials starting at ESCE = 0 mV in neutral electrolyte solution saturated with nitroaniline the CVs recorded with gold show waves at Ea,SCE = + 1164
mV, Ea,SCE = + 1013 mV, and Ea,SCE = + 1120 mV for o-NA, m-NA, and p-NA respectively
(Fig. 11a), nearly the same positions of these waves were observed with a platinum electrode under the same conditions (Fig. 11b). The close agreement of the sequence of these
waves with those obtained with an acidic electrolyte is obvious.
38
Chapter 3
I / mA
3
(a) At gold electrode
m-Nitroaniline
o-Nitroaniline
p-Nitroaniline
2
1
0
0.0
0.5
ESCE / V
1.0
1.5
1.0
1.5
4
(b) At platinum electrode
m-Nitroaniline
o-Nitroaniline
p-Nitroaniline
I / mA
2
0
-2
0.0
0.5
ESCE / V
Fig. 11. Cyclic voltammograms obtained with gold and platinum electrodes in 0.1 M
KClO4 saturated with nitroaniline, scanned toward positive potentials; dE/dt = 0.1
V s-1, room temperature, nitrogen purged.
It is evident that the radical intermediate formed during electrochemical oxidation of
the amino-group of aniline is destabilized when the strong electron-withdrawing NO2
group is located in ortho and para positions, as illustrated in the following resonance structures (Scheme 3) of the aniline radical cation [210].
H
H
N
H
H
H
H
N
N
Scheme 3. Resonance structures of the aniline radical cation.
39
H
H
N
Chapter 3
The order of electrochemical oxidation observed in this study is strongly supported by
a previous study of the oxidation potential of these isomers by semiempirical and DFT
methods [172]. The good agreement between the oxidation potentials obtained theoretically in this reference for nitroaniline isomers and our experimental data is evidenced by
the high correlation coefficient obtained in all cases (Fig.12).
1200
Experimental oxidation potential / mV
o -NA
p -NA
1120
m -NA
2
R = 0.9990
1040
2
R = 1.0000
2
R = 0.9822
2
R = 0.9851
960
950
1000
1050
1100
1150
1200
Computed oxidation potential / mV
(DFT caculations)
Fig. 12. Correlation of the computed oxidation potential of nitroaniline isomers (obtained
from reference [172]) with the experimental oxidation potential of nitroaniline isomers obtained in this study (■ and ——: at gold in 0.1 M KClO4, ♦and ⎯ - ⎯: at
gold in 0.1 M HClO4, × and ----: at platinum in 0.1 M KClO4, ▲and ⎯ ⎯ ⎯: at
platinum in 0.1 M HClO4.
The CVs obtained with p-NA with both electrodes in neutral electrolyte solution also
contain a poorly defined cathodic wave at Ec,SCE = + 1054 mV at (Fig. 11). It is usually assumed that the radical cation formed during electrochemical oxidation of aniline and substituted anilines [211,212] undergoes rapid head-to-tail coupling [213-215]. Copolymers of
aniline and nitroaniline have been prepared chemically [215]. For the p-NA isomer it was
40
Chapter 3
initially suggested that this coupling was obstructed by the presence of the nitro-group in
the para position; a study of the respective polymers prepared by chemical polymerization
showed this to be incorrect [214]. Nevertheless the, presumably, slower subsequent polymerization leaves some radical cations available for electroreduction. In the absence of steric
interactions, coupling occurs preferentially between atoms with highest spin densities
[216,217]. It can be seen from the data in Fig. 13 that the radical cation of m-NA and o-NA
could be expected to go into faster and easier head-to-tail linked dimer (as in aniline radical cations) than p-NA radical cation. The position of highest spin density carbon in p-NA
radical cation is blocked by the presence of the nitro-group.
-0.014
H
-0.014
-0.014
H
0.367
H
N
-0.010
H
H
-0.010
0.059
H
-0.012
H
0.017
-0.136
O
0.028
H
H
0.004
H
0.004
N -0.030
O
-0.009
0.070
H
0.199
O
0.246
0.023
-0.153
0.408
0.004
-0.008
N
0.158
H
N
-0.014
0.273
-0.014
0.377
O
0.229
-0.136
H
0.002
N
0.229
H
-0.014
-0.012
0.388
-0.019
-0.114
-0.050
0.406
H
0.001
H
-0.017
H
0.003
-0.120
0.400
H
-0.016
0.004
O
N
O
0.015
0.028
Fig. 13. Calculated electron spin densities of nitroanilines radical cattion accourding to
B3LYP/6-31G(d) method.
3.3 Phenylenediamine oxidation
In electrooxidation of the isomeric phenylenediamines at a gold electrode the CVs of pPDA implies stability of the oxidized species at least sufficient to enable detection of the
corresponding current wave assigned to reduction of this intermediate (Fig. 3b), this reduction current is not observed for o-PDA (Fig. 4b) and m-PDA (Fig. 5b). It is known from
previous work that polymerization of m-PDA and o-PDA occurs during electrochemical
oxidation [218-221]. For p-PDA this polymerization is inhibited by the blocking of the
para position of the benzene ring, which is, in most polyanilines, the preferred coupling
41
Chapter 3
position [222]. The same features were observed for phenylenediamines in CVs with a
platinum electrode in acidic electrolyte solution (Fig. 14).
4
I / mA
2
0
-2
I / mA
(a) p-PDA
st
1 Cycle
nd
2 Cycle
rd
3 Cycle
0.0
0.2
0.4
ESCE / V
0.6
0.8
0.6
0.8
3 (b) o-PDA
st
1 Cycle
nd
2
2 Cycle
rd
3 Cycle
1
0
I / mA
0.0
0.2
0.4
ESCE / V
2 (c) m-PDA
st
1 Cycle
nd
2 Cycle
1
rd
3 Cycle
0
-1
0.0
0.2
0.4 0.6 0.8
ESCE / V
1.0
1.2
Fig. 14. Cyclic voltammograms obtained with a platinum electrode in a solution of 0.1 M
HClO4 containing 2.7 mM of phenylenediamines, scanned toward positive potentials; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
The slow growth of the cathodic wave at Ec,SCE = + 251 mV assigned to reduction of an
electrochemically redox-active polymer was observed clearly in the second and third cycle
of CVs with p-PDA combined with slowly disappearing redox waves (Ea,SCE = + 570 mV,
Ec,SCE = + 420 mV) corresponding to oxidation of monomeric p-PDA and the reduction of
the reaction intermediate (presumably the radical cation) (Fig. 14a). The cathodic signal of
the growing polymer formed electrochemically was not discussed further in earlier investi-
42
Chapter 3
gations [223-227]. The CVs recorded for m-PDA and o-PDA at a platinum electrode reveal
that polymerization is faster than for p-PDA. The poorly defined anodic wave assigned to
oxidation of m-PDA disappeared in the second and third cycle, indicating inhibition of further monomer oxidation, whereas the cathodic wave at Ec,SCE = + 426 mV indicated redox
activity of the formed polymer (Fig. 14c). A similar rapid decrease, but not disappearance,
of the oxidation wave was observed for o-PDA (Fig. 14b). In this case no significant redox
activity of a conceivable polymer was observed.
Polymerization of phenylenediamines during electrochemical oxidation at gold and
platinum electrodes was also observed in neutral electrolyte. The anodic wave obtained
with o-PDA at Ea,SCE = + 454 mV (Fig. 15b) and the poorly defined wave obtained with mPDA at Ea,SCE = + 750 mV (Fig. 15c) disappeared rapidly during the second and third cycles, implying the formation of a product on the electrode surface inhibiting further electrooxidation. For p-PDA the amplitude of the current of the two anodic waves at Ea1,SCE =
+ 212 mV and Ea2,SCE = + 413 mV and the cathodic wave at Ec1,SCE = + 139 mV decreased
only slowly, at the same time another cathodic wave at Ec2,SCE = + 340 mV (Fig. 15a) appeared. It is highly probable that the growing cathodic wave corresponds to the polymer
slowly growing on the electrode surface as discussed for the case of the acidic solution; the
other three waves correspond to redox processes of p-PDA.
The data for isomeric nitroanilines show oxidation to be more difficult than for isomeric phenylenediamines (at less positive potentials, see Table 2). The oxidation potential
of mono-substituted aniline is depending on the substituent type and position. Substituents
tending to decrease the electron density at the amino-group of aniline lead to greater difficulty of oxidation, as is observed for o-NA and p-NA, whereas the opposite is true for oPDA and p-PDA in which the amino-group is in a position which increases the electron
density at the amino-group of aniline. For m-PDA the oxidation potential is observed at
higher oxidation potential compared to o-PDA and p-PDA. This is due to the fact that the
first amino-group of m-PDA cannot efficiently accommodate the electron donated by the
second one, which leads to lower electron density of both amino-groups in comparison
with other isomers. The opposite situation is true in case of the presence of a nitro-group in
the meta-position. The oxidation potential of m-NA has been discussed previously.
43
Chapter 3
6
I / mA
4
2
(a) p-PDA
st
1 Cycle
nd
2 Cycle
rd
3 Cycle
0
-2
-0.2
I / mA
4
0.0
0.2
0.4
ESCE / V
0.6
(b) o-PDA
st
1 Cycle
nd
2 Cycle
rd
3 Cycle
2
0
0.0
0.2
0.4
ESCE / V
0.6
6 (c) m-PDA
st
1 Cycle
nd
2 Cycle
rd
3 Cycle
I / mA
4
2
0
-2
0.0
0.2
0.4
0.6
ESCE / V
0.8
1.0
Fig. 15. Cyclic voltammograms obtained with a platinum electrode in 0.1 M KClO4 con-
taining 2.7 mM phenylenediamines, scanned toward positive potentials starting at
ESCE = 0 V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
Table 2.
Position of anodic waves (ESCE, mV) obtained in the first cycle of CVs recorded
with gold and platinum electrodes for isomeric nitroanilines and phenylenediamines in acidic and neutral electrolyte solutions.
In 0.1 M HClO4
In 0.1 M KClO4
At gold
At platinum
At gold
At platinum
p-NA
1123a
1100 a
1120 b
1110 b
o-NA
1150 a
1125a
1164 b
1170 b
a
a
b
1010
1013
1005 b
m-NA
1045
c
f
g, i
g, j
p-PDA
553
570
210 , 406
212 h, i, 413 h, j
568 f
451 g
454 h
o-PDA
559 d
e
f
g
~993
~713
~750 h
m-PDA
~870
a
b
c
: value obtained from Fig. 10; : value obtained from Fig. 11; : value obtained from Fig. 3b; d: value obtained from Fig. 4b; e: value obtained from Fig. 5b; f: value obtained from Fig. 14; g: value obtained from Fig.
6; h: value obtained from Fig. 15; i: 1st anodic wave; j: 2nd anodic wave.
Compound
44
Chapter 3
The generally accepted mechanism for the electropolymerization reaction of organic
molecules like o-PDA includes three steps. The first one is a one-electron oxidation of the
monomer to form the corresponding radical cation. The second step consists of a coupling
reaction of two radical-cations to yield a dihydro-dimer dication. In the third step, the
elimination of two protons leads to dimer formation. Jang et al. proposed a mechanism for
electropolymerization of o-PDA including these three steps [228] (Scheme 4).
NH2
NH2
-
-e
NH2
NH2
2
NH2
C-N coupling
NH2
H
N
- 2H+
NH2
NH2
NH2
-
- 2e
- 2H+
N
NH2
H
N
NH2
N
NH2
N
H
NH2
- 2e- 2H+
Scheme 4. Proposed mechanism of electropolymerization for o-PDA as reported in reference [228].
C-N couplings in electropolymerization of o-PDA described in Scheme 4 seem to be
preferable for several reasons. In the absence of any steric hindrances, C-N coupling is the
preferable coupling in electropolymerization of anilines in aqueous solution [222]. The
calculated spin density values for o-PDA radical cation are displayed in Fig. 16. The carbon and nitrogen atoms of the o-PDA radical cation which are involved in the C-N coupling, as shown in Scheme 4, have high spin density values. The spin density values increase with the probability of bond formation through radical-cation coupling, the high
values indicating that the electronic orbitals of neighboring radical cations should begin to
overlap. Depending on the spin density values of m-PDA radical cation atoms (Fig. 16) and
lower steric effect in coupling between two radical cations, we proposed a mechanism of
45
Chapter 3
the three first steps in electrochemical dimerization of m-PDA (Scheme 5). These three
steps include formation of radical cations, formation of dihydro-dimer dication via coupling between two radical cations and formation of dimer by the elimination of two protons. The electropolymerization then proceeds through successive electrochemical and
chemical steps according to the reported mechanism of electropolymerization of o-PDA
[228].
-0.010
H
-0.010
H
0.254
-0.007
-0.009
N
-0.003
-0.003
0.166
H
H
0.053
-0.006
H
0.053
0.053
0.053
H
0.166
-0.003
H
H
-0.006
-0.003
N
H
0.254
0.001
H
N
0.232
-0.003
-0.058
H
0.199
0.167
H
H
N
H
0.199
-0.058
H
-0.006
N
-0.003
-0.003
0.485
0.485
0.147
0.232
H
-0.025
N
-0.006
-0.007
0.000
H
0.147
-0.009
H
H
H
-0.020
H
-0.207
0.167
H
-0.006
H
-0.020
H
0.000
0.001
-0.010
-0.010
Fig. 16. Calculated electron spin densities for isomeric phenylenediamine radical cations
according to B3LYP/6-31G(d) method.
H2N
2
H2N
NH2
NH2
-e-
H2N
NH2
H2N
NH2
- 2H+
N
H
NH2
Scheme 5. Dimerization mechanism for m-PDA radical cations.
In contrast with o-PDA and m-PDA the p-PDA is oxidized to the monocation radical
[229,230] and the resulting radical cation is not going into dimerization or polymerization
via two radicals coupling. The position of highest spin density carbons in p-PDA radical
cation (Fig. 16) are blocked with the amino-group and the C-N coupling in this case lead to
very high steric hindrance. Solís et al. [231] proposed a mechanism for electrochemical
46
Chapter 3
oxidation p-PDA to produce p-phenylenediimine D (Scheme 6) as a conjugate oxidant of
p-PDA. This ECE mechanism involves two charge transfer steps coupled with acid-base
reactions. The same authors suggest that further process can take place after electrochemical oxidation of p-PDA. The very reactive diimine product D is going into 1,4 additions reaction to produce dimer E. The product, dimer E, can be oxidized easily at the applied potential with further 1,4 coupling giving rise to a cascade process producing polymer. The
ECE mechanism in Scheme 6 is also proposed for electrochemical oxidation of substituted
p-PDA [232].
H 2N
NH2
H2N
NH2
A
A + B
+
e-
B
H2N
+
NH3
H2N
NH
C
C
NH + H+ +
HN
e-
D
NH2
NH2
N
H
NH2
E
Scheme 6. Mechanism for p-PDA electrochemical oxidation and dimer structure of pPDA.
3.4 Polyvinylamine reduction
Cyclic voltammograms recorded with a gold electrode in a 0.1 M HClO4 electrolyte solution contain p-NA-substituted PVAms (PVAm1, PVAm2, PVAm3) scanned toward
negative potentials starting at ESCE = + 800 mV show a cathodic wave, which is caused by
47
Chapter 3
reduction of the nitro-group of the p-NA moiety of these polymers (Fig. 17). A redox wave
is observed in the second and subsequent cycles. The positions of these anodic and cathodic waves are summarized in Table 3.
I / mA
0.0
PVAm1
-0.2
-0.4
st
1 Cycle
nd
2 Cycle
-0.5
0.0
0.5
1.0
ESCE / V
I / mA
0.0
PVAm2
st
1 Cycle
nd
2 Cycle
-0.2
-0.4
-0.5
0.0
0.5
1.0
ESCE / V
I / mA
0.0
PVAm3
-0.2
st
1 Cycle
nd
2 Cycle
-0.4
-0.5
0.0
0.5
1.0
ESCE / V
Fig. 17. Cyclic voltammograms obtained with a gold electrode in a solution of 100 mL 0.1
M HClO4 + 0.05 g polymer, scanned toward negative potentials starting at ESCE =
0.8V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
48
Chapter 3
Table 3.
Position of anodic and cathodic waves obtained during the first and second cycle of the CVs recorded for p-NA (Fig. 3a) and p-NA-substituted-PVAms (Fig.
17).
Redox wave in 2nd cycle
Cathodic wave in 1st cycle
Ec, SCE (mV)
Ea, SCE (mV)
Ec, SCE (mV)
p-NA
-126
+536
+436
PVAm1
-172
+518
+439
PVAm2
-190
+520
+400
PVAm3
-214
+524
+390
Compound
The redox wave observed in the second and subsequent cycles of Fig. 17 cannot be assigned to reversible oxidation of the amino-groups located at the polymers backbone; the
oxidation potential of these amino-groups was observed at much higher electrode potential
(see section 3.5). Thus, this redox wave corresponds to reversible oxidation of the reduction product of these polymers. The most probable reduction product of these polymers is
p-PDA-substituted PVAm, which might be formed via a six-electron process as described
in terms of the ECE illustrated in Scheme 7. This conclusion is based on comparison of
this redox wave after polymer reduction with the redox wave obtained after reduction of pNA (see Table 3). The reduction of p-NA via six-electron process to p-PDA is discussed in
details above in this text. Compared to p-NA the electrochemical reduction potential of the
nitro-group of the p-nitroaniline moiety of the PVAm1, PVAm2 and PVAm3 is shifted to
more negative potentials. This small shift may be due to the influence of the high molecular weight of these polymers on one hand. On the other hand, the ionic properties of p-NA,
such as charge density and acid-base strength, are slightly different from these polymers.
Furthermore, these properties are different in each case of polymer according to the percentage of the p-NA substituent and the number of protonated primary amino-groups located at the polymer backbone. In order to be sure that the waves observed in the CVs of pNA-substituted PVAms (Fig.17) is caused by the p-NA moiety of these polymers, we recorded extra CVs with a gold electrode in a 0.1 M HClO4 electrolyte solution with and
without PVAm as shown in Fig. 18. From this figure we can see that no electrochemical
reduction takes place. Only the hydrogen evolution peak of the background is suppressed
in the presence of the adsorbate PVAm.
49
Chapter 3
X
NH2
Y
NH2
Y
X
Y
X
NH
NH2
NH
Y
X
N
NH2
+4e-, +4H+
NH
+2e-, +2H+
-H2O
-H2O
NO2
NH
NHOH
NH2
Scheme 7. Proposed ECE mechanism of the electrochemical reduction of p-NAsubstituted PVAms in acidic electrolyte at a gold electrode.
1
I / mA
0
-1
-2
-3
-0.5
0.0
0.5
ESCE / V
Fig. 18. Cyclic voltammograms obtained with a gold electrode in a solution of 0.1 M
HClO4 (dashed line) and 100 mL 0.1 M HClO4 + 0.05 g PVAm (solid line), scanned toward negative potentials starting at ESCE = 0.3 V; dE/dt = 0.1 V s-1, room
temperature, nitrogen purged.
In scans toward negative potentials of solutions containing o-NA-substituted PVAms
(PVAm4, PVAm5, PVAm6, PVAm7, PVAm8) in acidic electrolyte the CVs recorded with
a gold electrode show a cathodic wave, which is caused by the reduction of the o-nitrogroup of the o-nitroaniline moiety of these polymers (Fig. 19). A redox wave is observed
in the second and subsequent cycles. The positions of the cathodic and redox waves are
summarized in the Table 4. As discussed earlier in the electrochemical reduction of o-NA,
p-NA and p-NA-substituted PVAms in acidic electrolyte solution, the formation of o-PDAsubstituted PVAm as a final reduction product of o-NA-substituted PVAm is most likely in
this case. The o-NA moiety of these polymers can be reduced through the same ECE
50
Chapter 3
mechanism described for o-NA monomer. The prominent shift to less negative potentials
observed in the electrochemical reduction of these polymers compared to o-NA (see Table
4) may be due to the influence of molecular weight and ionic properties of these polymers
0.1
0.2
0.0
0.0
I / mA
I / mA
(see previous discussion).
-0.1
PVAm4
st
1 Cycle
nd
2 Cycle
-0.2
PVAm5
st
1 Cycle
nd
2 Cycle
-0.4
-0.6
-0.3
-0.5
0.0
ESCE / V
0.5
1.0
-0.5
0.2
0.0
ESCE / V
0.5
1.0
0.2
0.0
-0.2
I / mA
0.0
I / mA
-0.2
PVAm6
st
1 Cycle
nd
2 Cycle
-0.4
-0.5
0.0
ESCE / V
0.5
-0.2
PVAm7
st
1 Cycle
nd
2 Cycle
-0.4
-0.6
1.0
-0.5
0.0
ESCE / V
0.5
1.0
0.2
I / mA
0.0
-0.2
PVAm8
st
1 Cycle
nd
2 Cycle
-0.4
-0.6
-0.5
0.0
ESCE / V
0.5
1.0
Fig. 19. Cyclic voltammograms obtained with a gold electrode in a solution of 100 mL 0.1
M HClO4 + 0.05 g polymer, scanned toward negative potentials starting at ESCE =
0.8 V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
51
Chapter 3
Table 4.
Position of anodic and cathodic waves in the first and second cycle of CVs recorded for o-NA (Fig. 4a) and o-NA-substituted PVAms (Fig. 19).
Redox wave in 2nd cycle
Cathodic wave in 1st cycle
Ec, SCE (mV)
Ea, SCE (mV)
Ec, SCE (mV)
o-NA
-342
+546a
-
PVAm4
-202
+575
+423
PVAm5
-195
+579
+419
PVAm6
-197
+611
+407
PVAm7
-192
+585
+413
PVAm8
-184
+579
+419
Compound
a
: in the case of o-NA this anodic wave is irreversible.
It should be mentioned that the oxidation process of the reduction product of these
polymers has a reversible feature while it was irreversible in the case of o-NA monomer
(see Fig. 4, 19 and Table 4). This is because the intermediate formed after electrochemical
oxidation of the reduction product of these polymers is attached to the polymer backbone
(not free) and stays available for further reduction. But for o-NA monomer the intermediate formed after the electrochemical oxidation of its reduction product is free and consumed rapidly by a nonelectrochemical reaction as described previously in Scheme 4.
In contrast to our observations with nitroaniline-substituted PVAms, obviously no redox wave in the second and subsequent cycles is observed in CVs (Fig. 20) recorded under
the same condition for stilbene-substituted PVAm (PVAm11) and Wurster-substituted
PVAms (PVAm9, PVAm10). For PVAm11 the weak shoulder in the cathodic hydrogen
evolution potential region (at Ec,SCE ≈ -190 mV) indicates a possible reduction of the stilbene moiety of this polymer. A similar cathodic shoulder was obtained with a gold electrode and in the same electrolyte for 4-dimethylamino-4´-nitrostilbene as reported by
Holze [233]. For PVAm9 and PVAm10 the cathodic wave is observed at Ec,SCE ≈ -170 mV,
which is in the reduction potential region of the nitro-group of the aromatic substituent as
for other polymers. This wave cannot be caused by the reduction of the amino radical
cation group of the Wurster substituent. The redox wave for the Wursters amino-group occurs at more anodic potential as reported by Schwarzenbacher et. al. [234]. The CVs recorded for all polymers in acidic electrolyte at a platinum electrode in negative going potential scans showed only hydrogen evolution.
52
Chapter 3
1
I / mA
0
-1
PVAm9
st
1 Cycle
nd
2 Cycle
-2
-3
-0.5
0.0
ESCE / V
0.5
1.0
I / mA
0
PVAm10
-1
-2
st
1 Cycle
nd
2 Cycle
-0.5
0.0
ESCE / V
0.5
1.0
I / mA
0
PVAm11
st
1 Cycle
nd
2 Cycle
-2
-0.5
0.0
0.5
1.0
ESCE / V
Fig. 20. Cyclic voltammograms obtained with a gold electrode in a solution of 100 mL of
0.1 M HClO4 + 0.05 g polymer, scanned toward negative potentials starting at
ESCE = 0.8 V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
In neutral unbuffered electrolyte solution (0.1 M KClO4) results for PVAm1, PVAm2,
PVAm3, PVAm4, PVAm5, PVAm7, PVAm9, PVAm10 and PVAm12, at gold electrode,
were different from those obtained in scans toward negative potentials in acidic electrolyte
solutions. The cathodic waves observed for these polymers reduction (PVAm1 at Ec,SCE = 850 mV, PVAm2 at Ec,SCE = - 835 mV, PVAm3 at Ec,SCE = - 848 mV, PVAm4 at Ec,SCE = 858 mV, PVAm5 at Ec,SCE = - 815 mV, PVAm7 at Ec,SCE = - 750 mV, PVAm9 at Ec,SCE = 866 mV, PVAm10 at Ec,SCE = - 845 mV, PVAm11 at Ec,SCE = - 840 mV, Fig. 21) are irreversible in most cases except for PVAm3. This cathodic wave is caused by the reduction of
the nitro-group of the nitro-aromatic substituent of these polymers. This conclusion is
based on the absence of this wave in the CVs recorded with gold electrode in a 0.1 M
53
Chapter 3
KClO4 electrolyte solution with and without PVAm when scanned toward negative potentials (Fig. 22). The reduction of nitro-group to hydroxylamine via a four-electron process
is most likely in this case (see previous discussion in section 3.1).
0
-0.5
I / mA
I / mA
0.0
PVAm1
st
1 Cycle
nd
2 Cycle
-1.0
-1.0
-0.5
ESCE / V
0.0
-1.0
0.5
-0.5
ESCE / V
PVAm3
st
1 Cycle
nd
2 Cycle
-0.5
-1.0
-0.5
0.0
0.5
-1.0
-0.5
0.0
0.5
ESCE / V
0
0
I / mA
I / mA
0.5
PVAm4
st
1 Cycle
nd
2 Cycle
-1
ESCE / V
PVAm5
st
1 Cycle
nd
2 Cycle
-1
-1.0
-0.5
0.0
PVAm7
st
1 Cycle
nd
2 Cycle
-1
0.5
-1.0
-0.5
0.0
0.5
ESCE / V
ESCE / V
0
0
I / mA
I / mA
0.0
0
I / mA
I / mA
0.0
-1.0
PVAm2
st
1 Cycle
nd
2 Cycle
-1
PVAm9
-1
st
1 Cycle
nd
2 Cycle
-1.0
-0.5
0.0
ESCE / V
PVAm10
st
1 Cycle
nd
2 Cycle
-1
0.5
-1.0
-0.5
ESCE / V
0.0
0.5
I / mA
0
PVAm11
st
1 Cycle
nd
2 Cycle
-1
-1.0
-0.5
0.0
0.5
ESCE / V
Fig. 21. Cyclic voltammograms obtained with a gold electrode in a solution of 100 mL 0.1
M KClO4 + 0.05 g polymer, scanned toward negative potentials starting at ESCE =
0.3 V; dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
54
Chapter 3
I / mA
0.0
-0.5
-1.0
-1.0
-0.5
0.0
0.5
ESCE / V
Fig. 22. Cyclic voltammograms obtained with a gold electrode in a solution of 0.1 M
KClO4 (dashed line) and 100 mL 0.1 M KClO4 + 0.05 g PVAm (solid line), scanned toward negative potentials starting at ESCE = 0.3 V; dE/dt = 0.1 V s-1, room
temperature, nitrogen purged.
A weak pre-peak potential is observed in the first cycle of CVs of PVAm1 (Fig. 21) at
Ec,SCE = - 170 mV. This pre-peak might correspond to some reduction intermediates
formed during electrochemical reduction steps for nitro-group as described in previous
work [201]. The cathodic wave for PVAm3 is associated with redox wave at Ea,SCE = -74
mV and at Ec,SCE = - 120 mV (Fig. 21). This redox wave is in the range of the observed one
in the second subsequent cycle of electrochemical reduction for p-NA (see Fig. 6a). Accordingly, we can assume that this redox wave in the second subsequent cycle of the CVs
of PVAm3 is due to oxidation-reduction of the p-amino-N-phenylhydroxylaminesubstituted PVAm, the reduction product of this polymer. This redox feature is not observed in the case of electrochemical reduction of other polymers. This result suggests that
the reduction product of the PVAm3 exhibits a relative stability at the electrode surface
and does not diffuse rapidly into the bulk of the solution or is not consumed by a fast
chemical reaction. This stability of the reduction product of PVAm3 might be related to the
strong coordination of this polymer with the gold surface via the nitro-group as detected in
SERS measurements (see following chapter).
55
Chapter 3
The current of the cathodic waves of electrochemical reduction of the nitroaniline moiety of the PVAms is expected to be higher in the polymers with smaller chain length and
higher percentage of the nitroaniline. However, such a correlation was not observed here
(see CVs illustrated in Fig. 17, 19 and 21). The polymer layers and number of nitroaniline
substituents, which can be exposed to the electrode surface, is changed depending on the
conformation of the polymer in the electrolyte solution and at the electrode surface. In
acidic electrolyte (0.1 M HClO4, pH = 1.2), the amino-groups at the PVAms backbone are
completely hydrolized and the repulsion between the segments is dominant. In this situation the polymer adopt conformations with loops and tails (see the introduction). These
conformations increase the adsorption of the polymer and the adsorption restriction from
the long polymer chains become small. On the other hand, these conformations reduce the
amount of nitroaniline substituent which can be exposed to the electrode surface. In neutral
electrolyte (0.1 M KClO4, pH = 4.9), the PVAms are partially hydrolized. In this case the
repulsion between the polymer segments is reduced and a more flat conformation of the
polymer layers is favoured. In flat conformation the adsorption of the polymer on one hand
is hardly dependent on the molecular weight (see also introduction for the effect of cationic
monomer content) and on the other hand, the interaction between flat layers of the polymer
is possible and can be established via hydrogen bonding between nitroaniline substituents
and amino-groups. In such a way interactions control the amount of nitroaniline substituent, which can be exposed to the surface. These assumptions of polymer conformations
might be true in bulk solutions but not at the electrode surface. At the electrode surface the
conformation of polymer changes according to the monomer-surface contacts (Fig. 23). As
can be seen in Fig. 23 the conformation of the polymer at the surface is changed according
to the number of monomer-surface contacts and the distance d between each contact. Unfortunately, information about monomer-surface contacts is not available in the literature
and not studied here. In this study we can only determine the coordination type between
polymer substituent and gold surface as obtained from SERS measurements (see below).
Such monomer-surface contacts should be considered for PVAms in a future work. Atomic
force microscopy (AFM) has been used for such measurements in previous work [235].
56
Chapter 3
Model 1
d
Surface
Substituent
group
Monomer-Surface
Contact
Model 2
d
Surface
Model 3
d
Surface
Fig. 23. Schematic illustration of several monomer-surface contact models.
3.5 Polyvinylamine oxidation
In a scan toward positive potentials starting at ESCE = 0 V in 0.1 M HClO4 electrolyte
solution containing PVAms with a very low mole percentage of a nitroaniline substituent
(PVAm1, PVAm4) or none (PVAm) the CVs recorded with gold electrode show two anodic waves (Fig. 24a-c).
57
Chapter 3
PVAm
0.1 M HClO4
0
0.5
1.0
ESCE / V
-2
1.5
(b)
2
I / mA
PVAm1
0.1 M HClO4
0.0
0.5
ESCE / V
1.0
(e)
10
PVAm1
0.1 M HClO4
0
0.0
0.5
1.0
1.5
1.0
1.5
ESCE / V
(h)
PVAm1
0.1 M HClO4
5
0
0.0
0.5
1.0
1.5
-2
0.0
ESCE / V
4
(c)
I / mA
PVAm4
0.1 M HClO4
1
0
0.5
ESCE / V
1.0
10
PVAm4
0.1 M HClO4
0
0.5
1.0
1.5
-2
0.5
(i)
PVAm4
0.1 M HClO4
5
0
-1
0.0
0.0
ESCE / V
(f)
2
-5
1.5
I / mA
-2
I / mA
5
-5
1.5
I / mA
0.0
-1
-2
PVAm
0.1 M HClO4
0
0
2
(g)
10
0
-2
I / mA
PVAm
0.1 M HClO4
2
-1
1
15
(d)
I / mA
4
(a)
I / mA
I / mA
1
0.0
0.5
1.0
ESCE / V
ESCE / V
1.5
-5
0.0
0.5
1.0
1.5
ESCE / V
Fig. 24. Cyclic voltammograms obtained with a gold electrode in a solution of (a-i, dashed
line) 0.1 M HClO4, (a-c, solid line) 100 mL 0.1 M HClO4 + 0.05 g of polymer, (df, solid line) 100 mL 0.1 M HClO4 + 0.1 g of polymer and (g-i, solid line) 100 mL
0.1 M HClO4 + 0.6 g of polymer, scanned toward positive potentials; dE/dt = 0.1
V s-1, room temperature, nitrogen purged.
In Fig. 24a-c the first anodic current increase starts at ESCE = + 950 and the peak appears at Ea1,SCE ≈ + 1081 mV for the PVAm, PVAm1 and PVAm4 containing solutions.
This wave is in the range of gold hydroxide/-oxide formation. However, the large oxidation current observed in the presence of a small amount of polymer only can be attributed
to the oxidation of the amino-groups located at the polymer backbone. The amino-groups
are protonated in acidic electrolyte solution and the anodic oxidation might proceed starting with a deprotonation step as reported by Mohilner et. al. [236] for the electrochemical
oxidation of the protonated amino-group of aniline in sulfuric acid. This interpretation is
supported by the fact, that the cathodic current caused by the subsequent reduction of gold
58
Chapter 3
oxide layer is significantly reduced. The second anodic peak appears at Ea2,SCE ≈ + 1350
mV for the same polymers. This wave might correspond to the oxidative desorption of the
adsorbed species occurring after oxidation of the polymer in the first wave. These species
were detected in SERS measurements (see the following chapter) at an electrode potential
of ESCE = + 1000 mV. This assignment, in particular of the first wave, is further supported
by CVs (Fig. 24d-f) recorded under the same conditions with higher concentrations of
polymer (double the concentration used in Fig. 24a-c). In these CVs the first anodic wave
shows a significant increase of the peak current with only a small increase of the current of
the second peak. The intermediate formed in the electrochemical oxidation of these polymers is reduced in a cathodic wave at Ec,SCE ≈ + 738 mV in the subsequent cathodic sweep
(Fig. 24d-f). The peak current of this wave is smaller than that of the anodic one. This implies a chemical side reaction after the oxidation of these polymers. In a third experiment,
we recorded CVs of these polymers with very high concentration in the same electrolyte at
the same electrode (Fig. 24g-i). These CVs show clearly the huge anodic current of the
oxidation of these polymers at Ea,SCE ≈ + 1168 mV and its corresponding cathodic current
at Ec,SCE ≈ + 645 mV in the reverse scan. Moreover, the peak currents of the
electrochemical reduction of gold oxide species in the polymer CVs (Fig. 24, solid lines at
Ec,SCE ≈ + 840 mV) are identical. Only the peak currents of other waves depend on the
concentration of the polymer. These features strongly suggest that the high anodic current
waves correspond to the electrochemical oxidation of these polymers.
The presence of the polymers PVAm, PVAm1, PVAm4 and their electrooxidation at
gold electrodes from a solution of 100 mL 0.1 M KClO4 + 0.05 g polymer can be derived
from the features in the CVs of Fig. 25a-c. The wave at Ea,SCE = + 924 mV assigned to the
gold hydroxide oxide layer is suppressed: instead a significantly higher current of two
broad overlapping anodic waves are observed at Ea1,SCE ≈ + 1153 mV and Ea2,SCE ≈ + 1345
mV. The first anodic wave can be attributed to the oxidation of the amino-groups located at
the polymer backbone and the second one to the oxidative desorption feature, as already
has been observed and discussed previously in the acidic electrolyte solution case. This assignment in particular of the first wave is further supported by CVs (Fig. 25d-f) recorded
under the same conditions with higher concentrations of polymer (double the concentration
used in Fig. 25a-c). In these CVs the first anodic wave shows a significant increase of the
peak current with only a small increase of the current of the second peak. On the subsequent negative sweep the oxidation product of these polymers is reduced in the reverse
59
Chapter 3
scan in the potential range of Ec, SCE ≈ 662-160 mV. Also in the subsequent negative sweep
the current waves assigned to the gold oxide reduction are correspondingly diminished in
most cases.
0.0
0.5
I / mA
1.0
ESCE / V
0
-1
1 (c)
0.0
0.5
ESCE / V
1.0
0.5
0.5
1.0
1.5
1.0
1.5
1.0
1.5
ESCE / V
(e)
PVAm1
0.1 M KClO4
1
0
0.0
0.5
ESCE / V
2
(f)
PVAm4
0.1 M KClO4
0.0
0.0
-1
1.5
1
0
-1
0
2
PVAm1
0.1 M KClO4
PVAm
0.1 M KClO4
-1
1.5
I / mA
1 (b)
I / mA
1
PVAm
0.1 M KClO4
0
-1
I / mA
(d)
(a)
I / mA
I / mA
1
1.0
0
-1
1.5
PVAm4
0.1 M KClO4
0.0
0.5
ESCE / V
ESCE / V
Fig. 25. Cyclic voltammograms obtained with a gold electrode in a solution of (a-f, dashed
line) 0.1 M KClO4, (a-c, solid line) 100 mL 0.1 M KClO4 + 0.05 g of polymer and
(d-f, solid line) 100 mL 0.1 M HClO4 + 0.1 g of polymer, scanned toward positive
potentials starting at ESCE = 0 V; dE/dt = 0.1 V s-1, room temperature, nitrogen
purged.
The electrooxidation of the polymers PVAm, PVAm1 and PVAm4 at a platinum electrode and in 0.1 M HClO4 shows an irreversible anodic wave at Ea,SCE ≈ + 1334 mV (Fig.
26a-c). These waves can be attributed to the irreversible oxidation of the amino-groups located at the polymer backbone. In 0.1 M KClO4 the electrooxidation of these polymers
shows only high anodic current over the potential range of Ec,SCE ≈ 764-1258 mV (Fig.
26d-f), which might be caused by adsorbed polymer layers. In the subsequent negative go-
60
Chapter 3
ing sweep the current waves assigned to the platinum oxide reduction are diminished in all
cases and in both electrolytes.
2
1
2
0.0
0.5
ESCE / V
1.0
0.5
PVAm1
0.1 M HClO4
0
2
0.5
ESCE / V
1.0
0.5
ESCE / V
1.5
1.0
1.5
1.0
1.5
PVAm1
0.1 M KClO4
0.0
0.5
PVAm4
0.1 M HClO4
0.0
1.0
0.0
0.5
ESCE / V
0
-1
0.5
ESCE / V
(e)
-0.5
1.5
(c)
1
0
0.0
(b)
0.0
PVAm
0.1 M KClO4
1.5
I / mA
I / mA
I / mA
0
-1
I / mA
(d)
PVAm
0.1 M HClO4
1
I / mA
I / mA
(a)
1.0
PVAm4
0.1 M KClO4
0.0
-0.5
1.5
(f)
0.0
0.5
ESCE / V
Fig. 26. Cyclic voltammograms obtained with a platinum electrode in a solution of (a-c,
dashed line) 0.1 M HClO4, (d-f, dashed line) 0.1 M KClO4, (a-c, solid line) 100
mL 0.1 M HClO4 + 0.05 g of polymer and (d-f, solid line) 100 mL 0.1 M KClO4
+ 0.05 g of polymer, scanned toward positive potentials scan starting at ESCE = 0
V, dE/dt = 0.1 V s-1, room temperature, nitrogen purged.
The electrooxidation of the polymers PVAm2, PVAm3, PVAm5, PVAm6, PVAm7,
PVAm8, PVAm9, PVAm10 and PVAm11 at a gold electrode and in 0.1 M HClO4 electrolyte solution shows a single anodic wave (Fig. 27); the position of this wave for every
polymer is summarized in Table 5. This wave can be attributed to the irreversible oxidation
of the primary amino-groups located at the polymer backbone for several reasons. The
electrooxidation of the nitroaniline amino-group and stilbene are observed at much lower
potential range (see Table 5). Moreover, the secondary amino-group of the aromatic substituent of the PVAms can hardly be exposed to the electrode surface because of its
position between the polymer backbone and benzene ring. Also the current wave assigned
61
Chapter 3
to the gold hydroxide/oxide formation at Ea,SCE = + 1090 mV and the wave current assigned to the gold oxide reduction at Ec,SCE = + 835 mV, in all cases, are smaller than those
obtained with supporting electrolyte solution alone.
PVAm2
0.1 M HClO4
-1
0
-1
0.0
0.5
1.0
-2
1.5
0.0
PVAm6
0.1 M HClO4
0.5
ESCE / V
1.0
0
-2
1.5
PVAm9
0.1 M HClO4
I / mA
-1
0.0
0.5
1.0
ESCE / V
0.5
1.0
1.5
PVAm10
0.1 M HClO4
ESCE / V
0.0
0.5
1.0
1.5
1.0
1.5
ESCE / V
PVAm11
0.1 M HClO4
1
0
-2
1.5
0
-2
1.5
-1
0.0
1.0
-1
1
0
0.5
PVAm8
0.1 M HClO4
1
I / mA
0.0
0.0
ESCE / V
I / mA
I / mA
I / mA
-2
1.5
-1
1
I / mA
1.0
PVAm7
0.1 M HClO4
1
-1
-2
0.5
ESCE / V
0
-2
0
-1
ESCE / V
1
PVAm5
0.1 M HClO4
1
I / mA
0
-2
PVAm3
0.1 M HClO4
1
I / mA
I / mA
1
0
-1
0.0
0.5
1.0
ESCE / V
1.5
-2
0.0
0.5
ESCE / V
Fig. 27. Cyclic voltammograms obtained with a gold electrode in a solution of (dashed
line) 0.1 M HClO4 and (solid line) 100 mL 0.1 M HClO4 + 0.05 g of polymer,
scanned toward positive potentials starting at ESCE = 0 V; dE/dt = 0.1 V s-1, room
temperature, nitrogen purged.
Table 5.
Position of anodic waves in CVs recorded with PVAms, p-NA, o-NA, p-PDA,
o-PDA and stilbene.
Polymer
Ea, SCE / mV
Polymer
Ea, SCE / mV
Compound
Ea, SCE / mV
a
b
c
PVAm, PVAm1, PVAm4 + 1081 , + 1350
PVAm7
+ 1347
p-NA
1123d
c
c
PVAm2
+1 236
PVAm8
+ 1365
o-NA
1150 d
c
c
PVAm9
+ 1347
p-PDA
553 e
PVAm3
+ 1353
PVAm10
+ 1324 c
o-PDA
559 f
PVAm5
+ 1272 c
c
c
PVAm6
+ 1342
PVAm11
+ 1324
stilbene
540g
a, b
st
nd
c
d
: 1 and 2 anodic waves obtained from Fig. 24a-c; : single anodic wave obtained from Fig. 27; : single
anodic wave obtained from Fig. 10a; e: single anodic wave obtained from Fig.3b; f: single anodic wave obtained from Fig. 4b; g: single anodic wave obtained from reference [233].
62
Chapter 3
From Table 5 one can note that the oxidation of PVAm, PVAm1 and PVAm4 is easier
than of PVAm2, PVAm3, PVAm5, PVAm6, PVAm7, PVAm8, PVAm9, PVAm10 and
PVAm11. This is evident from the lower oxidation potential of the first anodic wave of
PVAm, PVAm1 and PVAm4. The higher oxidation potential observed for PVAm2,
PVAm3, PVAm5, PVAm6, PVAm7, PVAm8, PVAm9, PVAm10 and PVAm11 might be
due to the higher percentage of the nitro-aromatic or stilbene substituent located at the
backbone of these polymers. The oxidative desorption features in the second anodic wave
of the PVAm, PVAm1 and PVAm4 occur in the potential range of oxidation of the primary
amino-groups of other polymers in most cases. Thus, the single anodic wave observed for
PVAm2, PVAm3, PVAm5, PVAm6, PVAm7, PVAm8, PVAm9, PVAm10 and PVAm11
should correspond to the oxidation of the primary amino-groups of these polymers and
oxidative desorption of the adsorbed species occurring after oxidation of these polymers.
The latter electrooxidation of all PVAms amino-groups (more difficult) compared to the
electrooxidation of the p- and o-phenylenediamine amino-groups (see Table 5) can be attributed to the following reasons. The first one is the direct attachment of the benzene ring
to the amino-group in phenylenediamine, which increases the stability of the radical cation
formed after electrooxidation of the amino-group through a resonance effect. The second
reason is the presence of nitroaniline, radical cation Wurster and stilbene substituent at the
polymer backbone. These substituents tend to decrease the electron density of the polymer
amino-groups and lead to greater difficulty (higher oxidation potential) of oxidation of
these groups.
The reversible feature, shown in Fig. 24 for electrooxidation of the PVAm, PVAm1
and PVAm4, could not be observed for electrooxidation of other polymers as shown in Fig.
27. This result suggests that an increase of the amount of nitro-aromatic or stilbene substituents at the PVAm backbone destabilizes the intermediate formed after elecrooxidation
of these polymers. The irreversible electrooxidation of the PVAm2, PVAm3, PVAm5,
PVAm7, PVAm9, PVAm10 and PVAm11 polymers is also observed in CVs recorded for
these polymers at gold electrode in 0.1 M KClO4 (Fig. 28). The irreversible oxidation
wave of PVAm2, PVAm3, PVAm5, PVAm7, PVAm9, PVAm10 and PVAm11 in Fig. 28
was observed as a shoulder at Ea,SCE ≈ + 1166, + 1239, + 1233, + 1206, + 1248, +1241 and
+ 1267 mV, respectively. In the case of CVs of PVAm2, PVAm3, PVAm9 and PVAm10,
extra anodic currents were observed in the potential range of Ea,SCE ≈ 400-860 mV. This
anodic current might be due to adsorption feature of these polymers.
63
Chapter 3
PVAm2
0.1 M KClO4
I / mA
0
-1
0.0
I / mA
0.5
ESCE / V
1.0
0.0
0.0
1.0
-1
1.5
0.0
0.5
0.5
1.0
1.0
1.5
0
-1
1.5
0.0
0.5
ESCE / V
ESCE / V
1.0
PVAm7
0.1 M KClO4
0.5
I / mA
1.5
PVAm11
0.1 M KClO4
1
I / mA
I / mA
PVAm5
0.1 M KClO4
0.0
1.0
ESCE / V
0
-1
1.5
0
ESCE / V
1
1.0
PVAm10
0.1 M KClO4
1
0.5
0.5
ESCE / V
0
-1
0
-1
1.5
PVAm3
0.1 M KClO4
1
PVAm9
0.1 M KClO4
1
I / mA
I / mA
1
0.0
-0.5
-1.0
0.0
0.5
1.0
1.5
ESCE / V
Fig. 28. Cyclic voltammograms obtained with a gold electrode in a solution of (dashed
line) 0.1 M KClO4 and (solid line) 100 mL 0.1 M KClO4 + 0.05 g of polymer,
scanned toward positive potentials starting at ESCE = 0 V; dE/dt = 0.1 V s-1, room
temperature, nitrogen purged.
Fig. 29 shows the CVs recorded with a platinum electrode in a positive going potential
scan in a solution of 0.1 M HClO4 in the absence and presence of PVAm2, PVAm3,
PVAm5, PVAm6, PVAm7, PVAm8, PVAm9, PVAm10 and PVAm11. The platinum hydroxide and oxide formation are strongly inhibited and the corresponding reduction of
these surface layers are also reduced to a great extent, which strongly implies adsorption of
polymer layers even at the low concentration employed. The adsorbed polymer species
64
Chapter 3
show an anodic current response distributed over the potential range of Ea,SCE ≈ 900-1300
mV in all cases.
PVAm2
0.1 M HClO4
PVAm3
0.1 M HClO4
0
-1
0
-1
0.0
0.6
1.2
1.8
0.0
1.8
0.0
0
0
-1
0.6
1.2
1.8
0.0
0.6
1.2
1.8
0.6
1.2
1.2
1.8
PVAm11
0.1 M HClO4
1
0
-1
ESCE / V
0.6
ESCE / V
I / mA
I / mA
-1
0.0
0.0
PVAm10
0.1 M HClO4
1
0
1.8
0
ESCE / V
PVAm9
0.1 M HClO4
1.2
-1
ESCE / V
1
0.6
ESCE / V
PVAm8
0.1 M HClO4
1
I / mA
I / mA
I / mA
1.2
PVAm7
0.1 M HClO4
1
-1
I / mA
0.6
ESCE / V
PVAm6
0.1 M HClO4
0.0
0
-1
ESCE / V
1
PVAm5
0.1 M HClO4
1
I / mA
1
I / mA
I / mA
1
0
-1
0.0
0.6
ESCE / V
1.2
0.0
0.6
1.2
ESCE / V
Fig. 29. Cyclic voltammograms obtained with a platinum electrode in a solution of
(dashed line) 0.1 M HClO4 and (solid line) 100 mL 0.1 M HClO4 + 0.05 g of
polymer, scanned toward positive potentials starting at ESCE = 0 V; dE/dt = 0.1 V
s-1, room temperature, nitrogen purged.
In 0.1 M KClO4 the adsorbed species of the polymers PVAm2, PVAm3, PVAm5,
PVAm7, PVAm9, PVAm10 and PVAm11 also show anodic current over the potential
range of Ea,SCE ≈ 800-1220 mV (Fig. 30). Blocking the electrode surface upon adsorption
of polymer layers in this case is similar with that obtained and discussed with an acidic
electrolyte solution.
65
Chapter 3
1
I / mA
0.5
I / mA
PVAm2
0.1 M KClO4
0.0
0.0
0.5
1.0
PVAm7
0.1 M KClO4
0
1.5
0.0
0.5
ESCE / V
1.0
1.5
1.0
1.5
1.0
1.5
ESCE / V
PVAm3
0.1 M KClO4
0.5
1.0
0.0
0
0.5
0.5
ESCE / V
1
PVAm5
0.1 M KClO4
0.0
0.0
1.5
ESCE / V
1
I / mA
I / mA
0
0.0
PVAm9
0.1 M KClO4
0.5
I / mA
I / mA
1
1.0
PVAm10
0.1 M KClO4
0
1.5
0.0
0.5
ESCE / V
ESCE / V
I / mA
1
PVAm11
0.1 M KClO4
0
0.0
0.5
1.0
1.5
ESCE / V
Fig. 30. Cyclic voltammograms obtained with a platinum electrode in a solution of
(dashed line) 0.1 M KClO4 and (solid line) 100 mL 0.1 M KClO4 + 0.05 g of
polymer, scanned toward positive potentials starting at ESCE = 0 V; dE/dt = 0.1 V
s-1, room temperature, nitrogen purged.
66
- Chapter 4 4 UV-Visible spectroscopy
We recorded UV-Vis spectra of solutions of 0.1 M KClO4 and HClO4 saturated with
nitroanilines. A set of spectra is shown in Fig. 31. The major band attributed to the π → π*
transition is found at 428 nm, 375 nm, and 395 nm for o-NA, m-NA and p-NA, respectively, for aqueous solutions in 0.1 M KClO4. In HClO4 these bands were observed for oNA and p-NA only―for m-NA the band is very weak, possibly because of a protonation
effect.
(a) In 0.1 M KClO4
p-NA
o-NA
m-NA
1.0
Absorption / -
Absorption / -
3
2
(b) In 0.1 M HClO4
p-NA
o-NA
m-NA
0.5
1
0.0
0
400
600
800
400
600
800
Wavelength / nm
Wavelength / nm
Fig. 31. UV-Vis spectra obtained from saturated with aqueous solution of nitroanilines in
(a) 0.1 M KClO4 and (b) 0.1 M HClO4; 1-mm cell, resolution 0.1 nm.
UV-Vis spectra of phenylenediamines were also recorded in both electrolytes and are
shown in Fig. 32. The meta isomer shows very weak band in both electrolytes. This result
suggests that the amino-groups of this isomer are subject to strong protonation effects in
both electrolytes. The major band attributed to the π → π* transition is found at 446 nm
and 414 nm for p-PDA and o-PDA respectively in an aqueous solution of 0.1 M KClO4
(Fig. 32b). In 0.1 M HClO4 the band for p-PDA is shifted to the lower wavelength (394
nm, Fig. 32a), whereas for o-PDA the band is shifted to the higher wavelength (474 nm,
Fig. 32a). This shift in acidic medium might be attributed to the protonation effect of ortho
and para isomers, which is very small compared to the meta isomer as evident from its
weak absorption band.
67
Chapter 4
(a) In 0.1 M KClO4
(b) In 0.1 M HClO4
p-PDA
o-PDA
m-PDA
p-PDA
o-PDA
m-PDA
0.1
Absorption / -
Absorption / -
0.15
0.10
0.05
0.0
0.00
400
600
800
Wavelength / nm
400
600
800
Wavelength / nm
Fig. 32. UV-Vis spectra of an aqueous solution of (a) 0.1 M KClO4 and (b) 0.1 M HClO4,
containing 2.7 mM of phenylenediamines, 10 mm cell, resolution 0.1 nm.
Fig. 33 shows the UV-Vis spectra of nitroaniline-substituted PVAm polymers in both
electrolyte solutions. The absorption maximum of p-NA-substituted PVAm polymers shifts
(6 to 14 nm) to lower wavelengths in acidic electrolyte as compared to neutral one. This
shift is due to the protonation taking place at the secondary amine of a p-NA moiety of
these polymers. The interactions of proton and hydroxyl ions with p-NA moiety of the
PVAm chain and their influence on the UV-Vis absorption band as reported in previous
work [45] is illustrated in Scheme 8. The shift (6 nm) of the absorption bands in the case of
PVAm7 to the lower wavelength in acidic electrolyte compared to neutral one is relatively
smaller than that of p-NA-substituted PVAm polymers. This shift is not observed in the
case of PVAm5 and PVAm4. This means that the protonation of the secondary amine of a
o-NA moiety of these polymers is more difficult, which may be due to the formation of intramolecular hydrogen bonds between one oxygen of the nitro-group and the secondary
amino-group. Such intramolecular hydrogen bonding has been reported for o-NA in a
study of the optimized structures of substituted anilines by density functional theory (DFT)
[237].
68
Chapter 4
1.0
(a) In 0.1 M KClO4
(b) In 0.1 M HClO4
Absorption / -
Absorption / -
PVAm2, 398 nm
PVAm3, 400 nm
0.5
PVAm2, 384 nm
PVAm3, 386 nm
0.5
PVAm1, 392 nm
PVAm1, 398 nm
0.0
0.0
400
600
400
800
Wavelength / nm
0.4
800
0.4
(c) In 0.1 M KClO4
(d) In 0.1 M HClO4
Absorption / -
PVAm5, 432 nm
Absorption / -
600
Wavelength / nm
PVAm7, 438 nm
0.2
PVAm4, 438 nm
PVAm5, PVAm7
PVAm8, 432nm
0.2
PVAm6, 432nm
PVAm4, 438nm
0.0
0.0
400
600
800
400
Wavelength / nm
600
800
Wavelength / nm
Fig. 33. UV-Vis spectra of aqueous solutions of (a, c) 100 mL 0.1 M KClO4 + 0.05 g of
polymer and (b, d) 100 mL 0.1 M HClO4 + 0.05 g of polymer, 10 mm cell, resolution 0.1 nm.
+ OH
+H
H
N
H
NH
NH
NO2
NO2
NO2
λ = 394 nm
(pH 2)
λ = 403 nm
(pH 7)
OH
λ = 413 nm
(pH 12)
Scheme 8. Interactions of proton and hydroxyl ions with a p-NA moiety of the PVAm
chain and their influence on the UV-Vis absorption band as reported in reference [45].
69
Chapter 4
The UV-Vis spectra of stilbene-substituted PVAm (PVAm11) displayed in Fig. 34
show the same features as those of p-NA-substituted PVAm polymers. The absorption
maxima is shifted (13 nm) to the lower wavelength in acidic electrolyte compared to neutral one. This result suggests that the secondary amino-group of the stilbene substituent of
this polymer is subject to protonation in acidic electrolyte as described for p-NAsubstituted PVAm polymers in Scheme 8.
0.03
(a) In 0.1 M KClO4
(b) In 0.1 M HClO4
0.2
Absorption / -
Absorption / -
398 nm
0.02
0.01
385 nm
0.1
0.00
0.0
400
600
800
400
600
800
Wavelength / nm
Wavelength / nm
Fig. 34. UV-Vis spectra of aqueous solutions of (a) 100 mL 0.1 M KClO4 + 0.05 g of
PVAm11 and (b) 100 mL 0.1 M HClO4 + 0.05 g of PVAm11, 1 mm cell, resolution 0.1 nm.
The UV-Vis spectra of PVAms containing Wurster cation radical substituents
(PVAm9, PVAm10) in neutral and acidic electrolytes are illustrated in Fig. 35. The absorption maxima are shifted by 91 nm and 86 nm for PVAm9 and PVAm10, respectively to the
lower wavelength in acidic electrolyte compared to neutral. This is due to the complete
protonation of the amino-groups of these polymers in acidic electrolyte; the protonated and
unprotonated structures of these polymers are illustrated in Fig. 36. The sensitivity of the
absorption maximum of these polymers to the pH of the medium is currently under investigation using UV-vis and ESR spectroscopic methods in our institute [238].
Since the excitation wavelength of 647.1 nm used in SERS measurements is far from
absorption bands for all monomers and polymers SERS spectra were measured under offresonance condition.
70
Chapter 4
0.10
(b) In 0.1 M HClO4
Absorption / -
Absorption / -
(a) In 0.1 M KClO4
PVAm10, 506 nm
0.05
PVAm9, 417 nm
0.1
PVAm10, 420 nm
0.00
PVAm9, 508 nm
0.0
400
600
800
Wavelength / nm
400
600
800
Wavelength / nm
Fig. 35. UV-Vis spectra of aqueous solutions of (a) 100 mL 0.1 M KClO4 + 0.05 g of
polymer and (b) 100 mL 0.1 M HClO4 + 0.05 g of polymer, 1 mm cell, resolution
0.1 nm.
PVAm10
PVAm9
X
X
Y
NH2
Y
NH2
NH
NO2
NH
NO2
Unprotonated form in
0.1 M KClO4
NH2
N
H3C
CH3
PVAm9
PVAm10
ClO4
H3N
X
Y
X
H2N
ClO4
ClO4
NO2
H2N
ClO4
NO2
Protonated form in
0.1 M HClO4
N
H3C
H3N
Y
ClO4
CH3
Fig. 36. Protonated and unprotonated form of PVAm9 and PVAm10.
71
H3N
- Chapter 5 5 Adsorption of nitroanilines and phenylenediamines on gold
5.1 p-Nitroaniline adsorption on gold
Band positions from a Raman spectrum of solid p-NA (Fig. 37a), from calculated spectrum of p-NA single molecule in gas phase (Fig. 37b) and SER-spectra of p-NA adsorbed
from neutral (Fig. 37c) and acidic (Fig. 37d) electrolyte solutions are listed in Table 6; assignment of the observed lines is based on literature data [193,239-241] and calculated vibrational frequencies (the illustration of the optimized structure and the internal coordinate
list of p-NA according to DFT (B3LYP/6-31G(d)) are attached in appendix 1).
70
(a) Solid p-NA
-1
800
60
Raman intensity / s
-1
Raman intensity / s
600
400
(c) SER-spectra in 0.1 M KClO4
at ESCE = 200 mV
50
40
at ESCE = -100 mV
30
200
1500
1000
-1
Raman shift / cm
500
1500
1000
-1
Raman shift / cm
500
200
(d) SER-spectra in 0.1 M HClO4
200
-1
150
Raman intensity / s
Raman intensity / a.u.
(b) Calculated spectrum
100
50
150
at ESCE = 400 mV
100
at ESCE = -100 mV
50
0
1500
1000
500
1500
-1
1000
-1
500
Raman shift / cm
Raman shift / cm
Fig. 37. (a) Normal Raman spectrum of solid p-NA, capillary sampling technique, 488 nm
laser light; (b) calculated Raman spectrum of p-NA single molecule in gas phase
according to DFT (B3LYP/6-31G(d)); (c) and (d) SER-spectra of p-NA adsorbed
on a gold electrode at electrode potentials as indicated in neutral and acidic perchlorate solution, respectively, 647.1 nm.
72
Chapter 5
Table 6.
Spectral data of p-NA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for p-NA single molecule in
gas phase according to DFT (B3LYP/6-31G(d)).
Mode
Wilson
mode #
C-N-torsion
γ(CH)
γ(CH)
δ(ring)
δ(CH)
γ(ring)
νAu-O
n.a.
γ(ring)
βas(NO2)
γs(NH2)
δ(ring)
δ(ring)
γ(ring)
γs(NO2)
γ(CH)
ν (ring)
γ (CH)
βs(NO2)
11
10b
6a
9b
16a
16b
12
6b
4
10a
1
17b
-
γ(CH)
17a
Calc.vib.freq.
(R. int.),
Fig. 37b
p-NA
solid,
Fig. 39a
217 (0.93)
220 (1.53)
286 (0.01)
347 (1.42)
380 (0.19)
409 (0.07)
485 (0.03)
516 (2.76)
525 (1.21)
612 (1.87)
620 (6.31)
671 (1.17)
726 (0.96)
778 (7.01)
786 (3.63)
800 (0.03)
819 (43.89)
929 (0.49)
943 (2.44)
953 (1.20)
1023 (0.53)
1050 (74.49)
1114 (1.61)
1164 (11.73)
1276 (0.02)
1289 (1.38)
1335 (366.10)
1370 (40.91)
1462 (0.77)
1500 (19.66)
1513 (0.34)
1607 (59.16)
149
365
404
492
536
634
811
865
0.1 M KClO4,
Fig. 37c
ESCE = ESCE =
200 mV
100 mV
284
284
371
400
411
451
493
494
528
534
649
647
750
740
811
818
861
863
965
δ(CH)
18a
βas(NH2)
δ(CH)
18b
1110
ν(CH)
7a
δ(CH)
9a
1181
δ(CH)
3
1277
ν(ring)
14
1317
νs(NO2)
1339
ν(C─NH2)
1396
ν(ring)
19b
1453
ν(ring)
19a
1508
ν(ring)
8b
ν(ring)
8a
1595
γs(NO2) +
1635 (7.48)
δ(ring)
βs(NH2)
1672 (34.14)
1632
δ: in-plane deformation; γ: out-of-plane deformation; ν:
wagging; βs: scissoring; n.a.: not assigned.
73
0.1 M HClO4,
Fig. 37d
ESCE =
ESCE = 400 mV
100 mV
144
283
368
415
451
532
646
646
743
800
805
857
864
-
-
-
-
1114
1168
1252
1316
1334
1385
1497
1594
1111
1151
1251
1315
1383
1496
1595
1112
1153
1250
1335
1383
1501
1598
1113
1159
1251
1337
1391
1497
1607
-
-
-
-
1626
1634
stretching; νs: symmetric stretching; βas: rocking; γs:
Chapter 5
From the data displayed in Table 6, a comparison can be made between the calculated
Raman spectra of p-NA in gas phase and the recorded one in solid phase. Some of the calculated wavenumbers are not observed in the recorded Raman spectrum of the solid p-NA.
Some shift in the band positions is noted between theoretical and experimental frequencies.
These observations are due to intermolecular hydrogen bonding between p-NA molecules
in its crystal structure [21,25], which plays an important role in vibrational frequencies.
These intermolecular interactions are not included with the computational method used in
this study. We assumed the molecules under study to be single molecules in the gas phase.
Nevertheless, an overall comparison of the calculated frequencies with the experimental
one reveals a good agreement.
The SER-spectra of p-NA and its isomers were initially recorded only within a potential window between oxidation and reduction potentials to avoid contributions from reduction/oxidation products. In the SER-spectrum of adsorbed p-NA intensities of several inplane modes were increased relative to those of the out-of-plane modes in both electrolyte
solutions over the whole range of electrode potentials investigated. This implies perpendicular orientation of the adsorbed molecule; a similar conclusion has been reported for,
e.g., benzene and selected monosubstituted benzenes adsorbed on gold [242]. This orientation has also been deduced from limited experimental studies combined with DFT calculations for p-NA adsorbed on silver colloids [243] and from SER-studies with a silver electrode [244].
In the low-wave-number region an additional band not seen with solid p-NA is found at
411 and 415 cm-1 in neutral and acidic electrolyte solutions, respectively. This mode cannot be caused by the gold-oxygen mode of the adsorbed perchlorate, which was found
around 270 cm-1 in a previous study [190] and at 262 and 251 cm-1 in acidic and neutral
electrolyte, respectively, in this study (Fig. 38). Thus this band must be caused by interaction between the adsorbed p-NA and the gold surface. A variety of modes of interaction
between adsorbed p-NA and the electrode surface is conceivable and may involve the aromatic electron system of benzene ring, hydrogens atoms on the ring, or substituent groups.
Perpendicular orientation of adsorbed p-NA as mentioned above, and the coordinating capability of the substituent, makes interaction with the electrode surface via the oxygen atoms of the nitro-group or the free electron pair of the amino group likely. It has been reported that the gold-oxygen stretching mode occurs in the range from 400 to 500 cm-1 (e.g.
74
Chapter 5
approximately 412 cm-1 in [193]), whereas the gold-amino nitrogen stretching mode is
found at approximately 341 cm-1 [245] (on silver it is observed at 215 cm-1 [241]). In addition, the broadening and the downshift of the rocking, scissoring and symmetric stretching
modes of nitro-group at ESCE = 400 and 200 mV in acidic and neutral electrolyte, respectively, support the assumed adsorption via the nitro-group (Fig. 39).
Raman intensity / s
-1
90
at ESCE = 400 mV, in 0.1 M HClO4
at ESCE = 200 mV, in 0.1 M KClO4
80
70
60
50
40
100
200
300
400
500
-1
Raman shift / cm
Fig. 38. SER-spectra of adsorbed perchlorate anions on a gold electrode, 647.1 nm.
NH2
–
O
N+
O
Au
Fig. 39. Proposed structure of adsorbed p-NA at gold surface in both electrolytes and at
positive electrode potential.
75
Chapter 5
The gold-oxygen stretching mode was not observed at a more negatively charged surface at ESCE = - 100 mV. This is attributed to electrosorption of the anilinium cation of protonated p-NA in both acidic and neutral electrolytes (0.1 M KClO4, pH = 4.90, unbuffered
and only slightly acidic). Electrostatic adsorption of the anilinium cation without any of the
functional groups acting as anchors is also evident from the disappearance of the scissoring
mode of the amino-group on the negatively charged surface. A new strong band in acidic
solution and a weak one in neutral solution was, moreover, observed at 451 cm-1 for negative surface charge, presumably as a result of electrosorption of the anilinium cation (Fig.
40).
NO2
H
N
H
H
Au
Fig. 40. Proposed structure of adsorbed p-NA at gold surface in both electrolytes and at
negative electrode potential.
5.2 o-Nitroaniline adsorption on gold
Bands observed in a normal Raman spectrum of o-NA (Fig. 41a), a calculated Raman
spectrum of o-NA single molecule in gas phase (Fig. 41b) and SER-spectra of adsorbed oNA for neutral (Fig. 41c) and acidic (Fig. 41d) electrolyte solutions are listed in Table 7;
assignment of the observed lines is based on literature data [239,240] and calculated vibrational frequencies (the illustration of the optimized structure and the internal coordinate list
of o-NA according to DFT (B3LYP/6-31G(d)) are attached in appendix 2). As was observed for the p-NA isomer, the calculated Raman spectrum of o-NA reveals good agreement with observed spectra. The band position of the calculated frequencies is close to the
experimental ones in most cases (see Table 7). The o-NA adsorbs also in a perpendicular
76
Chapter 5
orientation as deduced from the SER-spectra, in which the in-plane modes are enhanced
when compared to the out-of-plane modes, which are relatively weak or absent.
800
(a) Solid
o-NA
(c) SER-spectra in 0.1 M KClO4
-1
600
Raman intensity / s
Raman intensity / s
-1
140
400
at ESCE = -100 mV
120
100
at ESCE = 200 mV
80
60
200
40
1500
1000
500
1500
-1
1000
500
-1
Raman shift / cm
Raman shift / cm
80
(b) Calculated spectrum
(d) SER-spectra in 0.1 M HClO4
90
-1
Raman intensity / s
Raman intensity / a.u.
60
40
20
at ESCE = 0 mV
80
70
60
0
50
1500
1000
500
at ESCE = 400 mV
1500
1000
500
-1
Raman shift / cm
-1
Raman shift / cm
Fig. 41. (a) Normal Raman spectrum of solid o-NA, capillary sampling technique, 647.1
nm laser light; (b) calculated Raman spectrum of o-NA single molecule in gas
phase according to DFT (B3LYP/6-31G(d)); (c) and (d) SER-spectra of o-NA adsorbed on a gold electrode at the electrode potentials indicated in neutral and
acidic perchlorate solution, respectively, 647.1 nm.
77
Chapter 5
Table 7.
Spectral data for o-NA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for o-NA single molecule in
gas phase according to DFT (B3LYP/6-31G(d)).
Mode
Wilson
mode #
Calc.vib.freq. o-NA
(R.int.),
solid,
Fig. 41b
Fig. 41a
0.1 M KClO4,
Fig. 41c
ESCE = ESCE =
200 mV
100 mV
0.1 M HClO4,
Fig. 41d
ESCE =
ESCE = 0
400 mV
mV
119 (2.75)
167
214 (1.08)
γ(CH)
10a
239 (1.57)
δ(CH)
15
292 (0.75)
298
262
δ(CH)
9b
377 (1.98)
δ(ring)
6b
391 (6.56)
392
νAu-O
406
404
410
16b
420 (0.21)
419
γ(ring)
n.a.
445
472
478
γ(ring)
16a
516 (0.30)
526 (2.91)
530
537
531
531
βas(NO2)
6a
548 (12.61)
560
569
564
561
569
δ(ring)
567 (0.78)
γs(NH2)
ν(ring)
1
657 (2.66)
670
668
667
662
670
γ(ring)
4
677 (1.45)
725 (2.61)
γ(CH)
11
780
744
744
747
755 (0.38)
δ(ring)
12
801 (29.75)
817
821
816
816
γ(CH)
17a
821 (4.12)
872
867
870
865
850 (0.54)
βs(NO2)
γ(CH)
17b
932 (0.80)
γ(CH)
5
946 (0.11)
961
938
936
936
934
δ(CH)
18b
999 (2.13)
1031
1023
1027
1031
1042 (18.68)
1067
1076
1076
βas(NH2)
ν(CH)
13
1077 (17.95)
1100
δ(CH)
18a
1143 (7.56)
1148
1137
1133
1144
δ(CH)
9a
1155 (7.44)
1176
1162
1176
7a
1259 (16.61)
1247
1231
1262
ν(CH)
ν(ring)
14
1269 (74.73)
1288
1287
1283
1295
1290
νs(NO2)
1343 (40.39)
1353
1347
1349
1321
1348
ν(ring)
1351 (79.47)
1370
1365
1369
n.a.
1388
ν(ring)
19a
1438 (2.24)
1446
1453
1455
1450
ν(ring)
19b
1474 (4.73)
1478
1494
1493
1527 (7.58)
1509
1524
1528
1529
νas(NO2)
ν(ring)
8a
1563 (23.50)
1569
ν(ring)
8b
1583 (8.61)
1601
1600
1606
1608
1601
1622 (7.36)
1625
1626
βs(NH2)
δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: asymmetric
stretching; βas: rocking; γs: wagging; βs: scissoring; n.a.: not assigned.
γ(CH)
10b
78
Chapter 5
In a solution of KClO4, the SER-spectra show the oxygen-gold mode at 406 cm-1. Appearance of this mode at lower electrode potentials does not agree with our observations in
case of p-NA (vide supra); this might be because of the lower degree of protonation of the
amino-group of o-NA than of p-NA. Protonation of o-NA is more difficult because of intramolecular hydrogen bonding between one oxygen of the nitro-group and the aminogroup, as observed in this study (see appendix 2) and reported in a study of the optimized
structures of substituted anilines by density functional theory (DFT) [237]. This interaction
is also reflected in the adsorption behavior of o-NA at a gold surface via one oxygen atom
only (Fig. 42) at a lower wavenumber compared with adsorption via two oxygen atoms for
p-NA.
In acidic electrolyte solution the oxygen-gold mode was observed at 410 cm-1 at more
positive electrode potential (ESCE = 400 mV). When we moved in the direction of negative
potential (ESCE = 0 mV) this mode disappeared and was replaced by a new strong mode at
445 cm-1. This might be caused by electrosorption of the anilinium cation of o-NA as discussed above for p-NA.
H
N
H
O–
N+
O
Au
Fig. 42. Proposed structure of adsorbed o-NA at gold surface in neutral electrolyte and at
positive and negative electrode potential.
5.3 m-Nitroaniline adsorption on gold
The overall Raman scattering intensities of m-NA are weaker than those of p-NA and
o-NA as shown in the normal Raman spectrum of solid m-NA and SER-spectra of ad-
79
Chapter 5
sorbed m-NA (Fig. 43, Table 8) [239,240,246]. This tendency is reproduced even in the
calculated Raman spectrum of m-NA (Fig. 43b) and may be associated with the order of
the average polarizablities of the isomeric nitroanilines (the calculated polarizabilities by
B3LYP/6-31G(d) are summarized for isomeric nitroanilines in the appendixes 1-3). It was
evident from Table 8 and Fig. 43a,b that experimental Raman data for solid m-NA fit with
calculated one as observed in case of the other nitroaniline isomers. This confirmed that
the calculated vibrational frequencies by the B3LYP method have very low average error
[192].
(a) Solid m-NA
(c) SER-spectra in 0.1 M KClO4
150
-1
Raman intensity / s
Raman intensity / s
-1
600
400
100
at ESCE = 100 mV
50
200
at ESCE = -100 mV
1500
1000
-1
Raman shift / cm
500
1500
(b) Calculated spectrum
500
-1
Raman intensity / s
Raman intensity / a.u.
-1
(d) SER-spectra in 0.1 M HClO4
140
100
1000
Raman shift / cm
50
at ESCE = 100 mV
120
at ESCE = -100 mV
100
80
0
1500
1000
-1
Raman shift / cm
60
500
1500
1000
-1
500
Raman shift / cm
Fig. 43. (a) Normal Raman spectrum of solid m-NA, capillary sampling technique, 647.1
nm laser light; (b) calculated Raman spectrum of m-NA single molecule in gas
phase according to DFT (B3LYP/6-31G(d)); (c) and (d) SER-spectra of m-NA adsorbed on a gold electrode at the electrode potentials indicated in neutral and
acidic perchlorate solution, respectively, 647.1 nm.
80
Chapter 5
Table 8.
Spectral data for m-NA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for m-NA single molecule in
gas phase according to DFT (B3LYP/6-31G(d)).
Mode
γ(CH)
δ(CH)
γ(CH)
δ(ring)
δ(CH)
νAu-O
n.a.
δ(ring)
γ(ring)
βas(NO2)
γs(NH2)
ν(ring)
γ(ring)
γs(NO2)
n.a.
γ(CH)
ν(CH)
Wilson
mode #
10b
9a
10a
6b
15
6a
16a
1
4
11
7b
Calc.vib.freq. m-NA
(R..int.),
solid,
Fig. 43b
Fig. 43a
0.1 M KClO4,
Fig. 43c
ESCE =
100 mV
423
532
673
730
809
ESCE = 100 mV
537
673
736
809
0.1 M HClO4,
Fig. 43d
ESCE =
100 mV
332
424
536
674
729
810
ESCE = 100 mV
469
612
814
161 (3.53)
178
211 (0.65)
221 (1.15)
231
363 (4.08)
385
394 (1.40)
497 (4.84)
514
515 (4.62)
555
541 (1.52)
562 (4.53)
616
659 (0.45)
664 (4.01)
677
721 (2.09)
739
770
778 (1.82)
791
796 (15.12)
817
850 (1.58)
γ(CH)
17b
879 (2.58)
γ(CH)
17a
915 (6.33)
929
935
940
935
936
γ(CH)
5
940 (0.96)
δ(ring)
12
975 (22.93)
999
1000
999
1001
1001
δ(CH)
18a
1050 (0.30)
δ(CH)
18b
1074 (14.64)
1099
1095
1098
1096
βas(NH2)
1098 (4.76)
δ(CH)
9b
1155 (3.12)
1164
1160
1160
ν(CH)
13
1258 (18.19)
1269
1243
1243
1242
1250
δ(CH)
3
1300 (14.12)
1309
ν(ring)
14
1336 (0.29)
1335
1346
νs(NO2)
1350
1350
1341
1344
1363
(165.20)
ν(ring)
19a
1451 (1.81)
1407
1407
ν(ring)
19b
1479 (0.83)
1483
1464
1478
νas(NO2)
1565 (19.39)
1533
1533
ν(ring)
8a
1581 (41.33)
1585
1593
1593
1596
1581
1616 (2.74)
βs(NH2)
1631
1633 (20.34)
δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: asymmetric
stretching; γs: wagging; βs: scissoring; n.a.: not assigned.
The nitro-group of m-NA cannot efficiently accommodate the electron donated by the
amino-group, which leads to lower electron density at the nitro-group oxygen atoms. This
is reflected in the shift of the observed oxygen-gold mode to higher wavenumbers at 423
81
Chapter 5
cm-1 in the SER-spectra of m-NA in neutral electrolyte solution at ESCE = 100 mV compared with the position of this mode for p-NA and o-NA. The existence of this band implies an adsorption via the nitro function (Fig. 44). Comparison of the position of this band
under otherwise the same conditions provides some insight into the effects of moleculesolvent interaction as well as simultaneous electronic resonance effects. The effect of
molecule-solvent interaction [247] in the absence of any resonant charge redistribution results in m-NA having the relatively highest negative charge density on the nitro-group and
–in turn–the strongest interaction with the gold surface, as is evident from the highest
wavenumber (at 423cm-1). For p-NA this effect is much less pronounced, the band is redshifted to 411 cm-1. With o-NA the resonance charge transfer is practically absent; in addition, one oxygen is interacting intramolecularly with a hydrogen of the amino-group (presumably only one oxygen instead of two is interacting with the metal) causing a further
red-shift to 406 cm-1. At a less positive electrode potential of ESCE = - 100 mV this band
disappeared completely; at the same time the asymmetric stretching mode of NO2 at 1533
cm-1 and also the symmetric stretching of this group appeared and became more intense
and sharp. The absence of the oxygen-gold mode and these other changes seem to indicate
a mode of adsorption with the nitro function not engaged as an anchoring group. The absence of any other mode indicating an interaction via, e.g., the amino function leaves this
suggestion unsubstantiated at the moment.
In acidic electrolyte solution the oxygen-gold mode at 424 cm-1 was observed at positive electrode potentials, it again disappeared at negative electrode potentials, whereas the
wagging mode of NH2 at 612 cm-1 was strongly enhanced. In acidic electrolyte a poorly defined broad band around 469 cm-1 was observed for m-NA at less positive electrode potentials as observed for o-NA and p-NA.
H2N
–
O
N+
O
Au
Fig. 44. Proposed structure of adsorbed m-NA at gold surface in both electrolytes and at
positive electrode potential.
82
Chapter 5
5.4 p-Phenylenediamine adsorption on gold
Fig. 45 shows the normal Raman spectrum of solid p-PDA, calculated spectrum of pPDA single molecule in gas phase and SER-spectra of p-PDA adsorbed from neutral and
acidic electrolyte solutions. Their peak positions are listed in Table 9 along with appropriate vibrational assignments. Assignments of the vibrational modes are based on literature
sources [239,240,248] and calculated vibrational frequencies (the illustration of the optimized structure and the internal coordinate list of p-PDA according to DFT (B3LYP/631G(d)) are attached in appendix 4).
(a) Solid p-PDA
(c) SER-spectra in 0.1 M KClO4
at ESCE = 100 mV
-1
150
Raman intensity / s
Raman intensity / s
-1
600
100
at ESCE = 0 mV
vAu-N
400
200
50
1500
1000
500
1500
-1
Raman shift / cm
500
-1
100
60
(d) SER-spectra in 0.1 M HClO4
Raman intensity / s
-1
(b) Calculated spectrum
Raman intensity / a.u.
1000
Raman shift / cm
40
20
80
at ESCE = 200 mV
at ESCE = 0 mV
60
vAu-N
40
0
20
1500
1000
500
1500
-1
1000
500
-1
Raman shift / cm
Raman shift / cm
Fig. 45. (a) Normal Raman spectrum of solid p-PDA, capillary sampling technique, 647.1
nm laser light; (b) calculated Raman spectrum of p-PDA single molecule in gas
phase according to DFT (B3LYP/6-31G(d)); (c) and (d) SER-spectra of p-PDA
adsorbed on a gold electrode at the electrode potentials indicated in neutral and
acidic perchlorate solution, respectively, 647.1 nm.
83
Chapter 5
Table 9.
Spectral data for p-PDA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for p-PDA single molecule in
gas phase according to DFT (B3LYP/6-31G(d)).
Mode
γ(CH)
νAu-N
γas(NH2)
δ(CH)
γ(CH)
γ(Ring)
δ(CH)
δ(Ring)
γ(Ring)
δ(Ring)
γs(NH2)
n.a.
γ(Ring)
γ(Ring)
γ(CH)
γ(CH)
ν(Ring)
γ(CH)
γ(CH)
δ(CH)
βas(NH2)
δ(CH)
δ(CH)
ν(CH)
ν(CH)
ν(C─N)
δ(CH)
n.a.
n.a.
ν(Ring)
ν(Ring)
ν(Ring)
ν(Ring)
ν(Ring)
Wilson
mode #
11
15
10b
16a
9b
6a
16b
6b
4
12
10a
17b
1
17a
5
18a
9a
18b
7a
13
3
14
19b
19a
8b
8a
Calc. vib freq.
(R. int.),
Fig. 45b
p-PDA
solid,
Fig. 45a
0.1 M KClO4,
Fig. 45c
ESCE =
ESCE = 0
mV
100 mV
199
195
419
419
493
496
538
539
612
612
650
651
727
727
749
751
844
845
1169
1170
1250
1253
1330
1369
1370
1406
1447
1445
1506
1507
1557
1558
0.1 M HClO4,
Fig. 45d
ESCE = 0
ESCE =
mV
200 mV
215
215
418
419
477
477
575
577
614
618
831
831
939
938
1023
1021
1180
1186
1245
1250
1373
1374
1473
1481
1568
1567
144 (0.00)
138
212 (1.36)
305 (0.00)
339 (2.74)
369
400 (0.00)
409 (0.47)
435
456 (8.05)
471
496 (0.00)
637 (5.91)
560
648 (0.00)
663 (8.11)
650
717 (9.64)
703
779 (8.38)
736
802 (0.00)
829 (44.51)
848
877 (0.00)
881 (5.48)
932
989 (0.00)
1043 (0.00)
1098 (10.48)
1143
1125 (0.00)
1166 (7.53)
1177
1249 (0.00)
1263 (18.65)
1267
1320 (1.14)
1333
1321 (0.00)
1447 (0.00)
1424
1511 (0.00)
1508
1583 (0.42)
1520
1618 (12.77)
1628 (0.00)
βs(NH2)
1618
1636
1637
1635 (91.48)
δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; βas: rocking; γs: wagging; βs: scissoring;
γas: twisting; n.a.: not assigned.
84
Chapter 5
The overall Raman scattering intensities of p-PDA (Fig. 45) are very weak compared to
those of p-NA (Fig. 37); this might be due to the higher polarizability of p-NA than for pPDA (see appendixes 1 and 4). The higher polarizability of p-NA is attributed to the presence of the electron withdrawing nitro-group and electron donating amino-group in paraposition at an aromatic ring system. Several calculated bands assigned in Table 9 cannot
find their counterparts in the Raman spectra of solid p-PDA. These bands in most cases are
Raman inactive and their calculated Raman intensities are zero.
Information about the orientation of the molecule with respect to the electrode surface
and the mode of its attachment can be obtained from the shift of bands in surface spectra
compared with “normal spectra” and from the appearance and disappearance of bands ascribed to either direct molecule-surface interaction or to the effect of surface selection rules
[249]. The SER-spectra of p-PDA and its isomers were initially recorded only within a potential window below the oxidation potential to avoid contributions from oxidation products. In all SER-spectra (Fig. 45c, d) in-plane as well as out-of-plane bands are observed
(see Table 9). Thus an adsorbate orientation flat (with out-of-plane modes prominent) or
perpendicular (with in-plane modes being prominent) cannot be deduced straightforwardly.
However, the band broadening observed for several ring modes (19a, 19b, 1) at all electrode potentials implies a flat orientation. In flat orientation of adsorbed p-PDA both
amino-group and aromatic ring system can act as surface anchoring sites. The broadening
and downshift of the C-N stretching frequency from 1267 cm-1 in solid p-PDA to 245-1253
cm-1 in the adsorbed state indicates coordinative interaction of the nitrogen with a metal
[250]. This result suggests a bonding of the p-PDA molecule to the surface via aminogroup nitrogen rather than via π-interaction of the aromatic ring. The appearance of a new
band at more positive electrode potentials around 195-199 cm-1 in neutral electrolyte and at
215 cm-1 in acidic one (on silver it is observed at 215 cm-1 in [241] and at 195 cm-1 in
[244], on gold it is observed from 235 to 264 cm-1 in [251,252]) is also an indication of the
interaction of p-PDA through nitrogen atom. This is in agreement with simple electrostatic
considerations where an interaction of anodically polarized electrode with the Lewis-base
amine group is more favourable and with the findings of Hubbard and Soriaga obtained
with a platinum electrode [253]. The observed νAu-N mode in this case appears in broad
shape, low intensity and shifted to lower wavenumbers compared with literature data. This
circumstance is due to the fact that adsorption of p-PDA in flat orientation via nitrogen at-
85
Chapter 5
oms includes steric constraints on the surface bonding geometry (Fig. 46). This accounts
for the upshift in several ring vibration frequencies (6a, 12, 10a, 5, 19a) upon adsorption.
H2N
NH2
Au
Fig. 46. Proposed structure of adsorbed p-PDA at gold surface in both electrolytes and at
positive and zero electrode potential.
5.5 o-Phenylenediamine adsorption on gold
Bands observed in a normal Raman spectrum of o-PDA (Fig. 47a), calculated Raman
spectrum of o-PDA single molecule in gas phase (Fig. 47b) and SER-spectra of adsorbed
o-PDA for neutral (Fig. 47c) and acidic (Fig. 47d) electrolyte solutions are listed in Table
10; assignment of the observed lines is based on literature data [239,240,248] and calculated vibrational frequencies (the illustration of the optimized structure and the internal coordinate list of o-PDA according to DFT (B3LYP/6-31G(d)) are attached in appendix 5).
In contrast with p-PDA, the SER-spectra of adsorbed o-PDA showed very intense
bands. This can be attributed to the different position of the amino-groups between these
isomers. Adsorption of o-PDA via nitrogens of the ortho amino-groups (Fig. 48) is preferred and does not include steric constraints as for p-PDA. Taking into account the electronic hybridization of the nitrogen atom (sp3) for coordination via the lone electron pair a
slightly tilted orientation of the molecule with respect to the surface can be assumed. Support to this hypothesis is provided by the occurrence of a relatively strong SERS band in
the 219-239 cm-1 region in both electrolyte solutions and at more positive electrode potential. This band has no counterpart in the “normal spectrum” of solid o-PDA and is attributable to νAu-N mode fundamentals. Appearance of this nitrogen-gold interaction mode with
higher intensity and at higher wavenumbers in this case than that for p-PDA is reflection of
stronger adsorption of o-PDA compared with p-PDA. This interpretation is further supported by the presence of strong and downshifted νC-N SERS band of the adsorbed o-PDA.
86
Chapter 5
1000
(c) SER-spectra in 0.1 M KClO4
100
at ESCE = 100 mV
-1
800
Raman intensity / s
Raman intensity / s
-1
(a) Solid o-PDA
600
80
vAu-N
60
40
400
20
at ESCE = 0 mV
200
1500
1000
-1
Raman shift / cm
500
1500
30
80
1000
-1
Raman shift / cm
500
(d) SER-spectra in 0.1 M HClO4
(b) Calculated spectrum
-1
Raman intensity / s
Raman intensity / a.u.
at ESCE = 400 mV
20
10
60
vAu-N
40
20
0
at ESCE = 100 mV
1500
1000
500
1500
-1
1000
-1
500
Raman shift / cm
Raman shift / cm
Fig. 47. (a) Normal Raman spectrum of solid o-PDA, capillary sampling technique, 488
nm laser light; (b) calculated Raman spectrum of o-PDA single molecule in gas
phase according to DFT (B3LYP/6-31G(d)); (c) and (d) SER-spectra of o-PDA
adsorbed on a gold electrode at the electrode potentials indicated in neutral and
acidic perchlorate solution, respectively, 647.1 nm.
H2N
NH2
Au
Fig. 48. Proposed structure of adsorbed o-PDA at gold surface in both electrolytes and at
positive and zero electrode potential.
87
Chapter 5
Table 10. Spectral data for o-PDA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for o-PDA single molecule in
gas phase according to DFT (B3LYP/6-31G(d)).
Mode
n.a.
n.a.
γ(CH)
νAu-N
γ(CH)
δ(CH)
δ(CH)
n.a.
γ(ring)
δ(ring)
δ(ring)
γ(ring)
γs(NH2)
γ(ring)
γ(CH)
ν(ring)
γ(CH)
γ(CH)
δ(ring)
γ(CH)
δ(CH)
βas(NH2)
δ(CH)
δ(CH)
ν(CH)
ν(C─N)
δ(CH)
ν(ring)
ν(ring)
ν(ring)
ν(ring)
ν(ring)
Wilson
mode #
10b
10a
15
9b
16b
6b
6a
16a
4
11
1
17a
12
17b
18b
18a
9a
13
3
14
19a
19b
8a
8b
Calc.vib.freq.
(R.int.),
Fig. 47b
o-PDA
solid,
Fig. 47a
0.1 M KClO4,
Fig. 47a
ESCE = 0 ESCE =
mV
100 mV
219
221
353
349
382
382
480
481
535
538
591
591
758
751
820
815
938
938
1043
1041
1163
1163
1268
1268
1338
1342
1505
1505
1603
1601
0.1 M HClO4,
Fig. 47b
ESCE =
ESCE =
100 mV
400 mV
234
239
357
351
388
392
475
471
591
592
757
757
819
817
938
938
1035
1036
1154
1158
1267
1266
1375
1374
1471
1504
1504
1602
1605
105
117
185 (1.66)
137
275 (0.90)
301
303 (0.58)
337
421 (0.11)
351
446 (1.90)
519 (3.72)
544 (0.98)
557
572 (7.39)
585
691 (3.94)
706 (6.08)
724
745 (3.14)
749 (21.22)
768
792 (11.20)
789
829 (10.97)
867 (1.61)
868
909 (0.24)
935
1025 (19.74)
1039
1043 (0.34)
1103 (6.18)
1121
1145 (6.76)
1158
1226 (0.69)
1268 (12.96)
1282
1309 (0.29)
1344 (4.80)
1343
1458 (2.11)
1466
1498 (2.58)
1510
1583 (11.44)
1590 (12.18)
1600
1626 (11.41)
1628
βs(NH2)
1627 (33.24)
δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; βas: rocking; γs: wagging; βs: scissoring;
n.a.: not assigned.
88
Chapter 5
5.6 m-Phenylenediamine adsorption on gold
Bands observed in a normal Raman spectrum of m-PDA (Fig. 49a), calculated Raman
spectrum of m-PDA single molecule in gas phase (Fig. 49b) and SER-spectra of adsorbed
o-PDA for neutral (Fig. 49c,d) and acidic (Fig. 49e,f) electrolyte solutions are listed in Table 11; assignment of the observed lines is based on literature data [239,240,248] and calculated vibrational frequencies (the illustration of the optimized structure and the internal
coordinate list of o-PDA according to DFT (B3LYP/6-31G(d)) are attached in appendix 6).
(d) SER-spectrum in 0.1 M KClO4
at ESCE = 200 mV
250
-1
Raman intensity / s
Raman intensity / s
-1
(a) Solid m-PDA
150
100
200
150
100
50
1500
1000
500
1500
-1
300
(e) SER-spectrum in 0.1 M HClO4
at ESCE = 100 mV
-1
Raman intensity / s
Raman intensity / a.u.
(b) Calculated spectrum
20
10
250
200
0
1500
1000
500
1500
-1
120
500
Raman shift / cm
(c) SER-spectrum in 0.1 M KClO4
at ESCE = 0 mV
(f) SER-spectrum in 0.1 M HClO4
at ESCE = 400 mV
-1
500
Raman intensity / s
110
1000
-1
Raman shift / cm
-1
500
Raman shift / cm
30
Raman intensity / s
1000
-1
Raman shift / cm
100
90
80
400
300
200
70
1500
1000
500
1500
1000
500
-1
Raman shift / cm
-1
Raman shift / cm
Fig. 49. (a) Normal Raman spectrum of solid m-PDA, capillary sampling technique, 647.1
nm laser light; (b) calculated Raman spectrum of m-PDA single molecule in gas
phase according to DFT (B3LYP/6-31G(d)); SER-spectra of o-PDA adsorbed on a
gold electrode at the electrode potentials indicated in (c, d) neutral and (e, f) acidic
perchlorate solution, 647.1 nm.
89
Chapter 5
Table 11. Spectral data for m-PDA as a solid and adsorbed on a gold electrode at different
electrode potentials and in different electrolyte solutions; calculated vibrational
frequencies and Raman intensities (arbitrary unit) for m-PDA single molecule
in gas phase according to DFT (B3LYP/6-31G(d)).
Mode
n.a.
Wilson
mode #
Calc.vib.freq. m-PDA
(R. int.),
solid,
Fig. 49b
Fig. 49a
-
γ(CH)
10b
γ(CH)
δ(CH)
124
0.1 M KClO4,
Fig. 49c-d
ESCE =
ESCE = 0
mV
200 mV
-
0.1 M HClO4,
Fig. 49e-f
ESCE =100 ESCE =
mV
400 mV
148
-
208 (1.06)
-
-
-
10a
218 (2.31)
249
-
-
-
-
9a
275 (1.18)
-
-
-
299
-
δ(CH)
15
304 (1.16)
327
-
-
-
-
γ(ring)
16b
433 (0.15)
474
451
456
454
456
-
-
-
-
-
470
-
δ(ring)
6b
510 (3.82)
530
547
538
535
533
δ(ring)
6a
528 (5.32)
549
-
-
-
-
γs(NH2)
-
571 (8.72)
-
575
585
586
584
γ(ring)
16a
612 (5.56)
609
627
627
617
618
ν(ring)
1
654 (6.78)
-
-
-
-
-
γ(ring)
4
673 (0.42)
-
-
-
695
696
γ(CH)
11
737 (18.75)
761
742
731
733
732
γ(CH)
17b
794 (3.45)
-
-
-
790
798
γ(CH)
17a
910 (1.20)
-
-
-
-
-
γ(CH)
5
941 (0.16)
-
940
940
-
-
n.a.
-
258
δ(ring)
12
969 (28.60)
998
1000
1002
1002
1003
δ(CH)
18a
1048 (2.91)
1068
-
-
-
-
-
1099 (0.37)
-
-
-
-
-
βas(NH2)
δ(CH)
18b
1109 (6.70)
1145
1136
1144
-
-
δ(CH)
9b
1154 (3.98)
1165
-
-
1159
1161
ν(CH)
13
1194 (0.19)
1179
-
-
-
-
n.a.
-
-
-
-
-
1241
1243
ν(C─N)
-
1311 (10.57)
1322
1369
1369
1374
1374
ν(ring)
14
1329 (3.12)
-
-
-
-
-
ν(ring)
19a
1474 (1.43)
1452
1450
1460
1413
1417
ν(ring)
19b
1493 (0.33)
1488
1516
1517
1496
1511
ν(ring)
8a
1586 (6.83)
-
1567
1568
1578
1576
ν(ring)
8b
1608 (15.03)
1593
-
-
-
-
1631 (23.27)
1605
βs(NH2)
δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; βas: rocking; γs: wagging; βs: scissoring;
n.a.: not assigned.
90
Chapter 5
In contrast with p-PDA and o-PDA, no band corresponding to nitrogen-gold bonding
can be observed in the SER-spectra of adsorbed m-PDA. This reflects a strong protonation
effect in both electrolyte solutions as deduced from very weak absorption bands in the UVVis spectra of m-PDA (Fig. 32). Upon protonation the nitrogen is coordinated fourfold and
thus unable to establish any other coordinative interaction. The alternative binding site to
the gold surface is the aromatic ring, which is favorable in the absence of nitrogen-surface
interaction. The significant upshift in νC-N mode (ca. 47-52 cm-1) upon adsorption is indicative of ring-surface π orbital overlap. Band broadening and low intensity of the observed
ring modes in SER-spectra at all electrode potentials indicate flat orientation of adsorbed
m-PDA via benzene ring (Fig. 50).
H3N
NH3
Au
Fig. 50. Proposed structure of adsorbed m-PDA at gold surface in both electrolytes and at
positive and zero electrode potential.
91
- Chapter 6 6 Vibrational spectroscopy of solid polyvinylamines
Since solid samples of all polymers under investigation in this study showed no Raman
active features, we used IR-spectra (KBr pellet technique) to determine the vibrational frequencies of these polymers. In addition all results displayed in this chapter are used to assign the observed lines in the SER-spectra of these polymers in the next chapter. The IR
specra of solid PVAm is displayed in Fig. 51a. Assignment of the observed vibrational
bands is performed by combining the information obtained from the recorded spectra and
from the reported characteristic frequencies of the groups ─CH2─, ─NH2, CCN, CCCC,
CC and C─N in literature [239,240,248,254]. The band positions are listed in Table 12
with their assignments.
(c) PVAm2
(a) PVAm
Transmission / %
Transmission / %
40
30
50
45
40
20
2000
1500
1000
-1
35
2000
500
1500
1000
-1
500
Wavenumbers / cm
Wavenumbers / cm
(d) PVAm3
(b) PVAm1
Transmission / %
Transmission / %
55
50
45
70
60
50
40
2000
1500
1000
500
2000
-1
1500
1000
-1
500
Wavenumbers / cm
Wavenumbers / cm
Fig. 51. Infrared spectra for solid PVAm and p-NA-substituted PVAm polymers, KBr pellet, resolution 2 cm-1.
92
Chapter 6
The profile of the IR-spectrum of p-NA-substituted PVAm polymer with low percentage of p-NA substituent (PVAm1, Fig. 51b) is similar to the IR-spectrum of PVAm. As the
percentage of the p-NA substituent increases at the PVAm backbone the characteristic
modes of the benzene ring and nitro-group appear clearly in the IR-spectrum as for
PVAm2 and PVAm3 polymers (Fig. 51c,d).
Table 12. Assignment of vibrational modes of solid PVAm.
Frequency range
Solid PVAm,
Fig. 51a
CCN def.a
~ 436
476
a
~502
540
550-700
667
(755-820)
774
~830
820
837-905
850
930-1005
931
-
987
βas(NH2)
1020-1130
1117
a
1153-1163
1159
1200-1260
1230
1175-1310
1264
Mode
CCN def.
b
γs(NH2)
b
βas(CH2)
c
N─H def.
b,d
ν(CC)
b
ν(CCCC)
n.a.
b
ν(C─N)
b
γas(CH2)
d
βas(CH2) and γas (CH2)
b
β(CH)
1250-1430
1308
b
1310-1390
1363
b
βs(CH2)
1375-1470
1447
βs(NH2)a,b,c
1580-1650
γs(CH2)
1599
1632
n.a.
1668
a
b
: assignment according to reference [248]; : assignment according to reference [239]; c: assignment according to reference [254]; d: assignment according to reference [240]; ν: stretching; νs: symmetric stretching; βas:
rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned.
The assignments of the observed vibrational bands of the p-NA-substituted PVAm
polymers are based on the reported characteristic frequences of the groups (─CH2─,
─NH2, CCN, CCCC, C─N, ─NO2) and specific vibrations of p-disubstituted benzenes in
literature [239,240,248,254]. In order to get more accurate assingments a comparison was
made between the observed bands of the PVAm with those observed for the p-NA-
93
Chapter 6
substituted PVAm polymers. The final result of assignment of the p-NA-substituted PVAm
polymers is summarized in Table 13.
Table 13. Assignment of vibrational modes of solid p-NA-substituted PVAm polymers.
PVAm1,
PVAm2,
PVAm3,
Wilson
Fig. 51b
Fig. 51c
Fig. 51d
mode # e
a
CCN def.
~ 436
473
γ(Ring)b,d
455-570
16b
491
494
~ 502
540
530
530
CCN def.a
b,d
δ(ring)
605-650
6b
623
621
550-700
659
659
657
γs(NH2)b
670-735
4
698
698
γ(Ring)b,d
b
βas(CH2)
755-820
776
700-760
755
755
γs(NO2)b,d
~830
822
N─H def.c
b,d
γ(CH)
795-885
17b
840
837
838
930-1005
941
947
953
ν(CCCC)b
995-1030
18a
997
1028
δ(CH)b,d
δ(CH)b,d
1085-1130
18b
1113
1110
1110
1153-1163
1156
1160
ν(C-N)a
1175-1280
7a
1185
1186
ν(CH)b,d
b
1200-1260
1232
1228
1228
γas(CH2)
1265-1320
3
1286
1288
δ(CH)b,d
β(CH)b
1250-1430
1313
1308
1307
νs(NO2)a
1318-1357
1349
1352
1350
1310-1390
1365
1384
1387
γs(CH2)b
βs(CH2)b
1375-1470
1445
1441
1437
ν(Ring)b,d
1400-1480
19b
1478
1478
1473
1465-1530
19a
1508
1504
1507
ν(Ring)b,d
1550-1610
8b
1552
1561
ν(Ring)b,d
ν(Ring)b,d
1590-1630
8a
1596
1602
1603
βs(NH2)a,b,c
1580-1650
1631
1632
n.a.
1671
1665
1662
n.a.
1712
1711
a
: assignment according to reference [248]; b: assignment according to reference [239]; c: assignment according to reference [254]; d: assignment according to reference [240]; e: band corresponding to benzene ring; δ:
in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βas: rocking; γs:
wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned..
Mode
Frequency range
We repeated the same procedure for the recorded IR-spectrum of o-NA-substituted
PVAm polymers. The spectra of these polymers are illustrated in Fig. 69. It was notable
from Fig. 52 that the profile of the IR-spectrum of o-NA-substituted PVAm polymer with
low percentage of o-NA substituent (PVAm4) is similar to the IR-spectrum of PVAm. The
assignments of the observed vibrational bands (see Table 14) are based on the reported
94
Chapter 6
characteristic frequences of the groups (─CH2─, ─NH2, CCN, CCCC, C─N, ─NO2) and
specific vibrations of o-disubstituted benzene [239,240,248,254].
52
Transmission / %
Transmission / %
50
40
30
20
2000
50
48
46
PVAm4
1500
1000
PVAm5
44
2000
500
1000
500
-1
-1
Wavenumbers / cm
Wavenumbers / cm
50
60
Transmission / %
Transmission / %
1500
45
40
50
40
35
2000
PVAm7
PVAm6
1500
1000
500
2000
-1
1500
1000
500
-1
Wavenumbers / cm
Wavenumbers / cm
Transmission / %
30
28
PVAm8
2000
1500
1000
500
-1
Wavenumbers / cm
Fig. 52. Infrared spectra of solid o-NA-substituted PVAm polymers, KBr pellet, resolution
2 cm-1.
95
Chapter 6
Table 14. Assignment of vibrational modes of solid o-NA-substituted PVAm polymers.
Fig. 52
Frequency
range
Wilson
mode # e
PVAm4
PVAm5
PVAm6
PVAm7
PVAm8
350-560
6b
-
520
524
521
523
CCN def.
~ 502
-
539
-
-
-
-
γs(NH2)b
550-700
-
627
-
-
-
-
γ(Ring)b,d
680-735
4
-
697
696
698
697
γs(NO2)
700-760
-
-
740
744
743
740
βas(CH2)b
755-820
-
773
-
-
-
-
740-830
11
-
781
780
780
782
~830
-
821
-
-
-
-
γ(CH)b,d
840-900
17a
-
845
858
854
846
ν(CCCC)b
930-1005
-
937
950
966
966
953
δ (CH)
1000-1055
18b
1038
1035
1037
1036
1035
δ(CH)b,d
1075-1145
18a
1076
1079
1086
188
1077
1130-1180
9a
1152
1150
1154
1157
1151
γas (CH2)
1200-1260
-
1235
1231
1232
1233
1231
ν(CH)b,d
1200-1310
7a
-
1257
1263
1264
1262
νs(NO2)a
1318-1357
-
-
1352
1353
1356
1352
b
γs(CH2)
1310-1390
-
1363
1388
1384
1383
1384
ν(Ring)b,d
1415-1475
19a
-
1418
1420
1421
1415
βs(CH2)b
1375-1470
-
1445
1441
1437
1443
1443
νas(NO2)a,b
1480-1540
-
1504
1517
1512
1512
1503
1512
-
-
-
-
-
-
1553
ν(Ring)
1565-1610
8a
-
1573
1573
1576
1574
βs(NH2)a,b,c
1580-1650
-
1613
1620
1617
1619
1629
1632
-
-
-
-
n.a.
-
-
1671
1660
1663
1662
1660
n.a.
-
-
-
1710
1711
1708
1705
Mode
δ(Ring)b,d
a
b
γ(CH)b,d
c
N─H def.
b,d
δ (CH)b,d
b
n.a.
b,d
a
b
c
: assignment according to reference [248]; : assignment according to reference [239]; : assignment according to reference [254]; d: assignment according to reference [240]; e: band corresponding to benzene ring; δ:
in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric
stretching; βas: rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned.
96
Chapter 6
We have recorded IR spectra (Fig. 53) of solid Wurster-substituted PVAms (PVAm9,
PVAm10) to determine the vibrational frequencies of these polymers. Their peak positions
are listed in Table 15 along with appropriate vibrational assignments. The assignments of
the observed vibrational bands are based on the reported characteristic frequences of the
groups (─CH2─, ─CH3, ─NH2, CCN, CCCC, C─N, ─NO2) and specific vibrations of
Transmission / %
1,2,4-trisubstituted benzene [239,240,248,254].
40
35
PVAm9
30
2000
1500
1000
500
-1
Wavenumbers / cm
Transmission / %
46
44
42
40
PVAm10
38
2000
1500
1000
500
-1
Wavenumbers / cm
Fig. 53. Infrared spectra of solid Wurster-substituted PVAm polymers, KBr pellet, resolution 2 cm-1.
97
Chapter 6
Table 15. Assignment of vibrational modes of solid Wurster-substituted PVAm polymers.
Fig. 53
Frequency range
Wilson
mode # e
PVAm9
PVAm10
395-505
6b
464
467
~ 502
-
503
503
δ(Ring)b
440-570
6a
-
546
γ(Ring)b
530-620
16a
-
584
γs (NH2)
550-700
-
613
606
ν(Ring)b
630-740
1
-
704
βas(CH2)b
Mode
δ(Ring)b
a
R─NH2 (CCN def)
b
755-820
-
761
762
b
γ(CH)
780-855
11
808
801
γ(CH)b
840-915
5
-
858
ν(CH)b
820-1005
7b
-
945
γ(CH)
930-990
17b
972
965
βas(CH3)b
990-1070
-
1016
-
ν(CCCC)b
930-1005
-
1041
1045
δ(CH)
1065-1135
18b
-
1088
δ (CH)b
1130-1175
18a
1167
1159
b
b
γas(CH2)b
1200-1260
-
1230
1233
a
νs(NO2)
1318-1357
-
1346
1349
γs(CH2)b
1310-1390
-
1387
1384
1375-1470
-
1441
1439
νas(NO2)
1480-1540
-
1522
1522
ν(Ring)b
1545-1605
8a
1565
1577
ν(Ring)b
1585-1645
8b
1598
1598
1580-1650
-
-
1636
n.a.
-
-
1654
1655
n.a.
-
-
-
1702
βs(CH2)b
a,b
a,b,c
βs(NH2)
a
b
: assignment according to reference [248]; : assignment according to reference [239]; c: assignment according to reference [254]; d: assignment according to reference [240]; e: band corresponding to benzene ring; δ:
in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric
stretching; βas: rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned.
98
Chapter 6
IR spectrum of solid stilbene-substituted PVAm polymer was obtained and is displayed
in Fig. 54. The observed bands are collected in Table 16; the proposed assignments is
based on general literature data of characteristic frequences of the groups (─CH2─, ─NH2,
─NH, C═C, CCN, CCCC, C─N, ─NO2), specific vibrations of p-disubstituted and 1,2,4
trisubstituted benzene [239,240,248,254] and related compoud [233].
Transmission / %
45
40
35
PVAm11
2000
1500
1000
500
-1
Wavenumbers / cm
Fig. 54. Infrared spectrum of solid stilbene-substituted PVAm polymer, KBr pellet, resolution 2 cm-1.
99
Chapter 6
Table 16. Assignment of vibrational modes of solid stilbene-substituted PVAm polymers.
Mode
f,b
δ(Ring)
a
CCN def.
Frequency
range
395-505
Wilson
mode #
6b
PVAm11, Fig. 54
460
~ 502
-
502
g,b
455-570
16b
531
f,b
440-570
6a
547
b
550-700
-
616
f,b
630-740
1
688
b,e
700-760
-
702
γ(Ring)
δ(Ring)
γs (NH2)
ν(Ring)
γs(NO2)
c,e
γs(NH)
700-750
-
748
b
755-820
-
762
e,b,g
795-885
17b
804
e,b,g
800-860
10a
837
800-890
-
865
905-965
5
935
820-1005
7b
956
930-1005
-
1042
βas(CH2)
γ(CH)
γ(CH)
e,b
βs(NO2)
e,b,g
γ(CH)
f,b
ν(CH)
b
ν(CCCC)
n.a.
-
-
1067
e,b,g
1085-1130
18b
1108
e,b,g
1140-1190
9a
1159
b,g
1105-1295
13
1183
δ(CH)
δ(CH)
ν(CH)
b
γas(CH2)
1200-1260
-
1230
a,b,e
1318-1357
-
1339
b
1310-1390
-
1386
b
1375-1470
-
1437
e,b,g
1465-1530
19a
1511
b,e
1480-1540
-
1522
e,b,g
1550-1610
8b
1561
e,b,g
1590-1630
8a
1592
a,b,b,e
1580-1650
-
1625
b,e
1630-1660
-
1654
νs(NO2)
γs(CH2)
βs(CH2)
ν(Ring)
νas(NO2)
ν(Ring)
ν(Ring)
βs(NH2)
ν(C═C)
n.a.
1705
a
b
: assignment according to reference [248]; : assignment according to reference [239]; c: assignment according to reference [254]; d: assignment according to reference [240]; e: assignment according to reference [233];
f
: band corresponding to 1,2,4 tri-substituted-benzene ring; g: band corresponding to para-di-substitutedbenzene ring; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching;
νas: a symmetric stretching; βas: rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation;
n.a.: not assigned.
100
- Chapter 7 -
7 Polyvinylamines adsorption on gold
7.1 Polyvinylamine adsorption on gold
Band positions from SER-spectra (Fig. 55) of PVAm adsorbed on a gold electrode
from neutral electrolyte solution (0.1 M KClO4) are listed in Table 17; assignment of the
observed lines is based on the data displayed in chapter 6 (see Table 12). In all spectra the
bands corresponding to CH2 and NH2 groups are observed at all applied electrode potential. The intensity of these bands reached maximum at ESCE = 100 mV applied electrode
potential, which reveals strong adsorption of the PVAm layers at this potential.
Raman intensity / s
-1
80
70
ESCE =
600 mV
400 mV
60
300 mV
50
100 mV
0 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 55. SER-spectra of PVAm adsorbed on a gold electrode at the electrode potentials indicated in neutral perchlorate solution, 647.1 nm.
One of the most striking features of the SER-spectra of PVAm is the observed lowwavenumber mode around 222 to 231 cm-1, which is assigned to the metal-adsorbate vibration. Intensity and position as a function of electrode potential strongly support the assignment of this mode to a νAu-N. The vibration revealing the direct interaction of nitrogen atom
with several metal surfaces has been reported in previous works at 235-264 cm-1 for gold,
205-240 cm-1 for silver, 213 cm-1 for cadmium, 219 cm-1 for nickel, 241 cm-1 for copper
and 230 cm-1 for iron [252,255-261].
101
Chapter 7
Table 17. SERS spectral data for PVAm adsorbed on a gold electrode at different electrode potentials in neutral electrolyte solution.
0.1 M KClO4, Fig. 55
Mode
νAu-N
n.a.
ESCE = 0 mV
ESCE = 100 mV
ESCE = 300 mV
ESCE = 400 mV
ESCE = 600 mV
222
227
227
228
231
-
-
-
384
383
CCN def.
462
473
475
471
470
N─H def.
817
819
815
811
818
ν(ClO )
933
933
933
933
933
ν(C─N)
1148
1146
1149
1151
1149
γas(CH2)
1238
1235
1233
1241
1242
-
1297
-
1293
-
−
4
β(CH)
γs(CH2)
1232
1331
1339
1338
1339
βs(CH2)
1442
1445
1438
1446
1446
1590
1602
1607
1603
1600
βs(NH2)
1633
1635
1636
1630
1632
ν: stretching; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned.
Assignment of this low-wavenumber mode to the gold-perchlorate stretching mode is
unlikely. This conclusion is based on the obtained SER-spectra of adsorbed perchlorate
anions, which were recorded separately with electrolyte solutions containing the supporting electrolyte only (Fig. 56). It is obvious from figure Fig. 56 that the positions and intensities of the gold-perchlorate stretching mode as a function of electrode potential are different from those for the metal-adsorbate vibration observed in SER-spectra of PVAm.
Band positions, intensities and their respective changes as a function of electrode potential
for gold-perchlorate stretching mode are discussed in details previously by Kania and
Holze [262]. The Raman line at 933 cm-1 (Fig. 55) comes from the electrolyte anion ClO−4
as reported in previous studies [259,263-268]. Since the adsorbed PVAm is soaked in solution containing perchlorate anion the observation of this band might be an indication of
capped perchlorate anions inside the polymer chains. The presence of PVAm in unbuffered
neutral electrolyte (0.1 M KClO4, pH = 4.90, and only slightly acidic) support that small
part of the amino-groups located at the polymer backbone are protonated and the perchlorate anions are the counter ions of these groups. As consequence of what we observed in
the SER-spectra of PVAm and from our discussion above, the adsorption of PVAm at gold
surface is proposed and illustrated in Fig. 57.
102
Chapter 7
-1
ESCE = 100 mV, 256 cm
-1
ESCE = 400 mV, 250 cm
Raman intensity / s
-1
80
-1
ESCE = 700 mV, 253 cm
60 E = 0 mV, 247 cm-1
SCE
ESCE = -200 mV
40
-1
ESCE = -100 mV, 243 cm
400
300
200
100
-1
Raman shift / cm
Fig. 56. SER-spectra of adsorbed perchlorate anions (from 0.1 M KClO4 solution) on a
gold electrode, 647.1 nm.
ClO4
H3N
NH2
NH2
NH2
NH2
Au
Fig. 57. Proposed model of adsorbed PVAm at a gold surface in neutral electrolyte solution.
In 0.1 M HClO4 electrolyte, the SER-spectra of PVAm show a remarkable disappearance of the PVAm bands in the whole range of applied potential (Fig. 58). In this case, the
amino-groups of PVAm are subject to strong protonation effect and commonly ─ NH 3+
groups are located directly along the polymer backbone as mentioned in the literature [45].
Thus, the coordination between nitrogen atoms of the polymer amino-groups and the gold
surface can be ruled out in this case. On the other hand, the intensities of the two bands at
934 cm-1 and 258 cm-1, respectively, are increased and reached a maximum at ESCE = 100
103
Chapter 7
mV electrode potential. The strong band at 934 cm-1 corresponds to the symmetric vibration of the perchlorate anion as we mentioned above. The position of the other strong
SERS band at 258 cm-1 is close to the gold-perchlorate stretching mode, which was observed in SER-spectra recorded separately with electrolyte solutions containing the supporting electrolyte (0.1 M HClO4) only (Fig. 59).
100
-1
Raman intensity / s
-1
258 cm
ESCE =
100 mV
200 mV
-1 300 mV
934 cm
400 mV
500 mV
80
60
40
1500
1000
-1
Raman shift / cm
500
Fig. 58. SER-spectra of PVAm adsorbed on a gold electrode at the electrode potentials indicated in acidic perchlorate solution, 647.1 nm.
-1
ESCE = 400 mV, 260 cm
Raman intensity / s
-1
80
-1
ESCE = 500 mV, 259 cm
-1
ESCE = 300 mV, 256 cm
-1
ESCE = 200 mV, 251 cm
60 E = 100 mV, 251 cm-1
SCE
40
400
300
200
100
-1
Raman shift / cm
Fig. 59. SER-spectra of adsorbed perchlorate anions (from 0.1 M HClO4 solution) on a
gold electrode, 647.1 nm.
104
Chapter 7
The aforementioned spectroscopic observations demonstrate that the perchlorate anions
might mediate between the protonated amino-groups of PVAm and the metal surface. In
Fig. 60, a schematic illustration of a simultaneous interaction of perchlorate anions with
the ─ NH 3+ group of PVAm and the Au surface is given. Such a surface-adsorbate interaction has been reported in literature for poly(vinyl formamide-co-vinyl amine) and other
molecules [259,269,270]. However, the presence of a small amount of perchlorate anions
coadsorbed on the gold surface is also expected. The coadsorption features of perchlorate
anion are further discussed later in this chapter.
ClO4
ClO4
ClO4
H3 N
NH3
H3 N
NH3
NH3
O
O
Cl
O
O
O
O
O Cl
O
O
O
Cl
O
O
Au
Fig. 60. Simultaneous interaction of perchlorate anions with the ─ NH 3+ group of PVAm
and the Au surface.
At higher electrode potential in the range of oxidation of PVAm (at ESCE = 1000 mV),
the SER-spectra of 0.1 M HClO4 with and without PVAm are displayed in Fig. 61. From
this figure one can note that the three bands at 166, 337 and 356 cm-1 are observed only in
the presence of small amounts of PVAm. The band intensity of the gold-perchlorate
stretching mode at 267 cm-1 is reduced in the presence of PVAm compared to the observed
one in the SER-spectra of 0.1 M HClO4 alone. These results suggest that the PVAm is interfaced with the electrode surface via perchlorate anions and these three bands correspond
to the oxidized species of PVAm. Unfortunately, assignment of these bands is currently
impossible. Furthermore, these results suggest that the anodic wave observed around ESCE
= 1000 mV in the CV (Fig. 24a) recorded at gold electrode of PVAm in acidic electrolyte
solution is not only caused by the gold hydroxide/-oxide formation but also by the oxidation of the PVAm amino-groups (see section 3.5). If this anodic wave caused only by the
gold hydroxide/-oxide formation the SERS behavior of 0.1 M HClO4 with and without
105
Chapter 7
PVAm at ESCE = 1000 mV should be the same. As we can see from Fig. 61 the SERS behavior of 0.1 M HClO4 with and without PVAm at ESCE = 1000 mV are different.
90
Raman intensity / s
-1
80
-1
337 cm
-1
267 cm
-1
70
356 cm
-1
197 cm
60
50
-1
166 cm
40
500
400
300
200
100
-1
Raman shift / cm
Fig. 61. SER-spectra of adsorbed species on a gold electrode at ESCE = 1000 mV from a
solution of 0.1 M HClO4 (dashed line) and 100 mL 0.1 M HClO4 + 0.05 g PVAm
(solid line), 647.1 nm.
7.2 p-Nitroaniline-substituted polyvinylamine adsorption on gold
Fig. 62 shows the SER-spectra of PVAm1 adsorbed on a polycristalline gold surface in
0.1 M KClO4 at electrode potentials ranging from ESCE = 0 to ESCE = 800 mV. The assignment of the observed vibrational modes is collected in Table 18 and based on the data dis-
Raman intensity / s
-1
played in chapter 6 (see Table 13).
200
ESCE =
0 mV
300 mV
600 mV
100
800 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 62. SER-spectra of PVAm1 adsorbed on a gold electrode at the electrode potentials
indicated in neutral perchlorate solution, 647.1 nm.
106
Chapter 7
Table 18. SERS spectral data of PVAm1 adsorbed on a gold electrode at different electrode potentials and in different electrolyte solutions.
Mode
Wilson
mode # a
0.1 M KClO4, Fig. 62
ESCE = ESCE = ESCE =
ESCE =
300
600
800
0 mV
mV
mV
mV
215
218
221
248
250
251
261
409
455
457
460
463
537
537
538
610
610
611
613
0.1 M HClO4, Fig. 65
ESCE = ESCE = ESCE =
100
400
700
mV
mV
mV
258
269
266
462
458
-
νAu-N
νAu-OClO
νAu-ONO
CCN def.
CCN def.
δ(ring)
6b
βas( NH 3+)b
-
-
-
-
-
625
628
631
γs(NH2)
γ(Ring)
γs(NO2)
N─H def.
4
-
647
736
803
648
734
805
649
804
651
700
740
812
735
-
-
-
ν(ClO −4 )
-
933
934
934
932
933
933
933
n.a.
δ(CH)
n.a.
δ(CH)
ν(C─N)
ν(CH)
νs(NO2)
γs(CH2)
βs(CH2)
ν(Ring)
ν(Ring)
18a
18b
7a
19a
8a
1112
1154
1333
1387
1524
1596
1114
1155
1333
1389
1525
1597
1114
1157
1197
1337
1388
1525
1596
1053
1111
1156
1195
1350
1396
1421
1500
1601
974
1030
1161
1330
1448
1519
-
979
1029
1162
1318
1444
1526
-
982
1030
1162
1335
1443
1525
-
1624
1629
1628
δas( NH 3+)b
a
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer in
acidic electrolyte and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; def.: deformation; δas: a symmetric deformation; n.a.: not assigned.
The influence of the presence of small amounts of p-NA substituent (3% by mole) at
the polymer backbone is clearly notable from the stronger SERS bands for PVAm1 (Fig.
62) than those for PVAm (Fig. 55). From Fig. 62 it is also interesting to note that the profiles of PVAm1 SER-spectra at various electrode potentials are not identical. This difference in SERS lines is clearly observable between the two potentials ESCE = 300 mV and
ESCE = 800 mV. At ESCE = 300 mV the SER-spectra of PVAm1 present a metal-adsorbate
band at 218 cm-1. This band is assigned previously in this text to the gold-nitrogen stretching mode. This is an indication of the interaction between the amino-group substituent of
107
Chapter 7
PVAm1 and gold surface at this potential. It should be mentioned that the gold-perchlorate
stretching mode also appears as a weak shoulder at 250 cm-1 in the SER-spectra of PVAm1
at ESCE = 300 mV. This reveals that a small amount of coadsorbed perchlorate anions is
present at the electrode surface. Since the pH of the unbuffered solution of 0.1 M KClO4 is
4.9, which is in the acidic range (pH < 7), it is possible that some of ─NH2 groups of
PVAm1 are protonated in this electrolyte solution. Thus a simultaneous interaction of perchlorate anions with the ─ NH 3+ group of PVAm and the Au surface is possible. However,
the presence of a small amount of perchlorate anions as coadsorbed only at a gold surface
is also possible. Accordingly, the proposed model of the adsorbed PVAm1 polymer in the
presence of the coadsorbed perchlorate anion at ESCE = 300 mV electrode potential is illustrated in Fig. 63.
NO2
H2N
H2N ClO4
HN
NH3
NH3
O
NH2
O
O
NH2
O
Cl
O
O
Cl
O
O
Au
Fig. 63. Proposed model of adsorbed PVAm1 and perchlorate anions at a gold surface in
neutral electrolyte and at ESCE = 300 mV electrode potential.
In Fig. 62, one can also notice that the SERS signals changed sharply when the potential
moved from ESCE = 300 to ESCE = 800 mV. At ESCE = 800 mV the Au-N stretching mode
totally disappears demonstrating that the nitrogens of PVAm1 amino-groups were detached
from the gold surface. It has been additionally noted that new bands, not seen at less positive electrode potentials, are observed when the applied potential is near ESCE = 800 mV.
Two of these bands correspond to the benzene ring vibrations 4 and 7a, which appear at
700 and 1195 cm-1, respectively. Appearance of these bands suggests that the nitroaromatic portion of the PVAm1 polymer gets more closely to the metal surface. The third
108
Chapter 7
new band at 409 cm-1 is close to the gold-oxygen stretching mode, which was assigned
previously in this text to the interaction between the nitro-group of p-NA and the gold surface. Appearance of this band suggests that coordination between the nitro-group of p-NA
substituent and gold surface takes place at this potential (ESCE = 800 mV). The broadening
and low intensity of the symmetric stretching mode of nitro-group at ESCE = 800 mV support the assumed adsorption via the nitro-group. In addition, the prominent goldperchlorate stretching mode (SERS band at 261 cm-1) suggests that the two situations of
coadsorbed ClO−4 described in Fig. 63 are also possible in this case. From the above observations, the possible model of the adsorbed PVAm1 polymer in the presence of the coadsorbed perchlorate anion at ESCE = 800 mV electrode potential is illustrated in Fig. 64.
NH2
H2N
NH
H2N
NH3
O
O
+
O
N
O
O
O
O
Cl
O
Cl
O
O
Au
Fig. 64. Proposed model of adsorbed PVAm1 and perchlorate anions at a gold surface in
neutral electrolyte and at ESCE = 800 mV electrode potential.
In 0.1 M HClO4 electrolyte, the SER-spectra of PVAm1 show a remarkable decrease in
the band intensity in the whole range of applied potentials (Fig. 65, Table 18). Also two
strong SERS bands assigned previously to the gold-perchlorate stretching mode and
ν(ClO −4) mode are observed at ~260 cm-1 and 933 cm-1, respectively. These features are
quite similar to those observed in the SER-spectra of PVAm in the same electrolyte (Fig.
58). Based on this similarity and our previous discussion we can conclude that the primary
amino-groups of PVAm1 polymer are subject to strong protonation effects in this acidic
electrolyte solution as observed for PVAm. This hypothesis is further supported by the appearance of βas( NH 3+) and δas( NH 3+) bands in the SER-spectra of PVAm1 in acidic electro-
109
Chapter 7
lyte only (see Table 18). In addition, the secondary amino-groups are also subject to strong
protonation effects in acidic medium as discussed previously in chapter 4. On the basis of
the above arguments, we can assume that there is a simultaneous interaction of perchlorate
Raman intensity / s
-1
anions with the protonated amino-groups of PVAm1 and the Au surface (Fig. 66).
80
60
ESCE =
400 mV
700 mV
100 mV
40
2000
1500
1000
500
-1
Raman shift / cm
Fig. 65. SER-spectra of PVAm1 adsorbed on a gold electrode at the electrode potentials
indicated in acidic perchlorate solution, 647.1 nm.
ClO4
ClO4
ClO4
NH3
NH3
NH3
NH3
NH2
O2N
O
O
O
Cl
O
O
O
Cl
O
O
O
O
Cl
O
O
Au
Fig. 66. Simultaneous interaction of perchlorate anions with the protonated amino-groups
of PVAm1 and the Au surface.
110
Chapter 7
At higher electrode potential (at ESCE = 1000 mV) the SER-spectrum (Fig. 67) of
PVAm1 shows the same features observed in a SER-spectrum of PVAm at the same potential and in the same electrolyte (Fig. 61). It is clear from Fig. 67 that SERS behavior of
PVAm1 at ESCE = 1000 mV in 0.1 M HClO4 is different than that of free electrolyte alone.
This difference includes appearance of the SERS bands at 168, 334 and 358 cm-1 in the
SER-spectrum of 0.1 M HClO4 in the presence of PVAm1 only. According to the previous
discussion in the case of the SER-spectrum of PVAm at this electrode potential (Fig. 61),
these three SERS bands can be attributed to the adsorption of oxidized species of PVAm1.
It should be noted also that the electrochemical oxidation of PVAm1 exhibits the same features of PVAm (see chapter 3, Fig. 24).
Raman intensity / s
-1
90
80
70
-1
334 cm
-1
358 cm
-1
267 cm
-1
197 cm
60
50
-1
168 cm
40
500
400
300
200
-1
Raman shift / cm
100
Fig. 67. SER-spectra of adsorbed species on a gold electrode at ESCE = 1000 mV from a
solutions of 0.1 M HClO4 (dashed line) and 100 mL 0.1 M HClO4 + 0.05 g
PVAm1 (solid line), 647.1 nm.
The SER-spectra of PVAm3 adsorbed on a gold electrode at different electrode potentials in neutral and acidic electrolyte solutions are shown in Fig. 68. Assignments of the
observed bands are listed in Table 19 and based on the data displayed in chapter 6 (see Table 13). Intensity of SERS bands of PVAm3 is much higher than those of PVAm and
PVAm1. This enhancement feature of SERS bands might be due to a higher degree of pNA substituent located at the PVAm3 backbone (8.7 mole %) and implies strong adsorption behavior of this polymer at gold surface.
In neutral electrolyte (Fig. 68a, Table 18), vibrational bands of benzene ring as well as
other substituent groups are observed in most cases of applied electrode potentials ranging
from ESCE = 0 to 800 mV. From Fig. 68a, the prominent band at 410-412 cm-1 which was
111
Chapter 7
assigned to the gold-oxygen stretching mode, is a strong indication of the interaction
between nitro-group of aromatic substituent and gold surface. This band is observed at all
electrode potentials and its intensity reached maximum as well as that of other SERS bands
at ESCE = 200 mV. Also a weak band at 216 cm 1- assigned to νAu-N is observed at this potential. Assignment of this low-frequency mode was already discussed in section 7.1. Appearance of this mode suggests that the PVAm3 is also interfaced with the gold surface via
the primary amino-groups at this electrode potential. In addition, the coadsorbed perchlorate anion is detected in this case from the appearance of a weak SERS band at 266 cm-1.
As a result the coordination of PVAm3 with the gold surface via nitro-group of aromatic
substituent and amino-groups are illustrated in Fig. 69. Also, in unbuffered electrolyte (0.1
M KClO4, pH = 4.90) a small amount of the amino-groups of PVAm3 is protonated.
Therefore, it is possible that the perchlorate anion is adsorbed at the gold surface with or
without simultaneous interaction with the protonated amino-group of PVAm3.
(a) 0.1 M KClO4
Raman intensity / s
-1
1500
ESCE =
1000
800 mV
600 mV
400 mV
500
200 mV
0 mV
2000
1500
1000
500
-1
Raman shift / cm
Raman intensity / s
-1
500
(b) 0.1 M HClO4
400
ESCE =
300
800 mV
200
400 mV
100 mV
100
2000
1500
1000
500
-1
Raman shift / cm
Fig. 68. SER-spectra of PVAm3 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
112
Chapter 7
Table 19. SERS spectral data of PVAm3 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
0.1 M KClO4, Fig. 68a
ESCE = ESCE = ESCE =
200
400
600
mV
mV
mV
213
214
216
0.1 M HClO4, Fig. 68b
ESCE = ESCE = ESCE =
100
400
800
mV
mV
mV
-
Mode
Wilson
mode # a
νAu-N
-
-
νAu-OClO
-
265
266
266
268
271
255
262
274
n.a.
-
301
298
298
298
299
-
-
-
ESCE =
0 mV
ESCE =
800
mV
215
n.a.
-
329
330
329
330
331
-
329
329
νAu-ONO
-
412
412
412
412
410
410
411
410
n.a.
-
443
443
443
444
445
444
442
445
γ(Ring)
16b
508
-
-
-
-
505
508
-
CCN def.
-
531
530
530
530
529
535
530
527
n.a.
-
583s
565 m
583
583
583
584
-
585
δ(ring)
6b
631
630
631
631
631
-
630
629
γs(NH2)
-
676
677
677
678
679
-
-
-
n.a.
-
-
-
-
-
-
-
678
676
γ(Ring)
4
715
716
716
716
717
-
-
γs(NO2)
-
754
756
754
755
755
-
754
753
βas(CH2)
-
-
-
-
772
773
-
-
774
N─H def.
γ(CH)
ν(CCCC)
-
818
815
814
811
814
-
-
806
17b
-
843
843
843
-
-
-
-
-
959
958
961
961
961
-
960
960
δ(CH)
18a
1011
1010
1010
1010
1010
1011
1009
1009
δ(CH)
18b
1090
1091
1091
1091
1091
1091
1091
1091
-
1172
1171
1172
1171
1172
1176
1175
1175
n.a.
γas(CH2)
-
1215
1214
1215
1215
1213
1236
1215
1216
δ(CH)
3
1267
1265
1265
1260
-
1264
1266
1286
νs(NO2)
-
1335
1333
1336
1337
-
1336
1331
1340
γs(CH2)
-
1373
-
1373
1370
1365
-
-
1365
n.a.
-
1402
1398
1402
1397
1396
-
1409
-
βs(CH2)
-
-
1448
-
-
1456
-
ν(Ring)
19a
1509
1506
1510
1511
1512
1510
1511
1502
ν(Ring)
8b
1566
1564
1570
-
-
1566
-
-
ν(Ring)
8a
1592
1590
1590
1591
1590
1592
1591
1590
βs(NH2)
-
1628
1626
1628
1628
1626
-
-
-
+ b
3
1624
1623
1622
δas( NH )
a
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer in
acidic electrolyte and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; def.: deformation; δas: a symmetric deformation; n.a.: not assigned.
113
Chapter 7
NO2
ClO4
NH
H3N
NH2
NH2
NH
H3N
H2N
NH2
O
O
O
Cl
O
O
O
N+
O
O
NH2
Cl
O
O
Au
Fig. 69. Proposed model of adsorbed PVAm3 and perchlorate anions at a gold surface in
neutral electrolyte solution and at ESCE = 200 mV.
The SER-spectra of PVAm3 in acidic electrolyte (Fig. 68b, Table 19) give features
different from those observed for PVAm and PVAm1 in the same electrolyte (Fig. 58 and
65). Most vibrational bands of PVAm3 are observed at electrode potentials ranging from
ESCE = 100 to 800 mV. The intensity of these SERS bands are remarkably higher than
those observed for PVAm and PVAm1 in acidic electrolyte. This suggests that PVAm3
species interact directly with the metal surface. This hypothesis is supported by the occurrence of a relatively strong SERS band at 410-411 cm-1, which is related to the interaction
between nitro-groups of aromatic substituent and the gold surface as mentioned above. On
the other hand, the coadsorbed perchlorate anion is observed also in this case as indicated
by the appearance of the gold-perchlorate stretching band at 255-274 cm-1. The intensity of
this band decreased when the potential moved from ESCE = 100 to ESCE = 800 mV. This
demonstrate that the perchlorate coverage at the gold surfce is very small at ESCE = 800
mV. In contrast the intensity of most of the PVAm3 SERS bands reached maximum at this
electrode potential. This result indicates that the perchlorate coverage is suppressed by the
strong adsorption of the layers of PVAm3 at this electrode potential. It should be men114
Chapter 7
tioned that the SERS signals of PVAm3 in acidic electrolyte are relatively less intense than
those with neutral solution. This is due to the absence of the coordination of the primary
amino-groups of this polymer with the gold surface. The primary amino-groups of the
PVAm3 are subject to protonation effects in the acidic electrolyte solution as in the case of
PVAm. Upon protonation the nitrogens of the amino-groups of the PVAm3 are coordinated fourfold and thus unable to establish any other coordinative interaction. According to
these observations we can propose the expected model of adsorbed PVAm3 in Fig. 70.
ClO4
ClO4
ClO4
ClO4
NH3
NH3
H3N
H2N
NH3
O
O
O
N
+
O
Cl
O
O
O
O
Cl
O
O
Au
Fig. 70. Proposed model of adsorbed PVAm3 and perchlorate anions at a gold surface in
acidic electrolyte solution and at ESCE = 100 to 800 mV.
The SER-spectra of PVAm with higher mole percentage (11.3 mole %) of p-NA substituent (PVAm2) adsorbed on a polycristalline gold surface in neutral and acidic
electrolyte solutions at different electrode potentials are displayed in Fig. 71. Assignments
of the observed vibrational modes are collected in Table 20 and based on the data
displayed in chapter 6 (see Table 13). For instance, the SER intensity of all PVAm2
characteristic bands reached maximum at ESCE = 300 mV and ESCE = 200 mV in neutral
and acidic electrolyte solutions, respectively. These potentials, where the intensity of the
SER-spectra observed maximum, are close to those observed for PVAm1 (Fig. 62) and
PVAm3 (Fig. 68a) in neutral electrolyte solution. In neutral electrolyte solution, the SERspectra intensity of p-NA-substituted PVAm polymers increase in the following order
115
Chapter 7
PVAm2 < PVAm1 < PVAm3. The strong adsorption behavior observed for PVAm3
should be due to the mole percentage of p-NA substituent which is discussed later in this
text.
(a) 0.1 M KClO4
ESCE =
300 mV
Raman intensity / s
Raman intensity / s
-1
-1
110
150
105
100
600
400
200
-1
Raman shift / cm
ESCE =
100
300 mV
400 mV
600 mV
100 mV
2000
1500
1000
500
-1
Raman shift / cm
(b) 0.1 M HClO4
Raman intensity / s
-1
Raman intensity / s-1
150
140
ESCE = 200 mV
600
200
500
400
300
-1
Raman shift / cm
ESCE =
200 mV
400 mV
100
600 mV
800 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 71. SER-spectra of PVAm2 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
116
Chapter 7
Table 20. SERS spectral data of PVAm2 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
Wilson
mode # a
0.1 M KClO4, Fig. 71a
ESCE = ESCE = ESCE =
300
400
600
mV
mV
mV
158
160
0.1 M HClO4, Fig. 71b
ESCE
ESCE
ESCE
ESCE
= 200 = 400 = 600 = 800
mV
mV
mV
mV
-
n.a.
-
ESCE =
100
mV
158
νAu-N
-
-
221
-
-
-
-
-
-
νAu-OClO
-
-
275
277
-
254
262
262
270
n.a.
-
-
370
369
-
-
-
-
-
νAu-ONO
-
-
409
410
-
419
417
419
415
CCN def
-
-
453
-
-
451
455
455
460
γ(Ring)
16b
-
485
-
-
-
-
-
-
-
534
535
-
-
-
-
-
Mode
CCN def
+ b
3
βas( NH )
-
-
-
-
-
631
629
629
-
γs(NH2)
-
-
643
643
-
-
-
-
-
γ(Ring)
4
-
686
-
-
692
-
-
-
γs(NO2)
-
-
742
-
-
743
-
-
-
n.a.
-
-
800
-
-
804
-
-
-
N─H def.
-
815
820
819
820
817
818
818
816
17b
859
858
860
858
857
857
858
858
ν(ClO )
-
933
934
934
934
934
935
935
935
n.a.
-
-
992
-
-
-
-
-
-
δ(CH)
18a
-
1030
-
-
1031
1033
1030
1033
δ(CH)
18b
1112
1113
1114
1111
1114
1115
1114
1113
-
1146
1148
1150
1134
1140
1142
1142
1139
ν(CH)
7a
1185
1165
1167
1187
1193
1191
1191
β(CH)
-
1306
-
1308
-
-
1309
1308
1309
νs(NO2)
-
1324
1324
1324
1320
1326
1324
1325
1326
γs(CH2)
-
-
1382
1384
-
-
-
-
-
βs(CH2)
-
1447
1450
1445
1447
1442
1448
1443
1444
ν(Ring)
19a
1505
1500
1506
1501
1495
1520
1527
1526
ν(Ring)
8a
1597
1596
1598
1598
1594
1603
1603
1607
βs(NH2)
-
-
1625
1625
-
-
-
-
-
γ(CH)
−
4
n.a.
+ b
3
1629 1630 1635
δas( NH )
a
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer in
acidic electrolyte and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; def.: deformation; δas: a symmetric deformation; n.a.: not assigned.
117
Chapter 7
Further analysis of PVAm2 SER-spectrum at ESCE = 300 mV shows several metaladsorbate vibrations (νAu-N, νAu-ONO, νAu-OClO) in the low-frequency region (see the inset of
Fig. 71a and Table 20). Such metal-adsorbate vibrations were observed and discussed previously in the case of the adsorption of PVAm3 at a gold surface in neutral electrolyte
solution (Fig. 68a). Thus, we can assume that the proposed model of adsorbed PVAm2 at a
gold surface and in neutral electrolyte at ESCE = 300 mV is like that for PVAm3, which is
illustrated in Fig. 69. The prominent difference between PVAm2 and PVAm3 is the preferable anchoring site with the metal surface. The intensity of the νAu-N band in the SERspectrum of PVAm2 in neutral electrolyte at ESCE = 300 mV potential is higher than that of
νAu-ONO band. In contrast to PVAm2, the intensity of the νAu-N band in the SER-spectrum of
PVAm3 in neutral electrolyte at ESCE = 200 mV potential is lower than that of νAu-ONO
band. Accordingly, the preferable anchoring site with the metal surface in neutral electrolyte is via oxygens of nitro-group in the case of PVAm3 and via nitrogen of the primary
amino-groups in the case of PVAm2. The appearance of νs(NO2) as the most intense SERS
band and βs(NH2) as a weak shoulder in Fig. 71a at ESCE = 300 mV proves that the favorable adsorption of PVAm2 is through the primary amino-groups.
In acidic electrolyte solution and at ESCE = 200 mV the interaction between nitro-group
of the aromatic substituent and the gold surface causes the appearance of νAu-ONO as a weak
feature (see the inset of Fig. 71b and Table 20). On the other hand, bands due to
coadsorbed perchlorate anion are observed at 254 cm-1 (νAu-OClO) and 934 cm-1 (ν (ClO−4)).
The spectral features are similar to those observed for PVAm3 in acidic electrolyte
solution (Fig. 68b). On the basis of these observations, we assume that the PVAm2 is
interfaced with the gold surface via the nitro-group and perchlorate anion as described for
PVAm3 in Fig. 70. In contrast to PVAm3, the coordination of PVAm2 with the gold
electrode via the oxygens of the nitro-group is relatively weak and coadsorption of
perchlorate is enhanced. This conclusion is based on the intense feature of the νAu-OClO
band compared to the weak νAu-ONO band. It should be noted that the observed νAu-ONO at
409-419 cm-1 (Tables 18-20) in all SER-spectra of p-NA-substituted PVAms is close to
that observed for p-NA (Table 6).
118
Chapter 7
7.3 o-Nitroaniline-substituted polyvinylamine adsorption on gold
The SER-spectra of PVAm4 adsorbed on a gold electrode at different potentials in neutral and acidic electrolyte solutions are shown in Fig. 72. Assignments of the observed
vibrational modes are collected in Table 21 and based on the data displayed in chapter 6
(see Table 14).
Raman intensity / s
-1
(a) 0.1 M KClO4
120
ESCE =
900 mV
80
700 mV
400 mV
100 mV
0 mV
2000
1500
1000
500
-1
Raman shift / cm
240
Raman intensity / s
-1
(b) 0.1 M HClO4
160
ESCE =
100 mV
300 mV
500 mV
700 mV
80 900 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 72. SER-spectra of PVAm4 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solution, 647.1 nm.
119
Chapter 7
Table 21. SERS spectral data of PVAm4 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
0.1 M KClO4, Fig. 72a
ESCE
ESCE
ESCE
= 100 = 400 = 700
mV
mV
mV
243
242
243
0.1 M HClO4, Fig. 72b
ESCE
ESCE
ESCE
= 300 = 500 = 700
mV
mV
mV
-
Mode
Wilson
mode # a
νAu-N
-
ESCE
=0
mV
241
νAu-OClO
-
-
-
-
-
259
270
270
273
275
274
n.a.
-
-
-
-
330
-
-
-
-
-
n.a.
-
-
-
-
359
-
-
-
-
-
ESCE
= 900
mV
-
ESCE
= 100
mV
-
ESCE
= 900
mV
-
νAu-ONO
-
407
408
409
407
407
411
404
404
401
400
n.a.
-
453
453
460
-
460
-
-
-
462
-
CCN def.
-
529
530
532
529
532
-
-
-
-
-
561
556
559
-
561
560
564
563
-
-
-
-
-
-
-
594
-
-
597
-
-
-
-
-
-
635
-
634
639
633
676
681
679
667
-
671
671
-
671
-
n.a.
n.a.
+ b
3
βas( NH )
-
n.a.
γ(Ring)
4
-
-
-
693
695
-
-
-
-
-
γs(NO2)
-
743
743
-
746
748
-
-
-
-
-
N─H def.
-
815
816
811
816
817
818
822
825
822
820
17a
844
847
849
-
-
851
853
856
849
-
ν(ClO )
-
933
933
933
934
935
937
937
937
937
938
n.a.
-
991
991
991
993
-
-
-
-
-
-
18b
1030
1032
1032
1032
-
1046
1046
1043
1043
-
1120
1120
1122
-
1158
γ(CH)
−
4
δ(CH)
n.a.
δ(CH)
9a
1127
1125
1133
1144
-
1151
1154
1156
1154
-
-
-
1195
1192
-
-
-
-
-
1234
1231
1234
1233
1243
1236
1238
1236
1240
1240
7a
-
-
-
-
-
-
1281
1282
1283
1281
n.a
-
1327
1326
1327
-
-
-
-
-
-
-
νs(NO2)
-
1351
1351
1349
1355
1355
1355
1356
1357
1356
1357
γs(CH2)
-
-
-
-
-
-
-
-
1386
1389
1390
βs(CH2)
-
1442
1435
1444
1435
1433
1447
1451
1453
1451
1449
ν(Ring)
8a
-
-
-
1582
1580
-
-
-
-
-
βs(NH2)
-
1600
1602
1597
-
-
-
-
-
-
-
n.a.
γas(CH2)
ν(CH)
+ b
3
1605
1605
1608
1608
1602
δas( NH )
a
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer in
acidic electrolyte and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βas: rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; δas: a symmetric deformation; n.a.: not assigned.
120
Chapter 7
As generally observed in SER spectroscopy at electrodes, there is a clear dependence
on the applied electrode potential as shown for the PVAm4 SER-spectra in neutral electrolyte solution (Fig. 72a). In the low-frequency region the intensity of the pronounced band
at 241-243 cm-1 decreased with no significant change in position as the potential moved
positively from ESCE = 0 to 700 mV. Further positive shifting of the electrode potential to
ESCE = 900 mV increases the intensity of this band with shifting toward 259 cm-1. The
question is, however, whether there is such a large shift in the frequency of a single band
or a change in relative intensities of at least two adjacent overlapping bands (νAu-N and νAuOClO).
According to the SER-spectra of adsorbed ClO−4 obtained from neutral electrolyte
separately and at positive electrode potential (Fig. 56) the νAu-OClO appears at ≥ 250 cm-1.
Therefore, the band at 259 cm-1 (Fig. 72a) and at more positive electrode potential (ESCE =
900 mV) is attributed to the νAu-OClO. This reveals that at this potential a small amount of
coadsorbed perchlorate anion is present at the electrode surface alone or simultaneously interacting with the partially protonated amino-group of PVAm4 and the Au surface (see
discussion above). At less positive electrode potentials (≤ 700 mV) the band at 241-243
cm-1 (Fig. 72a) must be caused by another low-frequency surface mode, most likely of adsorbed PVAm4. As previously mentioned in this text for the interaction of nitrogen atom
with several metal surfaces this band must be caused by an Au-N mode of PVAm4 adsorbed on the gold surface via the primary amino-group. The proposed adsorptive interaction via the amino-group is further supported by the fact that the band at 1597-1602 cm-1
assigned to the βs(NH2) appears only in the potential range from ESCE = 0 to 400 mV and in
a broad feature. These results suggest that the coordination via the amino-group is decreasing as the potential positively moved to ESCE = 700 mV and disappeared at ESCE = 900 mV.
An additional metal-adsorbate band, albeit rather weak, is also observed at 407-409 cm-1
in the SER-spectra of PVAm4 in neutral electrolyte solution (Fig. 72a). This band is most
likely caused by an Au-O mode of o-NA substituent of the PVAm4 adsorbed on the gold
surface via the nitro-group. The corresponding band was found with adsorbed o-NA
monomer at 404-406 cm-1 (see section 5.2). The weak feature of the νAu-ONO in this case is
due to the very low mole percentage (1 mole %) of the o-NA substituent. The other reason
of the band weakness is that the coordination of the ortho-nitro-group with gold surface
occurs via one oxygen. This is due to the intramolecular hydrogen bonding between one
121
Chapter 7
oxygen of the nitro-group and the amino-group of the o-NA moiety of this polymer as we
mentioned previously in this text.
In the presence of an acidic electrolyte solution of 0.1 M HClO4 SER-spectra with a
weak feature of the νAu-ONO were obtained (Fig. 72b, Table 21). The coordination via the
amino-group is absent in this case because of the strong protonation effect as we have discussed in the case of PVAm. Thus, the simultaneous interaction of perchlorate anions with
the protonated amino-groups of PVAm4 polymer and the Au surface is expected in this
case. This is evident from the observable enhancement of the νAu-OClO and ν(ClO−4) bands at
all applied electrode potentials. At a higher electrode potential (ESCE = 1000 mV), the spectral features of PVAm4, presented in Fig. 73, are compatible with the presence of oxidized
species of this polymer near the electrode surface. The electrochemical oxidation and
SERS measurements of this polymer at ESCE = 1000 mV are observed and discussed previously in the case of PVAm and PVAm1.
140
-1
340 cm
Raman intensity / s
-1
120
-1
100
363 cm
-1
267 cm
80
-1
197 cm
60
-1
40
500
173 cm
400
300
200
100
-1
Raman shift / cm
Fig. 73. SER-spectra of adsorbed species on a gold electrode at ESCE =1000 mV from a so-
lutions of 0.1 M HClO4 (dashed line) and 100 mL 0.1 M HClO4 + 0.05 g PVAm4
(solid line), 647.1 nm.
The expected models of adsorbed PVAm4 in different cases and according to the above
mentioned observations are illustrated Fig. 74. It should be mentioned that the protonation
122
Chapter 7
at the secondary amine of the o-NA moiety of this polymer is absent because of intramolecular interaction.
ClO4
NH3
NH2
N
N+
At ESCE = 0-700 mV
In 0.1 M KClO4
NH2
H
O
O
NH2
Au
NH2
NH2
At ESCE = 900 mV
In 0.1 M KClO4
N
N+
NH3
H
O
O
O
O
O
O
Cl
O
Cl
O
O
O
Au
ClO4
ClO4
NH3
NH3
N
N+
H
At ESCE = 100-900 mV
In 0.1 M HClO4
NH3
O
O
O
O
O
Cl
O
O
O
Cl
O
O
Au
Fig. 74. Proposed model of adsorbed PVAm4 and perchlorate anions at a gold surface in
neutral and acidic electrolyte solutions and at different electrode potential.
123
Chapter 7
The SER-spectra of PVAm with a high mole percentage (25 mole %) of the o-NA
substituent (PVAm5) adsorbed on a gold electrode at different electrode potentials in
neutral and acidic electrolyte solutions are shown in Fig. 75. Assignment of the observed
vibrational modes is collected in Table 22 and based on the data displayed in chapter 6 (see
Table 14).
160
(a) 0.1 M HClO4
ESCE =
Raman intensity / s
-1
900 mV
120
700 mV
80
500 mV
300 mV
100 mV
40
2000
1500
1000
500
-1
Raman shift / cm
Raman intensity / s
-1
(b) 0.1 M KClO4
240
ESCE =
180
100 mV
300 mV
400 mV
500 mV
600 mV
120
2000
1500
1000
500
-1
Raman shift / cm
Fig. 75. SER-spectra of PVAm5 adsorbed on a gold electrode at the electrode potentials
indicated in (a) acidic and (b) neutral perchlorate solutions, 647.1 nm.
124
Chapter 7
Table 22. SERS spectral data of PVAm5 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
Wilson
mode # a
0.1 M KClO4, Fig. 75b
ESCE
ESCE
ESCE
= 300 = 400 = 500
mV
mV
mV
273
275
273
ESCE
= 600
mV
270
ESCE
= 100
mV
261
0.1 M HClO4, Fig. 75a
ESCE
ESCE
ESCE
= 300 = 500 = 700
mV
mV
mV
264
262
264
νAu-OClO
-
ESCE
= 100
mV
267
νAu-ONO
-
402
402
402
407
407
405
407
408
408
410
-
557
559
559
558
561
558
558
558
559
562
-
610
-
-
-
-
613
616
612
612
-
n.a.
-
676
678
674
674
670
667
667
667
667
-
γs(NO2)
-
-
-
-
-
-
742
746
746
744
-
N─H def.
-
808
810
820
823
824
816
818
816
814
816
17a
846
846
846
846
851
845
847
843
845
842
-
-
-
-
-
-
935
934
934
934
936
δ(CH)
18b
1028
1032
1032
1032
1032
1039
1042
1042
1037
1039
δ(CH)
18a
-
-
-
-
-
1092
1090
1094
1090
1090
δ(CH)
Mode
n.a.
+ b
3
βas( NH )
γ(CH)
ν(ClO −4 )
ESCE
= 900
mV
269
9a
1148
1148
1146
1153
1151
1145
1145
1150
1150
1147
γas(CH2)
-
1232
1233
1232
1228
1232
1229
1227
1226
1227
1231
n.a
-
1326
1328
1328
-
-
-
-
-
-
-
νs(NO2)
-
1351
1353
1351
1351
1353
1350
1349
1350
1350
1350
βs(CH2)
-
1445
1447
1447
1450
1447
1448
1446
1448
1446
1448
νas(NO2)
-
-
-
1524
1526
1530
-
-
-
-
-
+ b
3
1613
1611
1610
1613
1611
δas( NH )
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer and
the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; δas: a symmetric deformation; n.a.: not assigned.
a
From Fig. 75 one can notice that the strongest band in both electrolyte solutions and at
all applied electrode potential is observed at 261-273 cm-1. This band can be assigned to
the gold-perchlorate stretching mode (νAu-OClO) as we discussed above. An additional
metal-adsorbate band, weak in neutral electrolyte solution and strong in acidic one, is also
detectable in the SER-spectra of PVAm5 at 402-410 cm-1. This band is most likely caused
by an Au-O mode of the o-NA substituent of the PVAm5 adsorbed on the gold surface via
the nitro-group (see above). The intensity of this band demonstrates the strong coordination via the nitro-group in acidic electrolyte solutions compared to neutral ones. This result
is compatible with the strong SER-spectra of PVAm5 in acidic electrolyte solutions. The
other conspicuous feature of Fig. 75 is the complete absence of the coordination via the
amino-group in a neutral electrolyte solution. This reveals that the conformation of the ad-
125
Chapter 7
sorbed layers of this polymer makes this coordination hard to occur, which is in obvious
contrast with the observation with other polymers. Most amino-groups of this polymer are
not protonated as we mentioned above in this text. Therefore, the absence of this coordination is not due to protonation effects but should be due to the presence of a high percentage
of the o-NA substituent. These o-NA substituents interact strongly with the electrode surface as we mentioned above. Also it might be that the adsorbed layers of this polymer at
the gold surface adopt conformations with loops and tails in neutral electrolyte solution.
Under these conditions the number of primary amino-group substituents of this polymer,
which can be exposed to the electrode surface is very small because of steric hindrances.
This steric factor is not present in the case of other polymers because of the low percentage
of the nitroaniline substituent. As a consequence of what we observed from the SERspectra of PVAm5 and from our discussion above, the adsorption of PVAm5 at a gold surface in neutral and acidic perchlorate solutions is proposed and illustrated in Fig. 76.
NH2
NH2
At ESCE = 100-600 mV
In 0.1 M KClO4
N
N
NH3
H
O
O
O
O
Cl
O
O
O
Cl
O
O
O
Au
ClO4
ClO4
NH3
NH3
At ESCE = 100-900 mV
In 0.1 M HClO4
N
N
H
NH3
O
O
O
O
O
Cl
O
O
O
Cl
O
O
Au
Fig. 76. Proposed model of adsorbed PVAm5 and perchlorate anions at a gold surface in
neutral and acidic electrolyte solutions and at different electrode potential.
126
Chapter 7
Fig. 77 shows the SER-spectra of PVAm7 adsorbed on a polycristalline gold surface at
different electrode potentials in neutral and acidic electrolyte solutions. Assignment of the
observed vibrational modes is collected in Table 23 and based on the data displayed in
chapter 6 (see Table 14).
55
ESCE = 0 mV
Raman intensity / s
-1
(a) 0.1 M KClO4
Raman intensity / s
-1
270
ESCE =
180
50
45
40
400
350
300
250
200
-1
800 mV
Raman shift / cm
600 mV
400 mV
90
200 mV
0 mV
2000
1500
1000
-1
500
Raman shift / cm
(b) 0.1 M HClO4
Raman intensity / s
-1
150
ESCE =
800 mV
100
400 mV
50
300 mV
200 mV
100 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 77. SER-spectra of PVAm7 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
127
Chapter 7
Table 23. SERS spectral data of PVAm7 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
0.1 M KClO4, Fig. 77a
0.1 M HClO4, Fig. 77b
E
E
E
E
E
E
ESCE
ESCE
ESCE
ESCE
SCE
SCE
SCE
SCE
SCE
SCE
Mode
=0
= 200 = 400 = 600 = 800
= 100 = 200 = 300 = 400 = 800
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
νAu-N
221
221
224
219
219
νAu-OClO
253
255
250
252
268
254
257
268
271
272
n.a.
303
n.a.
332
334
329
332
332
331
332
330
329
νAu-ONO
408
409
410
410
410
408
409
410
409
410
n.a.
445
447
445
445
446
448
445
445
δ(Ring)
6b
510
509
509
498
501
505
507
CCN def.
534
537
532
533
532
538
539
537
537
539
n.a.
568
572
574
573
580
563
566
608
611
608
609
607
βas( NH 3+) b
b
635
632
630
633
630
γs (NH2)
γ(Ring)
4
680
680
680
681
681
687
685
686
685
n.a.
715
717
717
716
717
γs(NO2)
756
755
753
755
759
752
754
752
757
757
N─H def.
815
811
813
818
816
827
831
833
832
820
933
935
934
ν(ClO −4 )
ν(CCCC)
957
960
956
958
962
δ(CH)
18b
1013
1011
1012
1011
1011
δ(CH)
18a
1093
1088
1090
1090
1091
1092
1090
δ(CH)
9a
1165
1166
1166
1167
1170
1167
1170
1174
1174
1176
n.a.
1216
1218
1216
1215
1216
1194
1196
1194
1198
1237
1240
1237
1237
1233
1237
1233
γas(CH2)
ν(CH)
7a
1266
1265
1266
νs(NO2)
1340
1342
1344
1350
1355
1346
1348
1350
1355
1356
n.a.
1406
1404
1405
βs(CH2)
1452
1448
1450
1443
1448
νas(NO2)
1505
1507
1509
1509
1509
1511
1515
1520
1514
1510
ν(Ring)
8a
1587
1585
1584
1586
1587
1590
1591
1587
1587
1591
βs(NH2)
1626
1628
1623
1626
1626
1620
1618
1620
1619
1619
δas( NH 3+)b
a
: band corresponding to benzene ring; b: band corresponding to protonated amino-groups of this polymer and
the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; δas: a symmetric deformation; n.a.: not assigned.
Wilson
mode # a
SERS features of the adsorbed PVAm7 on a gold surface are similar to those observed
for the PVAm3 (see section 7.2). In neutral electrolyte, vibrational bands of benzene ring
as well as other substituent groups of the polymer are observed at all electrode potentials
ranging from ESCE = 0 to 800 mV (Fig. 77a, Table 23). The prominent band at 408-410 cm1
which has been assigned previously to the gold-oxygen stretching mode, is an indication
of the strong interaction between the nitro-group of o-NA substituent and the gold surface.
128
Chapter 7
This band is detected at all electrode potentials and its maximum intensity like all other
SERS bands is observed at ESCE = 0 mV. In addition, a weak band is observed at 219-224
cm-1. This band has been assigned previously to the νAu-N and is an indication of the interaction between the amino-groups of PVAm7 and the gold surface. The maximum intensity
of this band is observed at ESCE = 0 mV (see the inset of Fig. 77a). The features corresponding to coadsorbed perchlorate anions are detected by the presence of νAu-OClO band at
250-268 cm-1. The two expected types of coadsorbed perchlorate anions at the gold surface
in addition to the assignment of the νAu-OClO band has been discussed previously. As a result of the above observations the proposed model of the adsorbed PVAm7 at gold surface
in the presence of the coadsorbed perchlorate anions in neutral electrolyte solution is illustrated in Fig. 78.
NH2
ClO4
NH3
NH2
N
N+
H
NH2
NH3
O
O
O
O
O
NH2
O
Cl
O
O
Cl
O
O
Au
Fig. 78. Proposed model of adsorbed PVAm7 and perchlorate anions at a gold surface in
neutral electrolyte solution.
In acidic electrolyte, the observed metal-adsorbate bands (νAu-OClO, νAu-ONO) in the
strong SER-spectra of PVAm7 (Fig. 77b) are similar to those observed for PVAm3 (Fig.
68b) and PVAm5 (Fig. 75a). Therefore, the coordination modes of these polymers with the
gold surface in acidic electrolyte are identical (see Fig. 70 and 76). In contrast with
PVAm3, in this case, the intensity of the gold-perchlorate stretching band was almost independent of positively shifted electrode potential. In the SER-spectra of other o-NAsubstituted PVAms (PVAm4 (Fig. 72b), PVAm5 (Fig. 75a)) in acidic electrolyte solution,
the most intense SERS band is the gold-perchlorate stretching band. This feature is not observed in the case of SER-spectra of PVAm7 in acidic electrolyte solution. The coadsorption features of perchlorate anion are further discussed later in this text. The maximum in129
Chapter 7
tensity of the other metal-adsorbate band (νAu-ONO) is observed at ESCE = 400 mV. This result is compatible with the strong SER-spectra of PVAm7 observed at this electrode potential. At an electrode potential close to ESCE = 400 mV the SER-spectrum of PVAm5 in
acidic electrolyte solution shows the same result.
The SER-spectra of PVAm6 and PVAm8 adsorbed on a gold electrode at different
electrode potentials in an acidic electrolyte solution are shown in Fig. 79. Assignments of
the observed vibrational modes are collected in Table 24 and based on the data displayed
in chapter 6 (see Table 14).
(a) PVAm6
25
ESCE =
Raman intensity / s
-1
30
800 mV
20
600 mV
400 mV
200 mV
2000
1500
1000
500
-1
Raman shift / cm
(b) PVAm8
Raman intensity / s
-1
150
100
ESCE =
800 mV
50 600 mV
400 mV
200 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 79. SER-spectra of (a) PVAm6 and (b) PVAm8 adsorbed on a gold electrode at the
electrode potentials indicated in acidic perchlorate solution, 647.1 nm.
130
Chapter 7
Table 24. SERS spectral data of PVAm6 and PVAm8 adsorbed on a gold electrode at dif-
ferent electrode potentials and in acidic electrolyte solution.
Wilson
mode # a
PVAm6, Fig. 79a
ESCE
ESCE
= 400 = 600
mV
mV
267
269
νAu-OClO
-
νAu-ONO
-
405
407
208
409
409
407
405
405
n.a.
-
-
-
-
-
455
462
465
462
n.a.
-
-
-
584
-
-
-
-
-
n.a.
-
-
-
-
-
670
664
666
665
N─H def.
-
-
-
829
829
819
821
819
816
17a
856
854
851
854
853
856
856
855
ν(ClO )
-
935
934
936
935
935
936
936
936
n.a.
-
-
-
-
-
1006
1010
1008
1011
18b
1036
1034
1036
1034
-
1040
1041
1041
Mode
γ(CH)
−
4
δ(CH)
n.a.
δ(CH)
γas(CH2)
ESCE
= 800
mV
270
ESCE
= 200
mV
249
PVAm8, Fig. 79b
ESCE
ESCE
= 400 = 600
mV
mV
251
256
ESCE
= 200
mV
259
ESCE
= 800
mV
263
-
-
-
-
-
1117
1115
1116
1117
9a
1170
1168
1172
1167
1142
1144
1147
1152
-
1234
1236
1237
1232
1241
1243
1236
1234
n.a.
-
-
-
-
-
1292
1292
1290
1292
νs(NO2)
-
1355
1352
1356
1353
-
1354
1356
1355
-
-
-
-
-
1379
1378
1381
1384
b
γs(CH2)
n.a.
-
-
-
-
1394
-
-
-
-
βs(CH2)
-
1451
1451
1449
1451
1447
1451
1447
1452
νas(NO2)
-
-
-
1531
1534
1502
1505
1507
1508
ν(Ring)
8a
1582
1585
1590
-
-
-
-
-
+ b
3
1606 1608 1609 1611
δas( NH )
: band corresponding benzene ring; b: band corresponding to the protonated amino-groups of this polymer
and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane
deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric stretching; βas: rocking; γs: wagging; βs:
scissoring; γas: twisting; β: bend; δas: a symmetric deformation; n.a.: not assigned.
a
As shown in the SER-spectra of PVAm6 and PVAm8 in acidic electrolyte solution
(Fig. 79) both metal-adsorbate bands, νAu-OClO and νAu-ONO, were detected at all electrode
potentials ranging from ESCE = 200 to 800 mV. This behavior in the low-frequency region
is very similar to that obtained and discussed previously with SERS results of other o-NAsubstituted PVAms in acidic electrolyte solution. Consequently, adsorption of PVAm6 and
PVAm8 on the gold electrode from this acidic solution via the nitro-group and through
perchlorate anions can be concluded (see acidic case of Fig. 74 and 76).
131
Chapter 7
The observed νAu-ONO at 400-411 cm-1 (Tables 21-24) in all SER-spectra of o-NAsubstituted PVAms is very close to that observed for o-NA monomer (Table7). This metal
adsorbate band is relatively red-shifted as compared to that obtained in the SER-spectra of
p-NA-substituted PVAms (Tables 18-20). This is in agreement with our previous results
obtained and discussed for the adsorption behavior of o-NA and p-NA at gold surface (for
more details see sections 5.1 and 5.2). Through analysis and comparison of the intensity of
this band (νAu-ONO) as well as other polymer bands in the SER-spectra of nitroanilinesubstituted PVAms, we were able to deduce the influence of the nitroaniline substituent on
the adsorption behavior of PVAm. From the SERS results of PVAms, we can clearly see
that all SER-spectra of the nitroaniline-substituted PVAms show a higher intensity than
that of PVAm. The strong feature of the intensity of the SERS bands in the SER-spectra of
the nitroaniline-substituted PVAms must be due to the effect of the nitroaniline substituent.
In addition the intensity of the SERS bands of the nitroaniline-substituted PVAms varies
according to the mole percentage and type of the nitroaniline substituent. As we observed
from the SERS results the nitro-group of the nitroaniline substituent can act as a strong surface-anchoring site. Specifically, we note that the polymer bands are strongly enhanced
simultaneously with the observed strong feature of the gold-oxygen mode (νAu-ONO) in the
SER-spectrum of PVAms containing 8.7 and 12.7 mole % of p-NA (in PVAm3, Fig. 68)
and o-NA (in PVAm7, Fig. 77) substituent, respectively. Further enhancement of the
polymer bands is not observed in the SER-spectra of the other nitroaniline-substituted
PVAms. Thus, a plausible conclusion is that the nitroaniline-substituted PVAm containing
more or less mole percentage of nitroaniline substituent than these values (8.7 mole % of
p-NA, 12.7 mole % of o-NA) leads to the negative effect on the adsorption strength of this
polymer.
In the SER-spectra of the nitroaniline-substituted PVAms, in-plane as well as out-ofplan modes are observed in most cases. Interaction of the nitroaniline moiety of o- and pNA-substituted PVAms with a gold surface in a perpendicular orientation (with in-plane
modes being prominent) or in a flat orientation (with out-of-plane modes being prominent)
cannot be deduced straightforwardly. However, the coordination of the nitroaniline substituent of these polymers with the gold surface in perpendicular orientation is favorable.
This conclusion is based on the results obtained and discussed previously for the adsorption behavior of o-NA and p-NA monomers at gold surface.
132
Chapter 7
Further analysis of SERS results of PVAms shows the dominant presence of coadsorbed perchlorate anions from acidic electrolyte solution. This is due to the presence of
perchlorate anions as counter ions for the protonated amino-groups of these polymers,
which increase the coverage of perchlorate at the gold surface. However, this goldperchlorate stretching mode is relatively weak in the case of SER-spectra of PVAm3 and
PVAm7 in acidic electrolyte solution compared to that obtained in the SER-spectra of the
other PVAms in the same electrolyte. This result indicates that the perchlorate coverage is
suppressed by another strong adsorbate, most likely the layers of PVAm3 and PVAm7 (see
the substituent effect above).
In contrast with acidic electrolyte, the amino-groups of PVAms are only partially protonated in neutral electrolyte solution. In this case the presence of perchlorate anions as a
counter ion for this protonated amino-group is very small. Thus, the coadsorption feature
of perchlorate anions at the gold surface should be very weak in neutral electrolyte solutions. However, the decision in this case should be made according to the intensity of goldperchlorate stretching mode (strong, weak or not present) in comparison with the polymer
bands. In the SER-spectra of PVAm5 (Fig. 75b) the strongest band is the gold-perchlorate
stretching mode. Thus, the coadsorption feature of perchlorate anions at the gold surface is
very strong in this case. Which is due to the absence of the coordination of PVAm5 via
amino-groups with the gold surface in neutral electrolyte solution (see Table 22). This
leads to the low coverage of the PVAm5 layers at gold surface. In contrast with PVAm5,
the gold-perchlorate stretching mode is weaker than the polymer bands or not present in
the SER-spectra of PVAm (Fig. 55), PVAm1 (Fig. 62), PVAm2 (Fig. 71a), PVAm3 (Fig.
68a), PVAm4 (Fig. 72a) and PVAm7 (Fig. 77a) in neutral electrolyte solution. This observation indicates a weak coadsorption feature of perchlorate anions at the gold surface. In
both electrolyte solutions, simultaneous interaction of perchlorate anion with the ─ NH 3+
group of PVAms and the Au surface is most likely. However, presence of small amounts
of perchlorate anions as coadsorbed only at gold surface is also expected. The effect of salt
concentration, mentioned previously in introduction, on the adsorption behavior of the
PVAms under study is not considered in our discussion; the salt concentration is the same
in all cases.
133
Chapter 7
7.4 Wurster-substituted polyvinylamine adsorption on gold
Band positions from SER-spectra of PVAm9 adsorbed from neutral (Fig. 80a) and
acidic (Fig. 80b) electrolyte solutions are listed in Table 25; assignment of the observed
lines is based on the data displayed in chapter 6 (see Table 15). In neutral electrolyte solution, with the exception of the low-frequency mode, most of bands are corresponding to
modes of PVAm9. The remaining mode at 412-415 cm-1 is most likely caused by surfaceadsorbate mode. This suggestion is supported by a corresponding band at 400-419 cm-1
found with nitroaniline-substituted PVAm adsorbed on a gold electrode (see previous sections). Consequently, adsorption of PVAm9 on the gold electrode from this neutral solution via the nitro-group of Wurster substituent can be concluded. The high intensity of the
band attributed to the symmetric nitro-group-stretching mode supports this conclusion.
Raman intensity / s
-1
(a) 0.1 M KClO4
150 E =
SCE
800 mV
600 mV
100 400 mV
200 mV
0 mV
2000
1500
1000
500
-1
Raman shift / cm
(b) 0.1 M HClO4
Raman intensity / s
-1
80
70
ESCE =
700 mV
60
500 mV
300 mV
100 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 80. SER-spectra of PVAm9 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
134
Chapter 7
Table 25. SERS spectral data of PVAm9 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
Mode
Wilson
mode # a
0.1 M KClO4, Fig. 80a
ESCE
ESCE
ESCE
= 200 = 400 = 600
mV
mV
mV
412
415
415
-
ESCE
=0
mV
412
-
506
506
508
508
508
-
-
-
-
6a
16a
-
536
552
565
580
600
-
538
553
568
580
601
-
541
554
581
603
-
542
554
580
603
-
537
551
569
581
602
-
-
554
-
555
-
557
611
-
-
633
634
633
632
641
743
759
822
640
743
760
823
-
-
-
-
930
928
934
933
935
-
1224
1265
1306
1367
1406
1442
1498
1520
1568
1638
1270
1366
1403
1492
1566
1635
1218
1350
-
1221
1346
-
1248
1347
1501
1530
1574
-
1218
1347
1496
1528
1571
-
νAu-OClO
νAu-ONO
R─NH2
(CCN def)
δ(Ring)
n.a
n.a.
γ(Ring)
γs(NH2)
n.a.
βas( NH 3+)b
-
-
-
-
n.a.
n.a.
βas(CH2)
γ(CH)
11
641
739
760
821
642
741
762
823
641
741
761
822
ν(ClO −4 )
-
930
930
930
γas(CH2)
n.a.
n.a.
νs(NO2)
n.a
βs(CH2)
n.a.
νas(NO2)
ν(Ring)
βs(NH2)
8a
-
1363
1395
1440
1496
1567
1633
1224
1277
1304
1366
1404
1439
1496
1518
1568
1636
1222
1266
1306
1366
1404
1437
1500
1521
1566
1639
ESCE
= 800
mV
415
0.1 M HClO4, Fig. 80b
ESCE
ESCE
ESCE
ESCE
= 100 = 300 = 500 = 700
mV
mV
mV
mV
259
257
268
272
-
1625
1626
1624
δas( NH 3+)b
a
b
: band corresponding benzene ring; : band corresponding to the protonated amino-groups of this polymer in
acidic electrolyte and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; νas: a symmetric stretching; βas:
rocking; γs: wagging; βs: scissoring; γas: twisting; β: bend; def.: deformation; n.a.: not assigned; δas: a symmetric deformation.
In acidic electrolyte solution (Fig. 80b) the coadsorbed perchlorate anion is indicated
by the appearance of a low-frequency band (νAu-OClO) at 257-272 cm-1. In this case no other
surface-adsorbate mode revealing a direct interaction of this polymer with the gold surface
is detected. All SER-spectra of PVAm9 from this acidic solution show remarkable disappearance of the PVAm9 bands. Thus, the simultaneous interaction of ClO −4 with the protonated amino-group of this PVAm9 and Au surface can be concluded. This conclusion is
135
Chapter 7
based on the presence of perchlorate anions as counter ions for the protonated aminogroups of this polymer in acidic electrolyte solution (see previous sections). The appearance of two strong bands corresponding to the protonated amino-group (δas( NH 3+ ),
βas( NH 3+)) supports this conclusion. The proposed models of adsorbed PVAm9 at gold surface in both electrolyte solutions are illustrated in Fig. 81. In unbuffered electrolyte (0.1 M
KClO4, pH = 4.90) small amount of the amino-groups of PVAm9 are protonated. Therefore, the presence of perchlorate anions as counter ions for these groups should be considered in the model displayed in Fig. 81.
ClO4
NH3
NH2
NH2
NH
H3C
N
N+
CH3
O
O
In 0.1 M KClO4
Au
H3C
N
O2N
ClO4
CH3
ClO4
H2N
NH3
In 0.1 M HClO4
NH3
NH3
O
O
O
Cl
O
O
O
Cl
O
O
O
O
Cl
O
O
Au
Fig. 81. Proposed models of adsorbed PVAm9 on a gold surface in neutral and acidic elec-
trolyte solutions.
136
Chapter 7
Fig. 82 shows the SER-spectra of PVAm10 (2-nitro-p-phenylenediamine-substituted
PVAm radical cation) adsorbed on a polycristalline gold surface in different electrolyte solutions and at different electrode potentials. The assignment of the observed vibrational
modes is collected in Table 26 and based on the data displayed in chapter 6 (see Table 15).
ESCE =
(a) 0.1 M KClO4
Raman intensity / s
-1
40 700 mV
35 500 mV
200 mV
30
0 mV
25
2000
1500
1000
500
-1
Raman shift / cm
160
(b) 0.1 M HClO4
Raman intensity / s
-1
140
120
100
80 ESCE =
700 mV
500 mV
60 300 mV
100 mV
2000
1500
1000
500
-1
Raman shift / cm
Fig. 82. SER-spectra of PVAm10 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
In neutral electrolyte solution the SER-spectrum of PVAm10 at ESCE = 0 mV (Fig. 82)
shows a low-frequency mode attributed, as above, to the surface-adsorbate mode at 415
cm-1. The maximum intensity of this band and νs(NO2) as well as of other polymer bands is
observed at this electrode potential. According to our previous discussion this band must
be caused by an Au-O mode of PVAm10 adsorbed on the gold surface via the nitro-group
137
Chapter 7
of Wurster substituent. At ESCE = 700 mV all polymer bands and the band corresponding to
the interaction of the nitro-group with the gold surface are completely lost. This result
demonstrates that the oxygen of the nitro-group of Wurster substituent was detached from
the gold surface. Also at ESCE = 700 mV a new strong band is observed at 252 cm-1. This
band has been assigned previously to the gold-perchlorate stretching mode (νAu-OClO). It is
easy for the perchlorate anion to interact with the gold surface if it is present as counter ion
for the radical cation amino-group of the Wurster substituent of PVAm10 in neutral electrolyte solution. This feature is not observed with the other Wurster-substituted PVAm
(PVAm9). The interaction of perchlorate anion with radical cation amino-group of the
Wurster substituent of this polymer is obstructed by the presence of the two methyl group.
Thus, in contrast with PVAm10, the coadsorption of perchlorate anion was not observed at
all in the SER-spectra of PVAm9 in neutral electrolyte and at all electrode potentials (see
Fig. 80a).
Table 26. SERS spectral data of PVAm10 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
Wilson
mode # a
Mode
ESCE =
200 mV
-
ESCE =
500 mV
-
ESCE =
700 mV
252
0.1 M HClO4,
Fig. 82b
ESCE =
100 to 700 mV
250-266
0.1 M KClO4, Fig. 82a
νAu-OClO
-
ESCE = 0
mV
-
νAu-ONO
-
415
422
-
-
-
R─NH2 (CCN def)
-
506
503
-
-
-
11
818
815
819
820
-
-
935
936
936
936
933-936
γ(CH)
17b
-
962
-
-
-
δ(CH)
18b
1074
170
-
-
-
δ(CH)
18a
1147
1140
-
-
-
γas(CH2)
-
1240
-
-
-
-
νs(NO2)
-
1352
1346
1343
1357
-
n.a.
-
1418
1418
-
-
-
βs(CH2)
-
1452
-
-
-
-
νas(NO2)
-
1515
1518
-
-
-
ν(Ring)
8b
1601
1600
1606
1606
-
γ(CH)
−
4
ν(ClO )
βs(NH2)
1627
1616
a
: band corresponding benzene ring; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs:
symmetric stretching; νas: a symmetric stretching; βs: scissoring; γas: twisting; def.: deformation; n.a.: not assigned.
138
Chapter 7
With a solution of 0.1 M HClO4, the SER-spectra of PVAm10 (Fig. 82b) at all applied
electrode potentials are very similar to these recorded with a neutral solution at high positive electrode potential (see Fig. 80a, at ESCE = 700 mV). Thus, the simultaneous interaction of ClO−4 with the radical cation amino-group of the Wurster substituent of PVAm10
and Au surface can be concluded. The reasonable models of adsorbed PVAm10 at a gold
surface in different electrolyte solutions and at different electrode potentials are displayed
in Fig. 83.
ClO4
NH3
NH2
NH2
NH
ClO4
H2N
N+
In 0.1 M KClO4
at ESCE = 0-500 mV
O
O
Au
ClO4
NH3
NH2
ClO4
NH3
ClO4
NH3
ClO4
NH
H2N
NO2
NO2
In 0.1 M KClO4
at ESCE = 700 mV
NH2
NH2
In 0.1 M HClO4
at ESCE = 0-700 mV
O
O
O
O
Cl
O
Cl
O
O
Au
O
Au
Fig. 83. Proposed models of adsorbed PVAm10 on a gold surface in neutral and acidic
electrolyte solutions.
139
Chapter 7
7.5 Stilbene-substituted polyvinylamine adsorption on gold
Band positions from SER-spectra of PVAm11 adsorbed from neutral (Fig. 84a) and
acidic (Fig. 84b) electrolyte solutions are listed in Table 27; assignment of the observed
lines is based on the data displayed in chapter 6 (see Table 16). In both electrolyte solutions a low-frequency mode is observed at 248-252 cm-1. This mode cannot be caused by
the direct interaction of adsorbed PVAm11 with the gold surface. The observed high intensity of this mode at most applied electrode potentials is not compatible with the otherwise
weak feature observed for PVAm11 modes. According to our results discussed in previous
sections this mode is most likely caused by the Au-O of the coadsorbed perchlorate anion.
It is possible that the perchlorate anion is adsorbed at the gold surface with or without simultaneous interaction with the protonated amino-group of PVAm11.
(a) 0.1 M KClO4
Raman intensity / s
-1
50
40
30
20
ESCE =
500 mV
400 mV
300 mV
200 mV
2000
1500
1000
-1
500
Raman shift / cm
45
(b) 0.1 M HClO4
Raman intensity / s
-1
40
35
30
ESCE =
25
800 mV
700 mV
500 mV
100 mV
20
2000
1500
1000
500
-1
Raman shift / cm
Fig. 84. SER-spectra of PVAm11 adsorbed on a gold electrode at the electrode potentials
indicated in (a) neutral and (b) acidic perchlorate solutions, 647.1 nm.
140
Chapter 7
A second low-frequency mode is observed at 413-424 cm-1. A comparison with previously obtained results and assignments leads to the conclusion that this mode is most likely
caused by an Au-O mode of PVAm11 adsorbed on the gold surface via the nitro-group of
stilbene substituent. The intensity of this mode is strongly correlated with PVAm11 modes.
The observed band attributed to the symmetric nitro-group stretching mode in most cases
of applied potential supports this conclusion. Further support can be gained from the cyclic
voltammograms. A well defined current wave indicative of the nitro-group reduction is
found; this hints strongly at an adsorption via this group, which is a precondition for electroreduction [271]. The interaction of stilbene with gold surface via the nitro-group in
acidic and neutral electrolyte solutions has been reported in previous work [233].
Table 27. SERS spectral data of PVAm11 adsorbed on a gold electrode at different elec-
trode potentials and in different electrolyte solutions.
Wilson
mode # a
Mode
0.1 M KClO4, Fig. 84a
ESCE
ESCE
ESCE
ESCE
= 200 = 300 = 400 = 500
mV
mV
mV
mV
248
250
250
248
421
424
413
820
818
819
820
0.1 M HClO4, Fig. 84b
ESCE
ESCE
ESCE
ESCE
= 100 = 500 = 700 = 800
mV
mV
mV
mV
252
252
250
421
414
416
821
822
822
νAu-OClO
νAu-ONO
γ(CH)a
10a
ν(ClO −4 )
-
940
937
937
940
938
938
838
935
18b
13
19a
8a
-
1346
1446
1511
1592
1638
1110
1341
1443
1508
1592
1636
1336
1449
1594
1630
1340
1448
1597
1626
1184
-
1182
1343
-
1176
1345
-
1346
-
a
δ(CH)
ν(CH)a
νs(NO2)
βs(CH2)
ν(Ring)a
ν(Ring)a
βs(NH2)
1612 1615 1617
δas( NH 3+)b
a
: band corresponding para-di-substituted-benzene ring; b: band corresponding to the protonated aminogroups of this polymer and the assignment of the band according to the reference [239,254]; δ: in-plane deformation; γ: out-of-plane deformation; ν: stretching; νs: symmetric stretching; βs: scissoring; δas: a symmetric deformation.
All ring vibrations obtained from the SER-spectra of PVAm11 correspond to those of
para-di-substituted-benzene rings instead of 1,2,4 tri-substituted-benzene rings. Therefore,
the interaction of stilbene substituent with the gold surface must proceed via the p-nitrogroup attached to the para-di-substituted-benzene ring. The proposed models of adsorbed
PVAm11 at a gold surface in both electrolyte solutions are displayed in Fig. 85.
141
Chapter 7
ClO4
H3N
NH2
H2N
H2N
NH2
NH
H2N
NH2
NO2
In 0.1 M KClO4
NH3
O
O
O
O
Cl
O
O
N+
O
Cl
O
O
O
Au
ClO4
H3N
ClO4
ClO4
NH3
ClO4
NH3
H3N
ClO4
ClO4
H3N
NH3
H2N
ClO4
NH3
NO2
ClO4
In 0.1 M HClO4
NH3
O
O
O
Cl
O
O
O
N+
O
Cl
O
O
O
Au
Fig. 85. Proposed models of adsorbed PVAm11 and perchlorate anions on a gold surface
in neutral and acidic electrolyte solutions.
142
Summary
Summary
A rapid developing field in chemistry is the application of aniline, substituted anilines
and polymers with aromatic pendant groups in photoconducting and electronic devices.
Thus several investigations have been concentrated, recently, on the electrochemical and
spectroelectrochemical behavior of these materials at surfaces. Chapter 1 gives an overview of these investigations. This thesis describes the behavior of nitroanilines,
phenylenediamines and polyvinylamines containing several types of aromatic substituent
at gold and platinum surfaces, which is based on electrochemical and spectroelectrochemical measurements. The experimental parameters of these measurements are summarized in
chapter 2.
Chapter 3 summarizes the results obtained with cyclic voltammetry for isomeric nitroanilines and phenylenediamines at gold and platinum electrodes in neutral (0.1 M
KClO4, pH = 4.90, unbuffered) and acidic (0.1 M HClO4, pH = 1.2) electrolyte solutions.
The oxidation and reduction potentials for these monomers are obtained and discussed in
detail. The o- and p-nitroaniline are reduced to o- and p-phenylenediamine, respectively, in
acidic electrolyte solution according to the following ECE (electron transfer, chemical reaction, electron transfer) mechanism.
NHOH
NO2
NH
+ 2e-, + 2H+
+ 4e-, + 4H+
- H2O
NH2
NH2
- H2O
NH2
NH
NH2
In neutral electrolyte solution the final product of electrochemical reduction of o- and
p-nitroaniline are o- and p-amino-phenylhydroxylamine, respectively, and no further reduction occurs. This electrode reaction is discussed in terms of products and intermediates
using data obtained from quantum chemical calculations. The influence of substituent position on the electrochemical oxidation of nitroanilines is discussed. The order of increasing
oxidation potential of nitroanilines is m-nitroaniline > p-nitroaniline > o-nitroaniline. A
possible mechanism for electropolymerization started after electrochemical oxidation of
phenylenediamines is presented. The mechanism is further supported by electron spin den143
Summary
sity calculation for the phenylenediamine radical cation. The conclusions obtained from the
electrochemical measurements of these monomers are used as reference for the electrochemical measurements of nitroaniline-substituted polyvinylamines, which are displayed
in the same chapter. As a result, the o- and p-nitroaniline-substituted polyvinylamine are
reduced to o- and p-phenylenediamine-substituted polyvinylamine, respectively, in acidic
electrolyte solution according to the following ECE (electron transfer, chemical reaction,
electron transfer) mechanism.
X
NH2
Y
NH2
Y
X
Y
X
NH
NH2
NH
+4e-, +4H+
NH2
NH
+2e-, +2H+
-H2O
NO2
Y
X
N
-H2O
NH
NHOH
NH2
In neutral electrolyte solution the final product of electrochemical reduction of o- and
p-nitroaniline-substituted polyvinylamine are o- and p-amino-phenylhydroxylaminesubstituted polyvinylamine, respectively. The electrochemical oxidation of these polymers
corresponds to the primary amino-group substituents.
In Chapter 4 UV-Vis spectra measured in neutral and acidic electrolyte solutions of all
investigated monomers and polymers are presented. The influence of pH on the absorption
maximum is discussed.
The adsorption behavior of isomeric nitroanilines and phenylenediamines at a gold surface as studied with surface enhanced Raman spectroscopy in both neutral and acidic electrolyte solutions is described in chapter 5. Vibrational frequencies were calculated for these
monomers at the B3LYP level theory using 6-31G(d) basis sets in order to get accurate assignments for the observed lines in the normal Raman and SER-spectra. The surface-metal
interaction for these monomers is determined for every isomer. The nitroanilines are adsorbed on a gold surface in a perpendicular orientation via the nitro-group at positive electrode potentials in both electrolyte solutions as shown in the following model.
144
Summary
NH2
H
H2N
N
H
O–
+
–
O
N
N+
–
O
O
O
N+
O
Au
Au
Au
p-nitroaniline
o-nitroaniline
m-nitroaniline
The m- and p-phenylenediamine adsorbed via benzene ring and nitrogen atoms in a flat
orientation with respect to the metal surface, whereas o-phenylenediamine adsorption is
taking place via nitrogen atoms and with tilted orientation as illustrated in the following
models.
H2N
H2N
NH2
NH2
Au
o-phenylenediamine
Au
p-phenylenediamine
H3N
NH3
Au
m-phenylenediamine
Vibrational spectra of polyvinylamines investigated in this study have not been reported previously. IR spectra of polyvinylamines were obtained using standard techniques
and they are displayed in chapter 6.
145
Summary
Several modes of monomer-surface contact of adsorbed polyvinylamines at the gold
surface in both electrolyte solutions have been obtained and discussed in Chapter 7. A
comparison of these modes with those obtained for the adsorbed monomers is presented. A
variety of adsorption behaviors of these polymers at a gold surface was concluded; this
clearly depends on the applied electrode potential, pH of electrolyte (acidic or neutral),
type and mole percentage of the aromatic substituent. The adsorption of polyvinylamine at
a gold surface is strongly enhanced with 8.7 and 12.7 mole % of p- and o-nitroaniline substituent, respectively, at its backbone. The direct coordination of these polymers with the
gold surface occurred via the nitro-group of the aromatic substituent and primary aminogroup located at the polymer backbone.
146
Future work
Future work
Future investigation will require the employment of complementary in situ electron
spin resonance (ESR) spectroscopy. Many informations about the radicals generated during electrochemical processes can be gained from this in situ characterization method
[272,273]. These informations will give further insight into the mechanism of electrode reactions, which were discussed in chapter 5.
The study of adsorption behavior of polyvinylamines (polyelectrolytes) by surface enhanced Raman spectroscopy (SERS), which is one of the main area of this work, was taken
as a starting point. It is recommended for further investigation to use additional techniques.
The conformation of surface bound polyelectrolytes can be obtained from Monte Carlo
simulations [82] as a theoretical point of view. The statics and dynamics of single polymer
chains at surfaces can be studied by new experimental techniques such as atomic force microscopy (AFM) [235].
The research can be continued to examine the effects of changing experimental parameters on the electrochemical and spectroelectrochemical behavior of polymers and
monomers investigated in this study. Using several concentrations and pHs of the electrolyte solution [95] and other electrode surfaces (for example silver) is suggested. The effect
of these changes will give a better understanding of reaction mechanisms.
147
References
References
1
Y. Şahin, S. Perçin, M. Şahin, and G. Özkan, J. Appl. Polym. Sci. 91 (2004) 2302.
2
A. Cihaner and A. M. Önal, Polym. Int. 51 (2002) 680.
3
B. A. Deore, S. Hachey, and M. S. Freund, Chem. Mater. 16 (2004) 1427.
4
A. Buzarovska, I. Arsova, and L. Arsov, J. Serb. Soc. 66 (2001) 27.
5
R. Holze, J. Electroanal. Chem. 224 (1987) 253.
6
S. Neves and C. P. Fonseca, J. Braz. Chem. Soc. 15 (2004) 395.
7
E. C. Venancio, L. H. C. Mattoso, and A. J. Motheo, J. Braz. Chem. Soc. 12 (2001)
526.
8
S. Fauveaux and J. -L. Miane, Electromagnetics 23 (2003) 617.
9
V. Svetlicic, A. J. Schmidt, and L. L. Miller, Chem. Mater. 10 (1998) 3305.
10
R. Tomat, Chimica e L'Industria 52 (1970) 438.
11
D. E. Bartak, W. C. Danen, and M. D. Hawley, J. Org. Chem. 35 (1970) 1206.
12
S. S. Gitis, I. M. Sosonkin, and A. Y. Kaminskii, Teor. Eksp. Khim. 7 (1971) 253.
13
A. Darchen, C. R. Acad. Sci., Ser. C 272 (1971) 2193.
14
H. Nobutoki and H. Koezuka, J. Phys. Chem. A 101 (1997) 3762.
15
S. Yamada, K. Yamaguchi, K. Kamada, and K. Ohta, Mol. Phys. 101 (2003) 309.
16
M. Panhuis, R. W. Munn, and P. L. A. Popelier, J. Chem. Phys. 120 (2004) 11479.
17
F. Bertinelli, P. Palmieri, A. Brillante, and C. Taliani, Chem. Phys. 25 (1977) 333.
18
M. Stähelin, D. M. Burland, and J. E. Rice, Chem. Phys. Lett. 191 (1992) 245.
19
J. N. Woodford, M. A. Pauley, and C. H. Wang, J. Phys. Chem. A101 (1997) 1989.
20
F. L. Huyskens, P. L. Huyskens, and A. P. Persoons, J. Chem. Phys. 108 (1998)
8161.
21
L. Turi and J. J. Dannenberg, J. Phys. Chem. 100 (1996) 9638.
22
M. M. Szostak, B. Kozankiewicz, G. Wójcik, and J. Lipinski, J. Chem. Soc. Faraday Trans. 94 (1998) 3241.
23
M. Hurst and R.W. Munn, Special Publication-Royal Society of Chemistry (Org.
Mater. non-linear Opt.) 69 (1989) 3.
24
J. Hutter and G. Wagniere, J. Mol. Struct. 175 (1988) 159.
25
J. J. Dannenberg, ACS Symposium Series (Mater. Nonlinear Opt.) 455 (1991) 457.
26
A. Wesch, O. Dannenberger, C. Wöll, J. J. Wolff, and M. Buck, Langmuir 12
(1996) 5330.
148
References
27
E. D. Schmid, M. Moschallski, and W. L. Peticolas, J. Phys. Chem. 90 (1986)
2340.
28
K. Kumar and P. R. Carey, J. Chem. Phys. 63 (1975) 3697.
29
S. V. Zelentsov, N. V. Zelentsova, M.V. Kuznetsov, and I. V. Simdyanov, High
Energ. Chem. 38 (2004) 25.
30
J. L. Stevenson and A. C. Skapski, J. Phys. C: Solid State Phys. 5 (1972) L233.
31
D. W. Price, S. M. Dirk, A. M. Rawlett, J. Chen, W. Wang, M. A. Reed, A. G.
Zacarias, J. M. Seminario, and J. M. Tour, Mat. Res. Soc. Symp. Proc. 660 (2001)
JJ9.1.1/D7.4.1.
32
R. F. Nelson and R. N. Adams, J. Phys. Chem. 72 (1968) 740.
33
R. S. Begunov, T. N. Orlova, O. V. Taranova, and V. Y. Orlov, J. Appl. Spectrosc.
69 (2002) 807.
34
M. Guo, J. Chen, J. Li, L. Nie, and S. Yao, Electroanalysis 16 (2004) 1992.
35
S. V. Sasso, R. J. Pierce, R. Walla, and A. M. Yacynych, Anal. Chem. 62 (1990)
1111.
36
C. Malitesta, I. Losito and P. G. Zabonin, Anal. Chem. 71 (1999) 1366.
37
P. H. Schmidt and W. J. Plieth, J. Electroanal. Chem. 201 (1986) 163.
38
R. Holze: Surface and Interface Analysis: An Electrochemists Toolbox, SpringerVerlag, Heidelberg, in preparation.
39
R. Holze, Electrochim. Acta 36 (1991) 1523.
40
P. Gao and M. J. Weaver, J. Phys. Chem. 89 (1985) 5040.
41
Y. S. Li, X. Lin, and Y.H. Cao, Vib. Spectrosc. 20 (1999) 95.
42
Y. -S. Li, T. Vo-Dinh, D.L. Stokes, and Y. Wang, Appl. Spectrosc. 46 (1992) 1354.
43
J. Hu, B. Zhao, W. Xu, Y. Fan, B. Li, and Y. Ozaki, J. Phys. Chem. B 106 (2002)
6500.
44
J. Hu, B. Zhao, W. Xu, Y. Fan, B. Li, and Y. Ozaki, Langmuir 18 (2002) 6839.
45
I. Roth and S. Spange, Macromol. Rapid Commun. 22 (2001) 1288.
46
M. H. Davey, V. Y. Lee, L. -M. Wu, C. R. Moylan, W. Volksen, A. Knoesen, R. D.
Miller, and T. J. Marks, Chem. Mater. 12 (2000) 1679.
47
Y. -C. Shu, Z. -H. Gong, C. -F. Shu, E. M. Breitung, R. J. McMahon, G. -H. Lee,
and A. K. -Y. Jen, Chem. Mater. 11 (1999) 1628.
48
M. Ahlheim, M. Barzoukas, P. V. Bedworth, M. Blanchard-Desce, A. Fort, Z. -Y.
Hu, S. R. Marder, J. W. Perry, C. Runser, M. Staehelin, and B. Zysset, Science 271
(1996) 335.
149
References
49
C. J. Bloys van Treslong, Recl. Trav. Chim. Pays-Bas. 97 (1978) 13.
50
C. J. Bloys van Treslong and A. J. Staverman, Recl. Trav. Chim. Pays-Bas. 93
(1974) 171.
51
A. Katchalsky, J. Mazur, and P. Spitnik, J. Polym. Sci. 23 (1957) 513.
52
P. V. Prabhakaran, S. Venkatachalam, and K. N. Ninan, Eur. Polym. J. 35 (1999)
1743.
53
B. Martel, Y. Leckchiri, A. Pollet, and M. Morcellet. Eur. Polym. J. 31 (1995)
1083.
54
S. Kobayashi, K. -D. Suh, and Y. Shirokura, Macromolecules 22 (1989) 2363.
55
Y. Qiu, T. Zhang, M. Ruegsegger, and R. E. Marchant, Macromolecules 31 (1998)
165.
56
G. Crini, M. Morcellet, and G. Torri, J. Chromatogr. Sci. 34 (1996) 477.
57
G. Crini and M. Morcellet, J. Chromatogr. Sci. 34 (1996) 485.
58
Z. -H. Wu, S. -P Chen, and H. Tanaka, J. Appl. Polym. Sci. 65 (1997) 2159.
59
O. Kanie, H. Tanaka, A. Mayumi, T. Kitaoka, and H. Wariishi, J. Appl. Polym. Sci.
96 (2005) 861.
60
Y. Shirota, N. Noma, and H. Mikawa, Synth. Met. 18 (1987) 399.
61
I. R. Jeon, N. Noma, and Y. Shirota, Mol. Cryst. Liq. Cryst. 190 (1990) 1.
62
Y. Shirota, I. R. Jeon, and N. Noma, Synth. Met. 55 (1993) 803.
63
K. Nawa, K. Miyawaki, I. Imae, N. Noma, and Y. Shirota, Synth. Met. 55 (1993)
1176.
64
K. Nawa, I. Imae, N. Noma, and Y. Shirota, Macromolecules 28 (1995) 723.
65
H. W. Gibson, Polymer 25 (1984) 3.
66
H. Tanaka, L. Ödberg, L. Wigberg, and T. Lindstrom, J. Colloid Interface Sci. 134
(1990) 219.
67
L. Eriksson and B. Alm, Water Sci. Techn. 28 (1993) 203.
68
K. Cheng, M. -H. Yang, W. W. W. Chiu, C. -Y. Huang, J. Chang, T. -F. Ying, and
Y. Yang, Macromol. Rapid Commun. 26 (2005) 247.
69
R. C. Advincula, S. Inaoka, D. Roitman, C. Frank, W. Knoll, A. Baba, and F. Kaneko, MRS Proceeding, Flat-Panel Displays and Sensors-Principles, Materials and
Processes, 588 (1999).
70
O. A. Evers, G. J. Fleer, J. M. H. M. Scheutjens, and J. Lyklema, J. Colloid Interface Sci. 111 (1986) 446.
150
References
71
G. Durand-Piana, F. Lafuma, and R. Audebert, J. Colloid Interface Sci. 119 (1987)
474.
72
R. Denoyel, G. Durand, F. Lafuma, and R. Audebert, J. Colloid Interface Sci. 139
(1990) 281.
73
S. Minko, A. Kiriy, G. Gorodyska, and M. Stamm, J. Am. Chem. Soc. 124 (2002)
3218.
74
J. M. H. M. Scheutjens and G. J. Fleer, J. Phys. Chem. 83 (1979) 1619.
75
J. M. H. M. Scheutjens and G. J. Fleer, J. Phys. Chem. 84 (1980) 178.
76
H. A. van der Schee and J. Lyklema, J. Phys. Chem. 88 (1984) 6661.
77
M. R. Böhmer, O. A. Evers, and J. M. H. M. Scheutjens, Macromolecules 23
(1990) 2288.
78
T. Åkesson, C. Woodward, and B. Jönsson. J. Chem. Phys. 91 (1989) 2461.
79
M. K. Granfeldt, B. Jönsson, and C. E. Woodward, J. Phys. Chem. 95 (1991) 4819.
80
S. Beltrán, H. H. Hooper, H. W. Blanch, and J. M.Prasunitz, Macromolecules 24
(1991) 3178.
81
M. A. G. Dahlgren, Å. Waltermo, E. Blomberg, P. M. Claesson, L. Sjöström, T.
Åkesson, and B. Jönsson, J. Phys. Chem. 97 (1993) 11769.
82
L. Sjöström, T. Åkesson, and B. Jönsson, J. Chem. Phys. 99 (1993) 4739.
83
I. Borukhov, D. Andelman, and H. Orland, Eur. Phys. J. B 5 (1998) 869.
84
A. Shafir, D. Andelman, and R. R. Netz, J. Chem. Phys. 119 (2003) 2355.
85
M. Granfeldt, B. Jönsson, and C. E. Woodward, J. Phys. Chem. 96 (1992 ) 10080.
86
P. M. Claesson and B. W. Ninham, Langmuir 8 (1992) 1406.
87
P. F. Luckham and J. Klein, J. Chem. Soc., Faraday Trans. 1 80 (1984) 865.
88
T. Afshar-Rad, A. I. Bailey, P. F. Luckham, W. Macnaughtan, and D. Chapman,
Colloids Surf. 25 (1987) 263.
89
J. Marra and M. L. Hair, J. Phys. Chem. 92 (1988) 6044.
90
K. P. Ananthapadmanabhan, G. -Z. Mao, E. D. Goddard, and M. Tirrell, Colloids
Surf. 61 (1991) 167.
91
J. Marra and M. L. Hair, J. Colloid Interface Sci. 128 (1989) 511.
92
K. Kurihara and T. Kunitake, Langmuir 8 (1992) 2087.
93
J. M. Berg, P. M. Claesson, and R. D. Neuman, J. Colloid Interface Sci. 161 (1993)
182.
94
S. Dhoot, E. D. Goddard, D. S. Murphy, and M. Tirrell, Colloids Surf. 66 (1992)
91.
151
References
95
B. C. Bonekamp and H. A. Van der Schee, J. Lyklema, Croat. Chem. Acta 56
(1983) 695.
96
T. Ueda and S. Harada, J. Appl. Polym. Sci. 12 (1968) 2395.
97
M. A. C. Stuart, G. J. Fleer, J. Lyklema, W. Norde, and J. M. H. M. Scheutjens,
Adv. Colloid Interface Sci. 34 (1991) 477.
98
J. Papenhuijzen, H. A. Van der Schee, and G. J. Fleer, J. Colloid Interface Sci. 104
(1985) 540.
99
F. T. Hesselink, J. Colloid Interface Sci. 60 (1977) 448.
100
G. Durand, F. Lafuma, and R. Audebert, Prog. Colloid Polym. Sci. 76 (1988) 278.
101
T. K. Wang and R. Audebert, J. Colloid Interface Sci. 121 (1988) 32.
102
P. E. Froehling and A. Bantjes, J. Colloid Interface Sci. 62 (1977) 35.
103
E. R. Hendrickson and R. D. Neuman, J. Colloid Interface Sci. 110 (1986) 243.
104
R. H. Pelton, J. Colloid Interface Sci. 111 (1988) 475.
105
R. J. Davies, L. R. Dix, and C. Toprakcioglu, J. Colloid Interface Sci. 129 (1989)
145.
106
M. Muthukumar, J. Chem. Phys. 86 (1987) 7230.
107
F. W. Wiegel, J. Phys. A: Math. Gen. 10 (1977) 299.
108
T. –C. Wen, C. Sivakumar, and A. Gopalan, Spectrochem. Acta, Part A 58 (2002)
167.
109
M. Thanneermalai, T. Jeyaraman, C. Sivakumar, A. Gopalan, T. Vasudevan, and T.
C. Wen, Spectrochim. Acta, Part A 59 (2003) 1937.
110
S. Vogel and R. Holze, Electrochim. Acta 50 (2005) 1587.
111
M. J. Simone, W. R. Heineman, and G. P. Kreishman, Anal. Chem. 54 (1982)
2382.
112
S. P. Best, R. J. H. Clark, R. C. S. McQueen, and S. Joss, J. Am. Chem. Soc. 111
(1989) 548.
113
R. Holze, J. Solid State Electrochem. 8 (2004) 982.
114
E. S. Brandt, Anal. Chem. 57 (1985) 1276.
115
M. Fleischmann, P.J. Hendra, and A.J. McQuilan, J. Chem. Soc., Chem. Comm.
(1973) 80.
116
R. Holze, Recent Res. Devel. Electrochem., 6 (2003) 101.
117
M. Fleischmann, P. J. Hendra, and A. J. McQuilan, Chem. Phys. Lett. 26 (1974)
163.
118
D. L. Jeanmaire and R. P. Vanduyne, J. Electroanal. Chem. 84 (1977) 1.
152
References
119
M. G. Albrecht and J. A. Creighton, J. Am. Chem. Soc. 99 (1977) 5215.
120
M. G. Albrecht, J. F. Evans, and J.A. Creighton, Surf. Sci. 75 (1978) L777.
121
T. E. Furtak, J. Electroanal. Chem. Interfacial Eelectrochem. 150 (1983) 375.
122
J. Gersten and A. Nitzan, J. Chem. Phys. 73 (1980) 3023.
123
J. Gersten and A. Nitzan, J. Chem. Phys. 75 (1981) 1139.
124
M. Kerker, D. S. Wang, and H. Chew, Appl. Opt. 19 (1980) 3373.
125
S. S. Jha, J. R. Kirtley, and J. C. Tsang, Phys. Rev. B 22 (1980) 3973.
126
T. E. Furtak, G. Trott and B. H. Loo, Surf. Sci. 101 (1980) 374.
127
H. Seki, J. Chem. Phys. 76 (1982) 4412.
128
H. Yang Y. Liu, Z. Liu, Y. Yang, J. Jiang, Z. Zhang, G. Shen, and R. Yu, J. Phys.
Chem. B 109 (2005) 2739.
129
K. Kim, S. J. Lee, and K. L. Kim, J. Phys. Chem. B 108 (2004) 16208.
130
P. Cao, R. Gu, and Z. Tian, Langmuir 18 (2002) 7609.
131
K. H. Yu, J. M. Rhee, Y. Lee, K. Lee, and S-. C. Yu, Langmuir 17 (2001) 52.
132
N. H. Jang, J. S. Suh, and M. Moskovits, J. Phys. Chem. B 101 (1997) 1649.
133
L. A. Sanchez, R. L. Birke, and J. R. Lombardi, J. Phys. Chem. 88 (1984) 1762.
134
J. Soto, D. J. Fernández, S. P. Centeno, I. L. Tocón, and J. C. Otero, Langmuir 18
(2002) 3100.
135
M. Kim and K. Itoh, J. Phys. Chem. 91 (1987) 126.
136
Y. H. Yim, K. Kim, and M. S. Kim, J. Phys. Chem. 94 (1990) 2552.
137
J. A. Crelghton, M. S. Alvarez, D. A. Weltz, S. Garoff, and M. W. Klm, J. Phys.
Chem. 87 (1983) 4793.
138
M. L. Patterson and M. J. Weaver, J. Phys. Chem. 89 (1985) 5046.
139
C. S. Allen and R. P. Van Duyne, J. Am. Chem. Soc. 103 (1981) 7497.
140
P. Gao, M. L. Patterson, M. A. Tadayyoni, and M. J. Weaver, Langmuir 1 (1985)
173.
141
S. Nie and S. R. Emory, Science 275 (1997) 1102.
142
K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari, and M. S.
Feld, Phys. Rev. Lett. 78 (1997) 1667.
143
M. J. Weaver, S. Zou, and H. Y. H. Chan, Anal. Chem. 72 (2000) 38A.
144
C. M. Hosten, R. L. Birke, and J. R. Lombardi, J. Phys. Chem. 96 (1992) 6585.
145
I. R. Nabiev, G. D. Chumanov, and E. A. Manykin, Fizika 28 (1985) 33.
146
A. Feofanov, A. Ianoul, V. Oleinikov, S. Gromov, O. Fedorova, M. Alfimov, and I.
Nabiev, J. Phys. Chem. 100 (1996) 2154.
153
References
147
T. Lu, T. M. Cotton, J. K. Hurst, and D. H. P. Thompson, J. Phys. Chem. 92 (1988)
6978.
148
T. Koyama, M. Yamaga, M. Kim, and K. Itoh, Inorg. Chem. 24 (1985) 4258.
149
S. A. Maskevich, G. T. Vasilyuk, I. R. Nabiev, I. F. Sveklo, L. N. Kivach, A. Feofanov, and M. Manfait, Proceedings of SPIE-The International Society for Optical
Engineering (Laser Chemistry, Biophysics, and Biomedicine) 2802 (1996) 124.
150
M. M. B. Pessôa and M. L. A. Temperini, J. Raman Spectrosc. 33 (2002) 50.
151
J. Zheng, Y. Zhou, X. Li, Y. Ji, T. Lu, and R. Gu, Langmuir 19 (2003) 632.
152
P. D. Enlow, M. Buncick, R. J. Warmack, and T. Vo-Dinh, Anal. Chem. 58 (1988)
1119.
153
C. Otto, F. F. M. de Mul, A. Huizinga, and J. Greve, J. Phys. Chem. 92 (1988)
1239.
154
A. A. Ooka and R. L. Garrell, Biopolymers (Biospectroscopy) 57 (2000) 92.
155
N. H. Jang, Bull. Korean Chem. Soc. 23 (2002) 1790.
156
E. J. Bjerneld, Z. Fö1ldes-Papp, M. Käll, and R. Rigler, J. Phys. Chem. B 106
(2002) 1213.
157
J. J. Laserna, A. Berthod, and J.D. Winefordner, Microchem. J. 38 (1988) 125.
158
S. R. Emory, W. E. Haskins, and S. Nie, J. Am. Chem. Soc. 120 (1998) 8009.
159
S. Jang, J. Park, S. Shin, C. Yoon, B. K. Choi, M. Gong, and S-. W. Joo, Langmuir
20 (2004) 1922.
160
P. Hohenberg and W. Kohn, Phys. Rev. 136 (1964) B864.
161
N. C. Handy, C. W. Murry, and R. D. Amos, J. Phys. Chem. 97 (1993) 4392.
162
G. Rauhut and P. Pulay, J. Phys. Chem. 99 (1995) 3093.
163
A. P. Scott and L. Radom, J. Phys. Chem. 100 (1996) 16502.
164
P. Bour, C. N. Tam, M. Shaharuzzaman, J. S. Chickos, and T. A. Keiderling, J.
Phys. Chem. 100 (1996) 15041.
165
P. J. Stephens, F. J. Devlin, C. F. Chabalowski, and M. J. Frisch, J. Phys. Chem. 98
(1994) 11623.
166
A. A. El-Azhary and H. U. Suter, J. Phys. Chem. 100 (1996) 15056.
167
C. S. Ashvar, F. J. Devlin, K. L. Bak, P. R. Taylor, and P. J. Stephens, J. Phys.
Chem. 100 (1996) 9262.
168
B. G. Johnson, P. M. W. Gill, and J. A. Pople, J. Chem. Phys. 98 (1993) 5612.
169
Y. Qin and R. A. Wheeler, J. Phys. Chem. 100 (1996) 10554.
154
References
170
R. A. King, J. M. Galbraith, and H. F. Schaefer III, J. Phys. Chem. 100 (1996)
6061.
171
J. R. Cheeseman, G. W. Truks, T. A. Keith, and M. J. Frisch, J. Chem. Phys. 104
(1996) 5497.
172
P. Winget, E. J. Weber, C. J. Cramer, and D. G. Truhlar, Phys. Chem. Chem. Phys.
2 (2000) 1231.
173
N. L. Allinger, L. R. Schmitz, I. Motoc, C. Bender, and J. K. Labanowski, J. Am.
Chem. Soc. 114 (1992) 2880.
174
J. M. Schulman and R. L. Disch, J. Am. Chem. Soc. 106 (1984) 1202.
175
S. R. Saraf, W. J. Rogers, M. S. Mannan, M. B. Hall, and L. M. Thomson, J. Phys.
Chem. A 107 (2003) 1077.
176
R. Notario, O. Castaño, J. L. M. Abboud, R. Gomperts, L. M. Frutos, and R.
Palmeiro, J. Org. Chem. 64 (1999) 9011.
177
R. L. Disch, J. M. Schulman, and R. C. Peck, J. Phys. Chem. 96 (1992) 3998.
178
P. C. Chen and Y. C. Chieh, J. Chin. Chem. Soc. 49 (2002) 791.
179
Z. Chen and T. Hamilton, J. Phys. Chem. 103 (1999) 11026.
180
M. J. S. Dewar and H. S. Rzepa. J. Am. Chem. A Soc. 100 (1978) 777.
181
M. J. S. Dewar and H. S. Rzepa. J. Am. Chem. Soc. 100 (1978) 784.
182
R. C. Bingham, M. J. S. Dewar, and D. H. Lo, J. Am. Chem. Soc. 97 (1975) 1285.
183
R. Holze and G. Fischer, Dechema-Monographie 117 (1989) 331.
184
M. J. Astle and W. V. McConnell, J. Am. Chem. Soc. 65 (1943) 35.
185
C. Ravichandran, D. Vasudevan, and P.N. Anantharaman, J. Appl. Electrochem. 22
(1992) 1192.
186
E. Jagganathan, M. Mohamed, K.A.B. Ahmed, and P.N. Anantharaman, J. Electrochem. Soc. India 37 (1988) 65.
187
E. Jagannathan, S. Chellammal, P. Tirunavukkarasu, and P. N. Anantharaman,
Trans. SAEST 25 (1990) 25.
188
M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, and J. J. P. Stewart, J. Am. Chem.
Soc. 107 (1985) 3902.
189
J. R. Smith, P. A. Cox, N. M. Ratcliffe, and S. A. Campbell, Trans. Inst. Met. Finish. 80 (2002) 52.
190
R. Holze, Surf. Sci. 202 (1988) L612.
191
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant, S.
155
References
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J.
Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S.
Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C.
Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J.
L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh, 2003.
192
A. P. Scott and L. Radom, J. Phys. Chem. 100 (1996) 16502.
193
R. Holze, Electrochim. Acta 35 (1990) 1037.
194
Z. J. Karpiński and Z. Kublik, Pol. J. Chem. 68 (1986) 269.
195
A. C. Testa and W. H. Reinmuth, J. Am. Chem. Soc. 83 (1961) 784.
196
J. Stradins and I. Kravis, J. Electroanal. Chem. 65 (1975) 635.
197
S. Bencheikh-Sayarh, P. Pouillen, A. -M. Martre, and P. Martinet, Electrochim.
Acta 28 (1983) 627.
198
S. Bencheikh-Sayarh, B. Cheminat, G. Mousset, and P. Pouillen, Electrochim. Acta
29 (1984) 1225.
199
see also: M. Mohammad, A. Y. Khan, M. Afzal, A. Nisa, and R. Ahmed, Aust. J.
Chem., 27 (1974) 2495.
200
D. H. Geske, J.I. Ragle, M.A. Bambenek, and A.I. Balch, J. Am. Chem. Soc. 86
(1964) 987.
201
R. D. Allendoerfer and P. H. Rieger, J. Am. Chem. Soc. 88 (1966) 3711.
202
M. E. Runner, J. Am. Chem. Soc. 74 (1952) 3567.
203
O. D. Shreve and E. C. Markham, J. Am. Chem. Soc. 71 (1949) 2993.
204
for an aquoeus tetrahydrofuran solution see: R. Parkash, R.K. Kalla, and R. S.
Verma, Proc. Indian Acad. Sci. 84A (1976) 64.
205
see also: A. I. Carastoian, F.G. Banica, and M. Moraru, Rev. Roum. Chim. 38
(1993 ) 287; 38 (1993) 615.
206
M. A. S. Ana, I. Chadwick, and G. Gonzalez, Bol. Soc. Chil. Quim. 27 (1982) 244.
207
M. A .S. Ana, I. Chadwick, and G. Gonzalez, J. Chem. Soc. Perkin Trans. II (1985)
1755.
208
D. Stočesová, Collect. Czech. Chem. Commun. 14 (1949) 615.
209
S. Wawzonek and T.W. Mclntyre, J. Electrochem. Soc. 114 (1967) 1025.
156
References
210
W. Shenglong, W. Fosong, and G. Xiaohui, Synth. Met. 16 (1986) 99.
211
L. Yang, X. Guang-Zhi, S. Jing-Gui, G. Jin-Liang and T. You-Qi, Acta Chim. Sin.
(Eng. Ed.) 4 (1989) 365.
212
E. M. Genies and M. Lapkowski, J. Electroanal. Chem. 236 (1987) 189.
213
J. Bacon and R.N. Adams, J. Am. Chem. Soc. 90 (1968) 6596.
214
for a study of the respective polymers prepared via chemical polymerization see: B.
C. Roy, M.D. Gupta, and J.K. Ray, Macromolecules 28 (1995) 1727.
215
copolymers of aniline and nitroaniline have been prepared chemically: E.P.
Koval’chuk, S. Whittingham, O.M. Skolozdra, P.Y. Zavalij, I.Y. Zavaliy, O.V. Reshetnyak, and M. Seledets, Mater. Chem. Phys. 69 (2001) 154; E.P. Koval’chuk, S.
Whittingham, O.M. Skolozdra, P.Y. Zavalij, I.Y. Zavaliy, O.V. Reshetnyak, and J.
Blazejowski, Mater. Chem. Phys. 70 (2001) 38.
216
S. Ando and M. Ueda, Synth. Met. 129 (2002) 207.
217
M. Fréchette, M. Belletête, J. -Y. Bergeron, G. Durocher, and M. Leclerc, Macromol. Chem. Phys. 198 (1997) 1709.
218
W. R. Heineman, H. J. Wieck, and A. M. Yacynych, Anal. Chem. 52 (1980) 345.
219
C. Malitesta, F. Palmisano, L. Torsi, and P. G. Zambonin, Anal. Chem. 62 (1990)
2735.
220
B. -X. Ye, W.-M. Zhang, and X.-Y. Zhou, Chin. J. Chem. 15 (1997) 343.
221
J. Xu, X. Sun, B. Liu, and F. Xu, Anal. Sci. 17 (2001) i1363.
222
R. L. Hand and R. F. Nelson, J. Am. Chem. Soc. 96 (1974) 850.
223
L. H. Piette, P. Ludwig, and R. N. Adams, Anal. Chem. 34 (1962) 916.
224
A. Kitani, Y.-H. So, and L. L. Miller, J. Am. Chem. Soc. 103 (1981) 7636.
225
A. Kitani and L. L. Miller, J. Am. Chem. Soc. 103 (1981) 3595.
226
K. Ravichandran and R. P. Baldwin, Anal. Chem. 55 (1983) 1586.
227
A. Duca and D. Bejan, Rev. Roum. Chim. 36 (1991) 439.
228
D. -H. Jang, Y. -S. Yoo, and S. M. Oh, Bull. Korean Chem. Soc. 16 (1995) 392.
229
R. L. Bent, J. C. Dessloch, F. C. Duennebier, D. W. Fassett, D. B. Glass, T. H. James, D. B. Jullan, W. R. RubyIinnu, J. M. Snell, J. H. Sterner, J. R. Thirtle, P. W.
Vittum, and A. Weissberger, J. Am. Chem. Soc. 73 (1951) 3100.
230
A. Laschewsky and M. D. Ward, Polym. 32 (1991) 146.
231
V. Solís, T. Iwasita and M. C. Giordano, J. Electroanal. Chem. 73 (1976) 91.
232
M. D. Levi and M. Lapkowski, Electrochim. Acta 38 (1993) 271.
233
R. Holze, Vib. Spectrosc. 3 (1992) 255.
157
References
234
G. Schwarzenbacher, B. Evers, I. Schneider, A. d. Raadt, J. Besenhard, and R. Saf,
J. Mater. Chem. 12 (2002) 534.
235
B. J. Haupt, J. Ennis, and E. M. Sevick, Langmuir 15 (1999) 3886.
236
D. M. Mohilner, R. N. Adams, and W. J. Argersinger, J. Am. Chem. Soc. 84 (1962)
3618.
237
M. E. Vaschetto, B. A. Retamal, and A.P. Monkman, J. Mol. Str. 468 (1999) 209.
238
I. Roth, A. A. Jbarah, R. Holze, M. Friedrich, and S. Spange, Macromol. Rapid
Commun. 27 (2006) 193.
239
G. Varsányi, Assignments for Vibrational Spectra of Seven Hundred Benzene
Deravatives, Adam Hilger, London 1974.
240
F. R. Dollish, W. G. Fateley, and F. F. Bentley, Characteristic Raman Frequencies
of Organic Compounds, Wiley, New York 1974.
241
M. Muniz-Miranda, J. Raman Spectrosc. 28 (1997) 205.
242
X. Gao, J. P. Davies, and M. J. Weaver, J. Phys. Chem. 94 (1990) 6858.
243
T. Tanaka, A. Nakajima, A. Watanabe, T. Ohno, and Y. Ozaki, J. Mol. Struct. 661662 (2003) 437.
244
R. Holze, Electrochim. Acta 32 (1987) 1527.
245
R. Holze, J. Electroanal. Chem. 250 (1988) 143.
246
M. M. Szostak, J. Raman Spectrosc. 8 (1979) 43.
247
H. Boggetti, J.D. Anunziata, R. Cattana, and J.J. Silber, Spectrochim. Acta, 50A
(1994) 719.
248
D. Lin-Vien, N. B. Colthup, W. G. Fateley, and J. G. Grasselli, The Handbook of
Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic
Press, Inc, New York 1991.
249
M. Moskovits, J. Chem. Phys. 77 (1982) 4408.
250
K. Nakamoto, Infrared spectra of Inorganic and Coordination Compounds, 2nd. Ed
Wiley, New Yourk, p.150 (1970).
251
B. Pergolese, M. Muniz-Miranda, and A. Bigotto, J. Phys. Chem. B 108 (2004)
5698.
252
A. A. Jbarah, A. Ihle, K. Banert, and R. Holze, J. Raman Spectrosc., 37 (2006) 123.
253
M. P. Soriaga and A. T. Hubbard, J. Am. Chem. Soc. 104 (1982) 2735.
254
G. Socrates, Infrared Characteristic Group frequencies, John Wiley & Sons, Ltd.
(1980).
255
L. Li, D. Zhou, J. Zhang, and G. Xue, J. Phys. Chem. B 108 (2004) 5153.
158
References
256
G. Cardini and M. Muniz-Miranda, J. Phys. Chem. B 106 (2002) 6875.
257
C. Zuo and P. W. Jagodzinski, J. Phys. Chem. B 109 (2005) 1788.
258
N. Neto, M. Muniz-Miranda, and G. Sbrana, J. Phys. Chem. 100 (1996) 9911.
259
A. Michota, A. Kudelski, and J. Bukowska, Langmuir 16 (2000) 10236.
260
L. Rivas, A. Murza, S. Sánchez-Cortés, and J. V. García-Ramos, Vib. Spectrosc. 25
(2001) 19.
261
M. P. Somashekarappa, J. Keshavayya, and S. Sampath, Pure Appl. Chem. 74
(2002) 1609.
262
for a detailled discussion see: S. Kania and R. Holze: GDCh-Monographie 14 (J.
Russow, G. Sandstede, R. Staab, Eds.) GDCh, Frankfurt 1998, p. 115;
S. Kania and R. Holze, Surf. Sci. 408 (1998) 252.
263
T. Chakraborty, S. S. Khatri, and A. L. Verma, J. Chem. Phys. 84 (1986) 1018.
264
P. Cao, J. Yao, B. Ren, R. Gu, and Z. Tian, J. Phys. Chem. B 106 (2002) 10150.
265
X. Cao, J. Raman Spectrosc. 36 (2005) 250.
266
A. Michota, A. Kudelski, and J. Bukowska, J. Raman Spectrosc. 32 (2001) 345.
267
A. G. Brolo, M. Odziemkowski, and D. E, Irish, J. Raman Spectrosc. 29 (1998)
713.
268
D. P. Schweinsberg, S. E. Bottle, and V. Otieno-Alego, J. Appl. Electrochem. 27
(1997) 161.
269
A. G. Brolo, P. Germain, and G. Hager, J. Phys. Chem. B 106 (2002) 5982.
270
I. Voigt, F. Simon, K. Esthel, S. Spange, and M. Friedrich, Langmuir 17 (2001)
8355.
271
A. T. Hubbard, Electrochemical Society Spring Meeting, Los Angelels, May 7-12,
1989, Extended Abstract, No. 458.
272
P. H. Rieger, I. Bernal, W. H. Reinmuth, and G. K. Fraenkel, J. Am. Chem. Soc. 85
(1963) 683.
273
A. H. Maki and D. H. Geske, J. Am. Chem. Soc. 83 (1961) 1852.
159
Appendixes
Appendix 1
H14
H13
C6
C5
O9
H16
N8
N7
C4
C1
H15
C2
O10
C3
H12
H11
Optimized structure of p-nitroaniline at B3LYP/6-31G(d)
Internal coordinates of p-nitroaniline optimized by B3LYP/6-31G(d). Distances in Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
O(9)
O(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(4)
C(1)
N(7)
N(7)
C(2)
C(3)
C(5)
C(6)
N(8)
N(8)
Bond
Length
1.411
1.385
1.397
1.397
1.411
1.455
1.380
1.235
1.235
1.087
1.083
1.083
1.087
1.011
1.011
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(3)
C(2)
C(4)
C(4)
C(1)
C(2)
C(4)
C(1)
C(1)
C(1)
First Angle
120.660
119.510
120.903
118.757
119.548
120.600
117.920
117.920
119.499
121.260
119.230
119.499
117.354
117.354
Third
Atom
C(1)
C(2)
C(3)
C(5)
C(6)
C(3)
O(9)
C(3)
C(4)
C(6)
C(5)
C(2)
H(15)
Second
Angle
0.028
-0.059
0.000
119.549
120.600
-179.912
124.160
119.841
119.230
121.260
119.840
20.670
113.725
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-S
Pro-S
Dihedral
Pro-S
Pro-R
Pro-S
Pro-S
Pro-S
Dihedral
Pro-S
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of p-nitroaniline calculated by
B3LYP/6-31G(d).
µX
7.6362
αXX
138.994
µY
0.0000
αYY
90.464
µZ
0.1408
αZZ
29.048
160
Appendixes
Appendix 2
H16
H15
H13
N8
C4
C5
O9
H14
C6
C3
N7
C2
O10
C1
H12
H11
Optimized structure of o-nitroaniline at B3LYP/6-31G(d)
Internal coordinates of o-nitroaniline optimized by B3LYP/6-31G(d). Distances in Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
O(9)
O(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(3)
C(4)
N(7)
N(7)
C(1)
C(2)
C(5)
C(6)
N(8)
N(8)
Bond
Length
1.381
1.405
1.424
1.419
1.406
1.449
1.358
1.246
1.232
1.085
1.083
1.087
1.087
1.010
1.008
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(2)
C(3)
C(3)
C(3)
C(2)
C(1)
C(4)
C(1)
C(4)
C(4)
First Angle
120.840
121.212
116.354
118.858
116.946
124.132
118.952
118.621
120.388
121.607
117.997
119.939
118.387
119.244
Third
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(2)
O(9)
C(6)
C(3)
C(6)
C(5)
C(3)
H(15)
Second
Angle
0.195
-0.340
0.059
121.841
119.503
178.747
122.427
120.754
117.553
120.219
119.109
7.568
120.386
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-R
Pro-S
Dihedral
Pro-R
Pro-S
Pro-R
Pro-S
Pro-R
Dihedral
Pro-S
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of o-nitroaniline calculated by
B3LYP/6-31G(d).
µX
4.1468
αXX
113.885
µY
2.2727
αYY
106.274
µZ
0.3306
αZZ
28.992
161
Appendixes
Appendix 3
H14
H11
C1
C6
H13
O10
C5
C2
N7
C3
O9
C4
H16
N8
H12
H15
Optimized structure of m-nitroaniline at B3LYP/6-31G(d)
Internal coordinates of m-nitroaniline optimized by B3LYP/6-31G(d). Distances in Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
O(9)
O(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(2)
C(4)
N(7)
N(7)
C(1)
C(3)
C(5)
C(6)
N(8)
N(8)
Bond
Length
1.393
1.391
1.403
1.408
1.394
1.474
1.391
1.231
1.231
1.082
1.084
1.087
1.086
1.012
1.012
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(1)
C(3)
C(2)
C(2)
C(2)
C(2)
C(4)
C(1)
C(4)
C(4)
First Angle
123.088
119.148
118.502
117.321
118.797
120.604
117.850
117.736
120.207
119.371
119.262
119.585
115.743
115.804
Third
Atom
C(1)
C(2)
C(3)
C(3)
C(5)
C(1)
O(9)
C(6)
C(4)
C(6)
C(5)
C(3)
H(15)
Second
Angle
-0.082
0.105
0.020
118.114
120.837
-179.466
124.414
122.472
121.480
119.863
119.348
-23.178
112.209
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-S
Pro-R
Dihedral
Pro-S
Pro-R
Pro-R
Pro-R
Pro-R
Dihedral
Pro-R
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of m-nitroaniline calculated by
B3LYP/6-31G(d).
µX
5.5482
αXX
115.181
µY
0.3247
αYY
99.487
µZ
1.1989
αZZ
29.613
162
Appendixes
Appendix 4
H12
H11
C5
C6
H14
H16
N8
N7
C1
C4
H15
H13
C3
C2
H10
H9
Optimized structure of p-phenylenediamine at B3LYP/6-31G(d)
Internal coordinates of p-phenylenediamine optimized by B3LYP/6-31G(d). Distances in
Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
H(9)
H(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(4)
C(1)
C(2)
C(3)
C(5)
C(6)
N(7)
N(7)
N(8)
N(8)
Bond
Length
1.402
1.393
1.402
1.402
1.402
1.410
1.410
1.088
1.088
1.088
1.088
1.014
1.014
1.014
1.014
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(3)
C(2)
C(1)
C(2)
C(4)
C(1)
C(4)
C(4)
C(1)
C(1)
First Angle
121.111
121.111
117.778
117.778
121.069
121.070
119.536
119.351
119.536
119.537
113.311
113.311
113.311
113.311
Third
Atom
C(1)
C(2)
C(3)
C(5)
C(6)
C(3)
C(4)
C(6)
C(5)
C(3)
H(13)
C(2)
H(15)
Second
Angle
0.034
-0.034
-0.034
121.070
121.069
119.351
119.537
119.351
119.351
-28.827
109.628
-28.830
109.628
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-R
Pro-R
Pro-S
Pro-R
Pro-R
Pro-R
Dihedral
Pro-R
Dihedral
Pro-R
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of p-phenylenediamine calculated by B3LYP/6-31G(d).
µX
0.0000
αXX
111.371
µY
0.0000
αYY
82.071
µZ
0.0000
αZZ
28.348
163
Appendixes
Appendix 5
H11
H16
C5
H12
N8
C6
C4
H15
H13
H9
C3
C1
N7
C2
H14
H10
Optimized structure of o-phenylenediamine at B3LYP/6-31G(d)
Internal coordinates of o-phenylenediamine optimized by B3LYP/6-31G(d). Distances in
Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
H(9)
H(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(3)
C(4)
C(1)
C(2)
C(5)
C(6)
N(7)
N(7)
N(8)
N(8)
Bond
Length
1.398
1.396
1.414
1.396
1.393
1.411
1.411
1.086
1.089
1.089
1.086
1.018
1.015
1.018
1.015
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(2)
C(3)
C(2)
C(1)
C(4)
C(1)
C(3)
C(3)
C(4)
C(4)
First Angle
120.859
119.331
119.331
119.796
123.035
117.585
119.709
120.116
119.022
120.489
111.801
112.776
111.801
112.776
Third
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(3)
C(6)
C(5)
C(2)
H(13)
C(3)
H(15)
Second
Angle
-0.914
1.676
-0.635
117.585
123.035
120.489
119.022
120.116
119.709
-136.621
109.707
45.957
109.707
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-R
Pro-S
Pro-R
Pro-S
Pro-R
Pro-S
Dihedral
Pro-S
Dihedral
Pro-S
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of o-phenylenediamine calculated by B3LYP/6-31G(d).
µX
1.2359
αXX
29.145
µY
0.0000
αYY
88.474
µZ
0.0000
αZZ
99.601
164
Appendixes
Appendix 6
H12
H11
C6
H9
C5
C1
C2
H16
H14
C4
N8
H15
C3
N7
H10
H13
Optimized structure of m-phenylenediamine at B3LYP/6-31G(d)
Internal coordinates of m-phenylenediamine optimized by B3LYP/6-31G(d). Distances in
Å, angels in degrees.
Atom
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
N(7)
N(8)
H(9)
H(10)
H(11)
H(12)
H(13)
H(14)
H(15)
H(16)
Bond
Atom
C(1)
C(2)
C(3)
C(4)
C(1)
C(2)
C(4)
C(1)
C(3)
C(5)
C(6)
N(7)
N(7)
N(8)
N(8)
Bond
Length
1.405
1.402
1.402
1.405
1.393
1.401
1.401
1.087
1.089
1.087
1.087
1.013
1.013
1.013
1.013
Angle
Atom
C(1)
C(2)
C(3)
C(2)
C(1)
C(3)
C(2)
C(2)
C(4)
C(1)
C(2)
C(2)
C(4)
C(4)
First Angle
119.342
120.968
119.342
119.408
120.443
120.148
119.937
119.516
119.937
119.234
114.484
114.199
114.484
114.200
Third
Atom
C(1)
C(2)
C(3)
C(3)
C(5)
C(6)
C(4)
C(6)
C(5)
C(1)
H(13)
C(3)
H(15)
Second
Angle
-0.020
-0.020
0.048
120.148
120.443
120.654
119.516
120.654
119.234
155.292
110.788
-27.676
110.788
Angle
Type
Dihedral
Dihedral
Dihedral
Pro-S
Pro-R
Pro-R
Pro-S
Pro-R
Pro-S
Dihedral
Pro-R
Dihedral
Pro-R
Dipole moments (µ; Debye) and polarizabilites (α; Bohr3) of m-phenylenediamine calculated by B3LYP/6-31G(d).
µX
0.0000
αXX
104.664
µY
-1.1350
αYY
87.495
µZ
0.0000
αZZ
28.076
165
List of publications
List of publications
1
Water-soluble species and heavy metal contamination of the petroleum refinery
area, Jordan, K. A. Momani, Q. M. Jaradat, A. A. Q. Jbarah, A. A. Omari, and I. F.
Al-Momani, J. Environ. Monit. 4 (2002) 990.
2
Fractionation and sequential extraction of heavy metals in street dust of an industrial area, Jordan. K. A. Momani, Q. M. Jaradat, A. A. Q. Jbarah, I. F. Al-Momani,
Abhath Al-Yarmouk, Basic Sciences and Engineering 12(2B) (2003) 503.
3
Inorganic analysis of dust fall and office dust in an industrial area of Jordan, Q. M.
Jaradat, K. A. Momani, Jbarah, A. A. Q. Jbarah, and A. Massadeh,. Environ. Res.
96(2) (2004) 139.
4
A Comparative Spectroelectrochemical Study of the Redox Electrochemistry of Nitroanilines, A. A. Jbarah and R. Holz, J. Solid State Electrochem. 10 (2006) 360.
5
The Electrosorption of 1,2,3-Triazole on Gold as Studied with Surface Enhanced
Raman Spectroscopy, A. A. Jbarah, A. Ihle, K. Banert, and R. Holze, J. Raman
Spectrosc. 37 (2006) 123.
6
2-Nitro-1,4-diaminobenzene-Functionalized Poly(vinyl amine)s as Water-Soluble
UV-Vis-Sensitive pH Sensors, I. Roth, A. A. Jbarah, R. Holze, M. Friedrich, and S.
Spange, Macromol. Rapid Commun. 27 (2006) 193.
7
A
Spectroelectrochemical
Study
of
the
Electrosorption
of
4-
Isopropylsulfanylmethyl-1,2,3-Triazole on Gold, A. A. Jbarah, K. Banert, and R.
Holze, Vib. Spectrosc. (2006), submitted.
166
Selbständigkeitserklärung
Selbständigkeitserklärung
Hiermit erkläre ich an Eides statt, die vorliegende Arbeit selbständig und ohne unerlaubte
Hilfsmittel durchgeführt zu haben.
Chemnitz, den 14.06.2006
M.Sc. Abdel Aziz Jbarah
167
Curriculum Vitae
Abdel Aziz Qasem Mohammad Jbarah
Nationality:
Jordanian
Date and Place of Birth: 27.11.1973 in Kuwait
Education
1992-1993
Al-Qabisy Secondary School (accumulative average 82.4), Zarqa, Jordan.
1993-1997
Mu’tah University, Kerak, Jordan, Bachelor Degree of Science (chemistry,
accumulative average 73.3).
1997-1999
Mu’tah University, Kerak, Jordan, Master Degree of Science (analytical
chemistry, accumulative average 85.7). Master Thesis: supervisor Prof. Dr.
Qasem Jaradat and Dr. Kamal Momani, Title of Master Thesis: “Speciation
and Fractionation of Heavy Metal in Soil, Street Dust, Dust-Fall, and Plant
of Jordanian Refinery Complex”.
2002-2006
Ph. D. student, Chemnitz University of Technology, supervisor Prof. Dr.
Rudolf Holze.
Work Experience
1997-1999
Teacher and Research Assistant, Department of Chemistry, Mu’tah University, Kerak, Jordan.
2000-2002
Teaching Assistant, Hashimet University, Zarqa, Jordan.
168