Vibrational Spectroscopy 43 (2007) 274–283
www.elsevier.com/locate/vibspec
Simultaneous observation of reagent consumption and
product formation with the kinetics of benzaldehyde
and aniline reaction in FTIR liquid cell
Hilmi Namli *, Onur Turhan
Department of Chemistry, University of Balikesir, Soma Cad., 10100 Balikesir, Turkey
Received 21 December 2005; received in revised form 22 February 2006; accepted 27 February 2006
Available online 18 April 2006
Abstract
Monitoring of a reaction in FTIR liquid cell has complex spectral futures due to the presence of solvent, unreacted reagents; acid catalysis and
other additives. The current work was performed to study the kinetics of the reaction between benzaldehyde and aniline by simultaneous time
dependent observation in FTIR liquid cell.
Since the FTIR liquid cell used in this work was a closed system, background defining method allowed us to observe the changes in the
concentration of each component in the liquid cell at different side of absorbance or transmittance axes. The changes in concentration arising from
the reaction provide both the spectra and amount of the product and reagent, independently without having to isolate them from the reaction
mixture. Therefore, it would be possible to follow the amount of decreases in concentration of the reagent and conversely, increases in
concentration of the product. Consequently, using this method the reaction of benzaldehyde and aniline was found to go to equilibrium in
chloroform.
# 2006 Elsevier B.V. All rights reserved.
Keywords: In situ time dependent FTIR; Background defining; Simultaneous observation of imine formation; Opposing reaction
1. Introduction
Simultaneous observation of a reaction in the reacting media
is the main goal to obtain more information about the
interaction of the reagents and the structure of the product. Real
time monitoring can allow following a reaction by eliminating
the workup effects to the product.
Imination of carbonyl and amine is a widely studied
chemical reaction because of its relevance to biological process
[1]. It is decisive to follow whole the reaction process in detail.
By use of Fourier transform (FT) techniques it is quite practical
obtaining a spectrum, which allows having the spectra in the
liquid or gas phase, consecutively at different conditions [2] by
FTIR [3]–RAMAN [4] and also NMR [5] spectroscopy. Liquid
flow measurements were also conducted to observe all stages of
the reaction [6]. However, each method has some advantages
* Corresponding author.
E-mail address: [email protected] (H. Namli).
0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.vibspec.2006.02.010
and drawbacks over others according to the equipments,
limitations and measuring systems.
We have recently demonstrated a new method to observe the
whole imination reaction by eliminating the complex features
of the FTIR spectroscopy [7]. In this method, since the reaction
media was defined as background, the peaks related to reagents
and solvent (components at background scanning time) were
eliminated. This allowed us to ignore all the peaks at that time
and a zero base line representing the zero absorbance was
obtained as a FTIR spectrum. As a result, only the constituents
where their concentration changes with time in the solution,
will appear in the spectra as a very clear development of the
corresponding peak intensities towards upward and/or downward depending on their production or consumption. Defining a
background by this way can be easily applicable even to the
enzymatic [8] and bimolecular reactions which have large
molecular structures [9]. Hauser et al. [10] have employed the
FTIR difference spectra for the interpretation of amide bands
observed during protein reactions using isotopically labeled
bacteriorhodopsin.
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
FTIR can also be used to observe the intermediates [11] as
well as to determine the consumption of reagents and formation
of products which may give valuable information about the
mechanism of the reaction [12] by monitoring the organic
reaction simultaneously. For this purpose more complicated
and expensive methods [13] like fiber optic probe system
capillary glass tube and micro machined flow cell [14] have
been applied to investigate some organic reactions. These
methods usually would not be able to ignore the unreacted
reagents in the media and also due to appearances of peaks
belonging to all components in the spectra it was difficult to
follow the small changes in the reaction such as a lesser amount
of product formation or slight shift of the peak frequency.
However, we have proved here that it is possible to obtain the
simultaneous observation of benzaldehyde and aniline reaction
with relatively cheap and easily by defining the background at
the beginning or at any time of the reaction in the FTIR liquid
cell. The changes in peak intensities, which are related to the
either increase or decrease in concentration of components,
have been used to study the kinetics of the reaction.
2. Experimental methods and results
2.1. Application of the method: background defining
Perkin-Elmer Model BX 1600 instrument with a 0.015 mm
path length liquid CaF2 cell was used to collect the FTIR
spectra.
1. FTIR spectra of equivalent concentration of benzaldehyde
and aniline in chloroform were recorded as the initial
spectrum (labeled as Ho).
2. The calibrations curves were obtained by relating the peak
heights to the concentration of the constituents by two
different approaches: (a) direct dilution (conventional
method): chloroform scanned as background, (b) background defined: the initial concentration (labeled as Co) in
chloroform scanned as background (Ho = 0).
275
3. Benzaldehyde and aniline solutions were mixed and scanned
in the liquid cell.
4. The scanned spectra of the mixture were recorded as a
background for the latter real time scans.
5. FTIR spectra were collected sequentially in 3 min intervals
for the continuing reaction.
6. The FTIR spectra of the isolated product were also compared
with the product side of the real-time background defined
spectra.
7. The increasing and decreasing peaks were compared with
the reagents and product, respectively.
8. The extra peaks appeared in the spectra were attributed to the
possible intermediates or interactions.
Although it is common to use alcohols as solvent for imine
reaction, chloroform provides better spectrum. The benzaldehyde (Fig. 1a) and aniline (Fig. 1b) used as reagents and Nbenzylideneaniline (Fig. 1c) as product are good soluble and no
limitation at the C–H, O–H or N–H region.
Fig. 1 shows very clear FTIR spectra of reagents benzaldehyde (a), reaction mixture (b), and aniline (c) in the range of
1250–1800 cm1 in chloroforms in absorbance (A) mode.
From the spectra of reaction mixture (Fig. 1b) it is easy to
identify the aldehyde and amine functional groups in the
mixture. Characteristic benzaldehyde peak was appeared at
1702 cm1 for carbonyl, characteristic aniline aromatic
stretching peak was perceived at 1619 cm1, and amine C–N
peak was observed at 1499 cm1 (Fig. 1c).
Since the initial reaction mixture was accepted as background all the peaks appeared due to the reagents and the
solvents were eliminated and a zero absorbance line was
obtained.
A new style of FTIR spectra were obtained after the initial
scan was defined as background. First of all, the changes in
relative peak intensities were clearly observed, indicating that
the reaction still continuous in the cell. In the spectra, it has also
been realized that the relative absorbance increases (over zero
base line) or decreases (under zero base line) depending on the
Fig. 1. A 1250–1800 cm1 range FTIR spectra of benzaldehyde (a); background defined time of reaction mixture (b); aniline (c); (spectra were arbitrary displaced in
vertical for clarity).
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H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
Fig. 2. A 1250–1800 cm1 region of the time resolved FTIR spectra of the benzaldehyde and aniline reaction in chloroform (isolated product (a), in situ real time
scans of reaction mixture (b), aniline (c), benzaldehyde (d) reagents and product spectra were arbitrary moved for clarity).
concentration changes of the materials in the solution. For
instance, the most intense peaks at 1702 cm1 for the C O and
at 1499 cm1 for the C–N bonds are observed at exactly the
same position as benzaldehyde and aniline in chloroform and
their peak intensities are going down with time, while the
intensity of the peak at 1630 cm1 for C N is going up because
of the product imine (N-benzylideneaniline) formation
(Fig. 2b).
From the comparison of the peak intensities in the time
dependent FTIR spectra (2b) with the (2a), (2c) and (2d), it has
been found that the decreasing peaks where the peak positions are
exactly in coincidence with the reagents (2c), (2d) correspond to
the decrease in concentration of the reagents, while the
increasing peaks are related to the isolated products (2a).
It is also possible to estimate the position of the decreasing
peaks between 2700–3200 cm1 from the pure benzaldehyde
and aniline spectra using chloroform as a solvent (Fig. 3), Some
broad peaks in region between 3200–3800 cm1 could not be
related to any isolated product or reagents, however, since the
imination reaction runs over carbinolamine intermediates [15]
(Scheme 1), these broad peaks can be attributed to the COH and
NH stretching or water release as well as to the solvent and
solute interaction [16], like hydrogen bonding. Except these
broad peaks, we were able to identify the spectra of the product
Fig. 3. Background defined FTIR spectra of the conversion at 2700–3800 cm1 (A–H) region.
Scheme 1. Imine formation reaction of benzaldehyde and aniline.
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
277
Fig. 4. FTIR spectra of isolated product (a); background defined time dependent reaction monitoring of aniline and benzaldehyde (b); aniline (c); benzaldehyde (d);
in chloroform in absorbance mode 1250–4000 region (spectra of reagents and products were arbitrary displaced in vertical for clarity).
and reagents, separately without isolating them from the
reaction mixture (Fig. 4b).
In addition, it has been realized that possibility of defining
the background at any time of the reaction allowed us to extract
both already formed products and unreacted reagents in the
mixture. These manifest themselves in the spectra as a decrease
in peak intensities due to decreasing reaction rate. The end of
reaction can also be estimated by defining the background at a
time where the relative changes in peak intensity become
negligible.
Fig. 4 shows the comparison of the time dependent FTIR
spectra of the reaction mixture with the reagents, benzaldehyde,
aniline, and product (N-benzylideneaniline) in chloroform from
1200 to 4000 cm1.
Following both sides of zero base line of the absorbance
axes, it is possible to trace the formation of a product by
observing the positive side of the axes and reagent consumption
by examining the negative side of the axes starting from the
beginning of the reaction without interruptions of the
measuring system.
In Fig. 4 the most intense peaks observed at 1702, 1499 and
1579 cm1 for benzaldehyde, aniline and product N-benzylideneaniline, respectively, were used for further analysis.
For the comparison of the peaks heights, two peaks
corresponding to reagents (1702, 1499 cm1) were plotted
against the peak of product N-benzylideneaniline (1579 cm1).
As seen from the Fig. 5, since background defining technique
eliminates all the peaks of the components the initial
concentration of the reagents should be zero. Excellent
2.2. Studying the peak heights
The reaction of benzaldehyde and aniline was investigated
by detailed examination of the relative decrease or increase in
peak intensities of components in the FTIR spectra for a certain
time interval. As soon as (0.25 M) benzaldehyde and aniline
solutions were mixed together, it was placed into CaF2 cell and
then the mixture was directly scanned as background.
Afterwards, the FTIR spectra of the solution was scanned
for every 3 min intervals and recorded to study the peak heights.
When the reaction was started to become slower, the time
period was increased to 10 min and the solution was scanned
for three times in 10 min intervals. After 8 and 10 h the
scanning showed a negligible difference in the absorbance
indicating that the reaction has reached to the end.
Fig. 5. The peak heights of 1579 and 1499 vs. 1702 cm1 plots in 3 min
intervals for overall 90 min.
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H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
correlations were obtained for both peaks indicating that the
peak heights manipulations can be used for various calculations
such as concentrations of the compounds in the reaction
mixture.
Because the reagents were plotted against the product, it is
expected to observe negative values in the figure due to
consumption of the reagent aniline and benzaldehyde.
Although the slope of the two curves seem to give different
concentration of reagents at a time, as will be discussed later
after introducing the concept of the calibrations all the peak
heights are truly related to the same concentration changes in
the cell. Thus, calculating the concentration [X] at any time
would give directly the concentration of product and
subtracting this from the initial reactant concentration it is
possible to calculate the rest of the reactant concentration.
Fig. 6 shows these peak heights as a function of time. Here, it
should also be noted that since these data have not been
calibrated yet, they could only give a rough estimation about the
rate of the reaction. As seen in the Figure, the increasing or
decreasing rate of curves are not the same but related to the
peak intensities for the same concentration by means of the
most intense peak for C O, which has the most lowest values.
Scheme 2. The conventional and background defined concentration relation of
the reaction.
while the peak heights and their areas under each spectrum of
the reagent and product increases as to negative and positive
values, respectively, due to the increasing concentration
difference [DX] (Scheme 2). This result in a correlation of
peak heights to peak heights (Fig. 5) during the reaction in
which they exactly fit to each other. This indicates that the
concentration changes affect all the peaks in a constant amount.
Thus, following any peak heights or areas under the spectrum
give the concentration difference of any component in solution
by Lambert–Bear law.
2.3. Peak heights and concentration relation
In a conventional concentration relation of a reaction, the
initial concentration of reagents [Co] will decrease, while the
concentration of the product is expected to increase as the
reaction proceeds. At any time of the reaction, when the product
concentration increases to [X] value, the concentration of the
reagents will decrease to [Co – X] (Scheme 2).
Since the initial concentration of the reagents recorded as
background (Co = 0), only the changes in concentration
(DC = X) will appear with lesser absorbance in the FTIR
spectra. Thus, peaks at the negative side of the absorbance
spectra will belong to the reagents due to its consumption
during the reaction, while peaks at the positive side of
absorbance will correspond to the formation of products. It has
been observed that peak positions did not change in each scan,
Fig. 6. Time to peak highs plots of the benzaldehyde and aniline reaction.
2.4. Calibration of peak heights to concentration and
reaction kinetics
In order to calculate the reacted or unreacted concentration
of the reagents in the solution during the reaction two types of
calibration techniques were applied as follows:
(a) Direct dilution: A 0.5 ml of 0.250 M stock solutions in
chloroform of benzaldehyde have been taken to a test tube.
A part of this solution was scanned for the initial
concentration (Ho). Then, the solution was diluted by
adding 0.5 ml of chloroform and scanned for the first
diluted concentration. This process (dilution of the solution
by adding equal amount of chloroform and scanning for the
corresponding concentration) were repeated for ten times
sequentially until the difference in two consecutive peak
heights become negligible smaller). Fig. 7 shows the direct
dilution calibration for benzaldehyde.
(b) Background defined dilution: in this method, a 0.5 ml of
0.250 M stock solutions were firstly scanned as background
(Ho = 0) and 0.5 ml benzaldehyde solution were diluted
successively with a 0.5 ml of chloroform in each time and
scanned as similar to the direct dilution method. Fig. 8
shows the background defined dilution calibration for
benzaldehyde (a) and aniline (b) in absorbance mode. The
normal dilution calibrations were also performed for aniline
peak at 1492 cm1 (Fig. 9) to compare peak heights with
the background defined method. Thus, this method can, in
general, be applied for any peaks appeared in the spectra.
As it is expected in the background defined dilution method
the more the dilution will results in the more negative
absorbance for reagents. This negativeness is actually due to the
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
279
Fig. 7. Direct dilution of benzaldehyde in absorbance mode (a 0.5 ml of 0.250 M solution was diluted by 0.5 ml chloroform addition in each curve).
difference in peak heights between initial (Ho) and diluted
solutions (h). These results indicate that the concentration of
the constituent can be calculated by in situ investigation of the
reaction (Ho = h + h0 ). The sum of peak heights corresponding
to the direct dilution and background defined dilution methods
at the same dilution concentration will exactly be the same with
the initial peak heights of the reagents. Fig. 10 clearly
illustrates the relation between the direct dilution and
background defined dilution methods for benzaldehyde in
the absorbance mode.
Fig. 8. Background defined dilution of (a) benzaldehyde (b) aniline in absorbance mode (a 0.5 ml of 0.250 M solution was diluted by 0.5 ml chloroform addition in
each curve, and the inlets show expanded region at about 1702 for (a) and 1499 cm1for (b)).
280
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
Fig. 9. FTIR direct dilution spectra of aniline (0.25 M + 0.5 ml CHCl3 addition in each line).
Fig. 10. FTIR peak heights relation of direct dilution and background dilution for benzaldehyde in 1280–1760 cm1 region.
Fig. 11 shows the calibration curves for benzaldehyde
peaked at 1702 cm1. The curve above the zero absorbance
base line was obtained from direct dilution method, and while
the calibration curve below the zero absorbance base line was
Fig. 11. Peak heights to concentration calibration plots for benzaldehyde at
1702 cm1.
obtained from background defined dilution method. As will
be noticed from the figure that the differences in peak heights
are becoming smaller as increasing the dilution concentration
and approaching the zero absorbance base line in the normal
direct dilution case, however, in contrary it is becoming larger
and approaching to Ho in the background defined dilution
method. However, the sum of absorption peaks in each
dilution concentration gives the 0.25 M of initial absorbance
of 0.86 values. This greatly shows us that it is possible to
calculate the reacted and unreacted species in any time of the
reaction by applying background defined dilution calibration
methods.
From the calibration figures, it is also possible to find the
molar absorption coefficient for the correspondence peak.
Then, the molar absorption coefficient can be used to calculate
in situ concentration of any components in the mixture during
the reaction. For instance, the molar absorption coefficient of
the C0 peak at 1702 cm1 can be extracted as about 3.4.
It should be noticed that the peak heights are not directly
related to the calculated solution concentration for the negative
absorbance, but the difference between initial and calculated
concentrations (h0 [X]), while peak heights for the positive
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
281
Fig. 13. Conversion of benzaldehyde to the N-benzylideneaniline in chloroform.
Fig. 12. The calibration plot for (a) benzaldehyde (1702 cm1 peak) and (b)
aniline (1499 cm1 peak).
absorbance are directly related to the reagent concentration in
the solution [Co X].
To verify the applicability of the calculated in situ
concentration, the same procedures can also be applied to
obtain the dilution ABS values for other reagents. The
calibration curves for benzaldehyde and aniline are shown in
Fig. 12.
The reacted [X] and unreacted [C] parts of the reagents were
calculated using the obtained calibration curve and measured in
situ FTIR peak heights of the reagents. Table 1 shows the
measured peak heights and calculated concentrations of the
reagents.
The concentration changes are becoming very small after
60 min. Even so, to approach the final point, where the
concentration changes become zero, the solution was scanned
for three times in 10 min interval and then, the FTIR scan was
taken after the solution was left for one hour. An important
point that should be realized in Table 1 the peak heights for
C O at 1702 as well as others (although it is not included in the
table) could not reach to the initial peak heights of the starting
material.
Fig. 13 shows the reaction for the conversion of the
benzaldehyde to the product N-benzylideneaniline. Although
the Ho was a value of 0.86 for CO peak, it has finished at a value
of 0.63 at the end of the reaction, indicating that the fully
conversion was not occurred due to the equilibrium.
In the earlier studies of this reaction using Raman
spectroscopy [17] in chloroform and NMR in deuterated
nitromethane [5] (CD3NO2) the second order reaction
kinetics were suggested. We have followed a similar study;
however, our observation showed that the reaction follows
the second order reaction kinetics for the first 60 min. Fig. 14
shows the 1/C as a function of time obtained by applying the
second order reaction kinetics. As is clearly seen in the inset
of the Fig. 14 the reaction fits the second order kinetics,
reasonably well, up to 60 min. However, beyond 60 min the
curve seem to be saturated and the reaction started to be far
from the second order reaction kinetics. This indicates that
the opposing reaction of imine formation should be taken
into account in order to fully understand the reaction
mechanism.
Fig. 14. Second order kinetics of the benzaldehyde and aniline reaction.
282
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
Table 1
In situ FTIR peak highs, calculated concentrations of benzaldehyde and aniline reaction in 3 min intervals
h at 1702 cm1
[C]
[X]
1/C
h
i
½Xð½Co 2Xe ÞþXe ½Co ln
½C ðXe XÞ
3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
48
51
54
57
60
63
66
69
72
75
78
81
84
87
90
0.0656
0.1414
0.2033
0.2552
0.2989
0.3363
0.3705
0.3991
0.4241
0.4459
0.4647
0.4817
0.497
0.5186
0.5299
0.5396
0.5482
0.5555
0.5627
0.5692
0.5751
0.5792
0.5833
0.5874
0.5907
0.5937
0.5963
0.599
0.6014
0.6039
0.2324
0.2114
0.1942
0.1797
0.1676
0.1572
0.1477
0.1397
0.1328
0.1267
0.1215
0.1168
0.1125
0.1065
0.1034
0.1007
0.0983
0.0963
0.0943
0.0925
0.0908
0.0897
0.0886
0.0874
0.0865
0.0857
0.0849
0.0842
0.0835
0.0828
0.0176
0.0386
0.0558
0.0703
0.0824
0.0928
0.1023
0.1103
0.1172
0.1233
0.1285
0.1332
0.1375
0.1435
0.1466
0.1493
0.1517
0.1537
0.1557
0.1575
0.1592
0.1603
0.1614
0.1626
0.1635
0.1643
0.1651
0.1658
0.1665
0.1672
4.3022
4.731
5.1502
5.5635
5.9667
6.3612
6.7706
7.1557
7.5301
7.8901
8.2294
8.5624
8.8859
9.3866
9.6718
9.9307
10.172
10.387
10.607
10.814
11.009
11.149
11.293
11.44
11.561
11.674
11.773
11.878
11.973
12.073
0.0655
0.1588
0.2506
0.3422
0.4329
0.5234
0.6196
0.7127
0.8061
0.8992
0.9904
1.0838
1.1789
1.3363
1.4328
1.5256
1.6175
1.7040
1.7988
1.8942
1.9907
2.0644
2.1447
2.2327
2.3102
2.3868
2.4587
2.5398
2.6182
2.7075
1020
0.6299
0.07559966
0.1744
13.228
Time (min)
o
Thus, we have included the opposing reaction equation
(Eq. (1)) into consideration and an excellent agreement
was obtained for the reaction in chloroform as seen in
Fig. 15:
Xð½Co 2Xe Þ þ Xe ½Co
ln
½Co ðXe XÞ
¼
ðk1 Þ2 ½Co ð½Co Xe Þ
t:
Xe
(1)
Fig. 15. Opposing reaction kinetic of the benzaldehyde and aniline.
3. Conclusion
Conventional methods for the observation of a reaction in the
liquid cell was performed by addition of one of the reagent to the
other or mixing the reagents by flow through the cell and
recording the changes [12,18]. However, liquid cell detection by
this way is inconvenient for continuous monitoring of a chemical
reaction since each time a sample must be taken for refilling or
the amount of the flowing liquids must be controlled for the
reagent solutions. Following each addition of components, the
changes are very difficult to analyze and also each time cleaning
process of the cell by passing trough solvent and refilling for the
next scan is necessary in order not to cause concentration
differences. The observation of the concentration changes in the
flow cell is not also possible due to the continuous change. The
background defining method can be overcome these difficulties
by adding all the reagents at once at the beginning and defining
the FTIR spectra as a background. Because the reactants should
be mixed first before the reaction can subsequently be followed
by FTIR spectroscopy. Some of the organic reactions are
however by far to fast for this procedure. The most important
advantage of our method is to allow us not to disturb the reacting
media until the end of the reaction.
In this work, the kinetic study of the imination reaction
between benzaldehyde and aniline was also studied. As a result,
H. Namli, O. Turhan / Vibrational Spectroscopy 43 (2007) 274–283
a series of in situ real time so called direct difference FTIR
spectrum were obtained for this reaction. The changes in peak
heights and thus concentration of the constituent in the CaF2
cell were investigated in a 3 min time interval sequences for
overall 90 min. The peak heights and concentration changes
showed that the reaction fit the second order kinetics up to
60 min with a good correlation. However, beyond 60 min the
curve seem to be saturated and the reaction started to be far
from the second order reaction kinetics. This indicates that the
opposing reaction of imine formation should be taken into
account in order to fully understand the reaction mechanism.
Acknowledgement
Authors are gratefully thanks to the Prof. Dr. Mahir ALKAN
and Assoc. Prof. Dr. Ali TEKE for their helpful discussion.
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