molecules
Article
Toward
Exploring Novel Organic Materials:
Article
MP4-DFT
Properties
of 4-Amino-3-Iminoindene
Toward Exploring
Novel
Organic Materials:
MP4-DFT Properties of 4-Amino-3-Iminoindene
Tareq Irshaidat
Tareq Irshaidat
Department
of Chemistry, College of Sciences, Al-Hussein Bin Talal University, P.O. Box 20, Ma’an, Jordan;
[email protected];
Tel.: +962-799-514-989
Department of Chemistry,
College of Sciences, Al-Hussein Bin Talal University, P. O. Box 20, Ma’an, Jordan;
[email protected];
+962-799-514-989
Academic
Editor: Luis R.Tel.:
Domingo
Received:
8 March
Accepted:
Academic
Editor:2017;
Luis R.
Domingo25 April 2017; Published: 30 April 2017
Received: 8 March 2017; Accepted: 25 April 2017; Published: 30 April 2017
Abstract: Tautomerism links with many applications and remains an attracting feature in exploring
novel
systems.
In this regard,
properties
of indene-based
HNCCCN
segments
haveinnot
received
Abstract:
Tautomerism
links with
many applications
and remains
an attracting
feature
exploring
novel
systems. In
this regard,
properties
of indene-based
havetonot
receivedthe
any
considerable
attention.
In this
computational
organicHNCCCN
chemistrysegments
study, first,
calculate
any considerable
attention.
In at
this
computational
organic
chemistry
study,anfirst,
to calculate
the
proton
transfer energy
barrier
a reasonable
cost,
the study
identified
accurate
forth order
proton transfer
energy barrier
at a reasonable
cost, the
study
identified protocol
an accurate
forth order
Møller–Plesset
perturbation
theory-density
functional
theory
(MP4-DFT)
equivalent
to the
Møller–Plesset
perturbation
theory-density
functional
theory (MP4-DFT)
protocol
to the of
outstanding
pioneering
benchmark
calculations.
The calculations
illustrate
that theequivalent
two tautomers
outstanding
pioneering
benchmark
calculations.
The
calculations
illustrate
that
the
two
tautomers
the 4-amino-3-iminoindene nucleus are separated by a considerable energy barrier while featuring
of the molecular
4-amino-3-iminoindene
nucleus are
separated
bydistribution,
a considerable
energy
barrier while
different
orbital characteristics;
frontier
orbital
λmax,
and energies,
which
featuring
different
molecular
orbital
characteristics;
frontier
orbital
distribution,
λmax,
and
are known basic requirements in molecular switching and logic circuit applications. The N-H/BH
2
energies, which are known basic requirements in molecular switching and logic circuit
substitution was found to have significant influence on the electronic structure of the skeleton.
applications. The N-H/BH2 substitution was found to have significant influence on the electronic
Similarities in the two tautomers and the boron derivative to properties of known molecular materials
structure of the skeleton. Similarities in the two tautomers and the boron derivative to properties of
have been found.
known molecular materials have been found.
Keywords: tautomerism; MP4(SDTQ); molecular switching; CAM-B3LYP; frontier orbitals;
Keywords: tautomerism; MP4(SDTQ); molecular switching; CAM-B3LYP; frontier orbitals;
semiconductor
semiconductor
1. Introduction
1. Introduction
Tautomerism is a fundamental concept in organic chemistry and an interesting phenomenon as it
Tautomerism is a fundamental concept in organic chemistry and an interesting phenomenon as
associates
with with
manymany
important
chemical
and biological
processes
[1]. When
tautomers
exhibitexhibit
different
it associates
important
chemical
and biological
processes
[1]. When
tautomers
physical
properties,
it
is
possible,
in
principle,
to
invest
this
in
molecular
switching
applications.
different physical properties, it is possible, in principle, to invest this in molecular switching
However,
in many
cases the
barrier
between
the tautomers
may
be low and,
thetherefore,
tautomers
applications.
However,
in many
cases
the barrier
between the
tautomers
maytherefore,
be low and,
exist
together
in
the
medium.
In
such
cases,
structure
modification
is
necessary
to
control
the tautomers exist together in the medium. In such cases, structure modification is necessary tothe
formation
of one
tautomer
a time
to recognize
thetoswitching
(on/offprocess
of the desired
control the
formation
of at
one
tautomer
at a time
recognizeprocess
the switching
(on/off physical
of the
properties)
(Figure 1)
[2]. Among
the promising
applications
is the molecular
computer
all of the
desired physical
properties)
(Figure
1) [2]. Among
the promising
applications
is theand
molecular
accompanying
of miniaturizing
the hardware.
computer andadvantages
all of the accompanying
advantages
of miniaturizing the hardware.
Structure-1
(Tautomer-1)
Stimuli-1
Stimuli-2
Physical
Property-1
Structure-2
(Tautomer-2)
Physical
Property-2
Examples on physical properties:
-molecular shape (which changes chemical reactivity),
-conductivity at the molecular level,
-color.
Figure1.1.AAsimplified
simplifiedillustration
illustration for the concept
Figure
concept of
of“molecular
“molecularswitching”.
switching”.
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Molecules 2017, 22, 720
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Switching based on tautomerization reactions has been recognized in many molecular systems [3–10].
Molecules 2017, 22, 720
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The first
recognized ideal tautomerism-based molecular switching process was identified
for the
porphyrins-category
of molecules
[11]. The
proton
from one
nitrogen
atomsystems
to another
Switching based
on tautomerization
reactions
hastransfers
been recognized
in many
molecular
can take
place
in
a
controllable
manner
by
applying
a
voltage
at
low
temperature
using
a
[3–10]. The first recognized ideal tautomerism-based molecular switching process was identifiedscanning
for
tunneling
microscope (STM).
between
the tautomers
causes
changeatom
in thetoconductivity
the porphyrins-category
of Switching
molecules [11].
The proton
transfers from
oneanitrogen
another
from can
onetake
levelplace
to another.
Later on,manner
another
the switching
between desired
in a controllable
byexample
applying of
a voltage
at low temperature
using atautomers
scanning was
tunneling
microscope
(STM). Switching
between
the tautomers
causes
a change
in the
conductivity
achieved,
leading
to a multi-control
of the
molecular
conductivity
[12].
Another
type
of promising
from one level tomolecular
another. Later
on, another
example
of the
switching
between
desired
tautomerswere
tautomerism-based
switching
is based
on the
quinone
core.
Nitrogen
derivatives
was
achieved,
leading
to
a
multi-control
of
the
molecular
conductivity
[12].
Another
type
of
studied on the surface of Cu(110) as a supporting platform. The study confirms that the
molecular
promising tautomerism-based molecular switching is based on the quinone core. Nitrogen
conductivity can be controlled in a similar way using the STM [13].
derivatives were studied on the surface of Cu(110) as a supporting platform. The study confirms that
The basic requirement to observe one dominant (main) tautomer is high activation energy for the
the molecular conductivity can be controlled in a similar way using the STM [13].
proton transfer
process.
In a previous
study
focused
on characterizing
(NCCCN
The basic
requirement
to observe
one [14]
dominant
(main)
tautomer is highmalonaldimine
activation energy
for
skeleton,
Figure
2A)
as
a
potential
core
for
asymmetrical
molecular
switching,
we
found
that
the
proton
the proton transfer process. In a previous study [14] focused on characterizing malonaldimine
transfer
energyskeleton,
barrier Figure
is, nearly,
order
increase the
selectivity
among we
asymmetrical
(NCCCN
2A) 9askcal/mol.
a potentialIn
core
for to
asymmetrical
molecular
switching,
found
tautomers
(containing
the NCCCN
core) is,
it is
necessary
to push
the activation
to a higher
that the
proton transfer
energy barrier
nearly,
9 kcal/mol.
In order
to increaseenergy
the selectivity
asymmetrical
(containing
NCCCN
core) it is can
necessary
to push the
activation
value.among
As can
be seen in tautomers
the previous
studiesthe
[3–10],
aromaticity
be employed
here
to generate
energy
to
a
higher
value.
As
can
be
seen
in
the
previous
studies
[3–10],
aromaticity
can
be
employed
asymmetrical tautomers with different electronic structures; one is aromatic and the other is not
here toAdditionally,
generate asymmetrical
tautomers
with different
electronic
structures; pushing
one is aromatic
andnitrogen
the
aromatic.
introducing
a skeletal
constraint
that guarantees
the two
other is not aromatic. Additionally, introducing a skeletal constraint that guarantees pushing the
atoms further apart from each other (compared to malonaldimine; N . . . N = 2.7088 Å) can participate
two nitrogen atoms further apart from each other (compared to malonaldimine; N…N = 2.7088 Å)
in making the proton transfer more difficult and, thus, energetically more expensive. After initial
can participate in making the proton transfer more difficult and, thus, energetically more expensive.
evaluations,
we evaluations,
found that we
indene
is askeleton
reasonable
candidate.candidate.
Attaching
amine amine
and imine
After initial
foundskeleton
that indene
is a reasonable
Attaching
groups
in
a
way
that
guarantees
the
transfer
of
protons
and
the
subsequent
formation
of
two
tautomers
and imine groups in a way that guarantees the transfer of protons and the subsequent formation
of
is possible
in
many
ways.
two tautomers is possible in many ways.
H
NH2
NH
HN
NH
(A)
Malonaldimine and its transition state
H
OH
O
O
O
(B)
Malonaldehyde and its transition state
OH
OH
O
O
(C)
H2N
R
R= BH2, CN, NO2
NH2
R
The six malonaldehyde derivatives
(D)
NH2
1-ABA
NH
NH
NH2
2-ABA
Figure
2. Structuresofofmalonaldimine
malonaldimine (A);
and
its its
derivatives
(C); (C);
and and
Figure
2. Structures
(A); malonaldehyde
malonaldehyde(B)(B)
and
derivatives
2-aminobenzaldimine
tautomers
(D).
2-aminobenzaldimine tautomers (D).
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Choosing 4-amino-3-iminoindene was inspired from astonishing contemporary syntheses.
Choosing
4-amino-3-iminoindene
was inspired
from astonishing
syntheses.
Recently,
the indispensable
review by Gabriele
and co-workers
presented contemporary
many new methods
for
Recently,
the
indispensable
review
by
Gabriele
and
co-workers
presented
many
new
methods
for
synthesis
of
indene
derivatives
[15].
Among
them,
Tsukamoto
and
co-workers
[16]
showed
that
it
is
Choosing 4-amino-3-iminoindene was inspired from astonishing contemporary syntheses.
synthesis
of
indene
derivatives
[15].
Among
them,
Tsukamoto
and
co-workers
[16]
showed
that
it
is
possible
to
produce
3-aminoindene
derivatives
starting
from
alkynylbenzaldehyde,
secondary
Recently, the indispensable review by Gabriele and co-workers presented many new methods for
possible
to produce
3-aminoindene derivatives starting from alkynylbenzaldehyde, secondary
amine,
R-B(OH)
2, and a palladium catalyst. On the other hand, Liu and co-workers [17] presented
synthesis
of indene
derivatives [15]. Among them, Tsukamoto and co-workers [16] showed that it is
amine,
R-B(OH)
2, and a palladium catalyst. On the other hand, Liu and co-workers [17] presented
another
on producing
similar
derivatives,
but used a different
strategyamine,
that
possible successful
to produce example
3-aminoindene
derivatives
starting
from alkynylbenzaldehyde,
secondary
another
successful
example
on
producing
similar
derivatives,
but
used
a
different
strategy
that
depends
thea coupling
iodobenzaldimine
and
alkynes
in the
presence[17]
of presented
zinc and aanother
cobalt
R-B(OH)2on
, and
palladiumofcatalyst.
On the other
hand,
Liu and
co-workers
depends
on
the
coupling
of
iodobenzaldimine
and
alkynes
in
the
presence
of
zinc
and
a
cobalt
catalyst.
Starting
with
a protected
amine
group onbut
theused
benzene
ring strategy
of the starting
material
successful
example
on producing
similar
derivatives,
a different
that depends
on
catalyst. Starting with
aiodobenzaldimine),
protected amine group
on by
thethe
benzene
ring of the
starting
material
(alkynylbenzaldehyde
or
followed
indene-forming
reaction,
oxidation,
the coupling of iodobenzaldimine and alkynes in the presence of zinc and a cobalt catalyst. Starting
(alkynylbenzaldehyde
iodobenzaldimine),
by
the indene-forming
reaction, oxidation,
then
of theor
amine
would followed
produce
derivatives
of amino-3-iminoindene.
The
with deprotection
a protected amine
group
ongroup
the benzene
ring
of the
starting material
(alkynylbenzaldehyde
then
deprotection
of
the
amine
group
would
produce
derivatives
of
amino-3-iminoindene.
The
simplest
form
of
the
4-amino-3-iminoindene
nucleus
was
adopted
for
this
study.
The
amine
group,
or iodobenzaldimine), followed by the indene-forming reaction, oxidation, then deprotection of
simplest
form
of
the
4-amino-3-iminoindene
nucleus
was
adopted
for
this
study.
The
amine
group,
being
on C4,
allows
this structure
exist in two
asymmetrical tautomeric
forms
(1-AIIform
and 2-AII;
the amine
group
would
produce to
derivatives
of amino-3-iminoindene.
The
simplest
of the
being
on
C4,
allows
this
structure
to
exist
in
two
asymmetrical
tautomeric
forms
(1-AII
and 2-AII;
Figures
3
and
4)
separated
by
the
transition
state
(TS-AII).
The
purpose
of
this
study
is
to
explore
the
4-amino-3-iminoindene nucleus was adopted for this study. The amine group, being on C4, allows this
Figures 3 structure
and 4) separated
byofthe
transition state (TS-AII).
Theremainder
purpose ofofthis
study
is
to
explore as
the
electronic
features
4-amino-3-iminoindene.
The
this
text
is
arranged
structure to exist in two asymmetrical tautomeric forms (1-AII and 2-AII; Figures 3 and 4) separated by
electronic
features of 4-amino-3-iminoindene.
The remainder of this
text is arranged
as
follows:
thestructure
computational
section,
calculate
the transition
state (TS-AII). details
The purpose
of selecting
this studya isreasonable
to explore MP4(SDTQ)
the electronicprotocol
structuretofeatures
of
follows:
the
computational
details
section,
selecting
a
reasonable
MP4(SDTQ)
protocol
to
calculate
the
proton transfer activation
energy section,
and isthe
frontierasorbitals
optical properties
4-amino-3-iminoindene.
The remainder
of this text
arranged
follows:and
thethe
computational
details
the proton transfer activation energy section, and the frontier orbitals and the optical properties
section.
section, selecting a reasonable MP4(SDTQ) protocol to calculate the proton transfer activation energy
section.
section, and the frontier orbitals and the optical properties section.
H2
H2
H1
N 1 H1 N H3
2
N1
N 2 H3
C4
C
C5 C4 C9 C3 3
C5
C9 C2
C2
C6
C8
C6 C7 C8 C1
C1
C(1-AII)
7
(1-AII)
H2
H2
H
N1 H1 N H3
N1 1 N2 H3
2
C4
C
3
C5 C4 C9 C3
C5
C9 C2
C2
C6
C8
C6 C7 C8 C1
C1
C7
(2-AII)
(2-AII)
H2
H2
H1
N1 H1 N H3
2
N1
N2 H3
C4
C
C5 C4 C9 C3 3
C5
C9 C2
C2
C6
C8
C6 C7 C8 C1
C1
C7
(TS-AII)
(TS-AII)
Figure
3. The
Thenumbering
numberingsystem
system
tautomers
(1-AII
and 2-AII)
and
the transition
state
Figure 3.
of of
thethe
twotwo
tautomers
(1-AII
and 2-AII)
and the
transition
state (TS-AII)
Figure of
3. (4-Amino-3-imino)indene.
The numbering system ofThe
theother
two tautomers
(1-AIIare
andomitted
2-AII) for
andsimplicity.
the transition state
(TS-AII)
hydrogen
atoms
of (4-Amino-3-imino)indene.
The other hydrogen
atoms are
omitted for
simplicity.
(TS-AII) of (4-Amino-3-imino)indene. The other hydrogen atoms are omitted for simplicity.
Figure
Figure 4.
4. The
The optimized
optimized structures
structures of
of the
the two
two tautomers
tautomers (1-AII
(1-AII and
and 2-AII)
2-AII) and
and the
the transition
transition state
state
Figure
4.
The
optimized
structures
of
the
two
tautomers
(1-AII
and
2-AII)
and
the
transition
state
(TS-AII)
(TS-AII) of
of (4-Amino-3-imino)indene.
(4-Amino-3-imino)indene.
(TS-AII) of (4-Amino-3-imino)indene.
2.
2. Computational
Computational Methods
Methods
2. Computational Methods
As
As recommended
recommended earlier
earlier [18],
[18], the
the Becke,
Becke, three-parameter, Lee-Yang-Parr exchange-correlation
exchange-correlation
As recommended
earlier
the
Becke,
three-parameter,
Lee-Yang-Parr
exchange-correlation
functional
B3LYP[19,20]
[19,20]
and
the
6-31G(d,p)
basis
setwere
[21]used
were
to optimize
all of the
functional
B3LYP
and
the[18],
6-31G(d,p)
basis
set [21]
to used
optimize
all
of the geometries.
functional
B3LYP
[19,20]
and
the
6-31G(d,p)
basis
set
[21]
were
used
to
optimize
the
geometries.
These
calculations
gave
real
frequencies
for
1-AII
and
2-AII,
which
indicates
that
is
These calculations gave real frequencies for 1-AII and 2-AII, which indicates that each all
iseach
aoftrue
geometries.
These
calculations
gave
real
frequencies
for
1-AII
and
2-AII,
which
indicates
that
each
aminimum.
true minimum.
thehand,
otherthe
hand,
the calculation
one imaginary
TS-AIIis
On theOn
other
calculation
gave onegave
imaginary
frequencyfrequency
for TS-AII for
associated
a
true
minimum.
On
the
other
hand,
the
calculation
gave
one
imaginary
frequency
for
TS-AII
associated
with theof
vibration
of the between
hydrogenthe
between
the twoatoms,
nitrogen
atoms,
whichthe
confirms
the
with the vibration
the hydrogen
two nitrogen
which
confirms
identity
of
associated
with
the
vibration
of
the
hydrogen
between
the
two
nitrogen
atoms,
which
confirms
the
identity
of state.
the transition
state.
The forth perturbation
order Møller–Plesset
perturbation theory;
the transition
The forth order
Møller–Plesset
theory; MP4(SDTQ)/6-31G(d,p)
[22],
identity ofwere
the performed
transition
forth
order
Møller–Plesset
perturbation
MP4(SDTQ)/6-31G(d,p)
[22], calculations
were
performed
to
obtain
single-point
energies.theory;
calculations
tostate.
obtainThe
single-point
energies.
The Gaussian03
(Revision
E The
0.1;
MP4(SDTQ)/6-31G(d,p)
[22],
calculations
performed
to obtain
single-point
energies.
The
Gaussian03
(Revision
E 0.1;
Gaussian,
Inc.:ofwere
Pittsburgh,
PA, used
USA)
of
programs
was used[23].
to
Gaussian,
Inc.:
Pittsburgh,
PA,
USA) suite
programs
was
tosuite
perform
these calculations
Gaussian03
(Revision
E
0.1;
Gaussian,
Inc.:
Pittsburgh,
PA,
USA)
suite
of
programs
was
used
The time-dependent (TD) calculations [24–26] were performed to estimate the vertical excitationto
Molecules 2017, 22, 720
4 of 11
energy using thecoulomb-attenuating method of the B3LYP functional CAM-B3LYP [27] along with
the 6-31G(d) basis set, as implemented in GAMESS suite of programs [28,29] (Gamess is maintained
by the members of the Gordon Research Group at Iowa State University, Ames, Iowa, USA).
3. Results and Discussion
3.1. MP4(SDTQ)/B3LYP Protocol to Calculate the Proton Transfer Activation Energy
The proton transfer activation energy of malonaldehyde (Figure 2B) was calculated using the
coupled-cluster with single and double excitations and an approximate treatment of triples at the
complete basis set; CCSD(T)/CBS, level of theory and found to be equal to 4.09 kcal/mol [30].
The availability of this value allows searching for less demanding alternative methods. Previously,
we found that both B3LYP/6-31++G(3df,3pd) and MP2/6-31++G(3df,3pd) protocols fail in predicting
the proton transfer activation energy of malonaldehyde [18]. Table 1 presents the results of various
basis set types along with the MP4(SDTQ) method. The reason behind avoiding larger basis sets is to
keep the calculations at as low a price as possible. Among all of the basis sets 6-31G(d,p) shows the
best performance (|∆∆Eb| = 0.02 kcal/mol) in terms of accuracy and the computation time.
2017, 22, 720
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TableMolecules
1. MP4(SDTQ)/(basis
set)//B3LYP/6-31G(d,p) calculations based on malonaldehyde5 transition
state. ∆Eb is the barrier height (in kcal/mol). |∆∆Eb| is the absolute difference with respect to the
2-R-Malonamide
reference value (4.09 kcal/mol).
FP-CCSD(T)
MP4(SDTQ)/6-31G(d,p)
Basis
Set |∆∆Eb|=∆Eb
(Average
0.02)
B3LYP/6-31G(d,p)
6-31G(d,p)
4.11
6-31++G(d,p)
(Average |∆∆Eb|=3.87
0.50)
6-311G(d,p)
3.81
1-AII → TS-AII
6-311++G(d,p)
3.64
MP4(SDTQ)/6-31G(d,p)
cc-pVDZ
3.37
CCSD(T)/D95(d,p)
QCISD(T)/D95(d,p)
B3LYP/6-31G(d,p)
0.60
0.58
|∆∆Eb|
(0.02)
0.21
0.02
0.22
(0.39)
0.28
0.45
22.41
0.72
22.26
1.40
1.40
Basis(0.00)
Set
0.43
0.46
|∆∆Eb|
(0.03)
∆Eb
0.61
D95(d,p)
3.70
EPR-II
3.05
(0.79)
SVP→ TS-AII3.83
2-AII
TZVP
3.32
MP4(SDTQ)/6-31G(d,p)
CCSD(T)/D95(d,p)
QCISD(T)/D95(d,p)
B3LYP/6-31G(d,p)
21.99
18.83
0.120.39
1.04
(0.31)
0.26
5.720.77
6.15
6.04
4.03
(FP)-CCSD(T) Energy
The MP4(SDTQ)/6-31G(d,p) level of theory value is equal to 4.11 kcal/mol, which is an excellent
estimation compared
theresults,
more demanding
CCSD(T)
[18] →
and
the quadratic
In light ofto
these
the energy barrier
of AII (1-AII
TS-AII,
Table 2) wasconfiguration
calculated usinginteraction
theand
MP4(SDTQ)/6-31G(d,p)//B3LYP
protocol.
As a consequence,
activation
of 1-AII [14],
(MP4 protocols.
with single
double excitations and
an approximation
ofthe
the
triples;energy
QCISD(T)
=
22.41
kcal/mol
compared
to
MP4
=
9.15
kcal/mol
in
malonaldimine)
indicates
that
the
Further evaluation of the MP4(SDTQ)/6-31G(d,p)//B3LYP/6-31G(d,p) protocol was performed
on
transformation from 1-AII to 2-AII is not spontaneous, yet kinetically possible. On the other hand,
six malonaldehyde derivatives (Figure 2C, Table 2) where the reference values are focal point (FP)
going in the opposite direction (2-AII to TS-AII) requires lower energy (5.72 kcal/mol) and the value
CCSD(T) indicates
energiesthat
[31].
The average
deviation
from(stimuli-1)
the accurate
point
values
(|∆∆Eb|) are
the absence
of the first
external stimuli
allows focal
retrieving
the first
tautomer
(1-AII)
by a smaller
energy
nature of stimuli-2
depends on
the operating
0.19 and 0.02
kcal/mol
for the
two (stimuli-2).
groups ofThe
malonaldehyde
derivatives.
Plotting
the MP4(SDTQ)
temperature.
A support to the energies
values of of
thethe
proton
activation
energies comes
the
energies against
the (FP)-CCSD(T)
six transfer
derivatives,
in addition
to thefrom
malonaldehyde
CCSD(T)/D95(d,p)//B3LYP/6-31G(d,p) and QCISD(T)/D95(d,p)//B3LYP/6-31G(d,p) protocols, as
value (Figure 5), can produce a simple linear relationship between the two with high precision.
recommended earlier [14,18]. In a previous work, we demonstrated that, for a specific molecular
Therefore,system,
the results
confirmisthat
MP4(SDTQ)
is in excellent
agreement
the accurate
tautomerism
a the
controllable
processprotocol
[32]. Therefore,
fine-tuning
of the with
barriers
(increasing/decreasing)
and
relative energies
of tautomers
may be
achieved by
suitable barrier of
FP-CCSD(T)
protocol and may
bethe
considered
a reasonable
protocol
to estimate
thea energy
the protonsubstituent.
transfer process.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
MP4(SDTQ) Energy
Figure 5. A
graph presenting a linear relationship between the MP4(SDTQ) and (FP)-CCSD(T) energies.
Figure 5. A graph presenting a linear relationship between the MP4(SDTQ) and (FP)-CCSD(T)
energies.
In order to obtain a qualitative idea about the contribution of the aromaticity and the N…N
distance in increasing the transition state energy the tautomerization reaction of
2-aminobenzaldimine (2-NH2-PhCH=NH, Figure 2; 1-ABA and 2-ABA) is compared, in addition to
malonaldimine. The N…N distance of 1-ABA is equal to 2.7122 Å, which is a negligible increase
Molecules 2017, 22, 720
5 of 11
Table 2. Performance of MP4(SDTQ)/6-31G(d,p)//B3LYP/6-31G(d,p) in estimating the proton transfer
barrier of six malonaldehyde derivatives. |∆∆Eb| refers to the absolute deviation of the calculated
value from the FP-CCSD(T) reference value.
2-R-Malonaldehyde
FP-CCSD(T)
MP4(SDTQ)/6-31G(d,p)
(Average |∆∆Eb|= 0.19)
B3LYP/6-31G(d,p)
(Average |∆∆Eb|= 1.32)
2-R-Malonamide
FP-CCSD(T)
MP4(SDTQ)/6-31G(d,p)
(Average |∆∆Eb|= 0.02)
B3LYP/6-31G(d,p)
(Average |∆∆Eb|= 0.50)
1-AII → TS-AII
MP4(SDTQ)/6-31G(d,p)
CCSD(T)/D95(d,p)
QCISD(T)/D95(d,p)
B3LYP/6-31G(d,p)
R = BH2
R = CN
R = NO2
∆Eb
2.62
2.75
(0.13)
1.55
(1.07)
∆Eb
3.56
3.74
(0.18)
2.06
(1.50)
∆Eb
3.34
3.59
(0.25)
1.96
(1.38)
0.60
0.58
(0.02)
0.21
(0.39)
1.40
1.40
(0.00)
0.61
(0.79)
2-AII → TS-AII
MP4(SDTQ)/6-31G(d,p)
CCSD(T)/D95(d,p)
QCISD(T)/D95(d,p)
B3LYP/6-31G(d,p)
0.43
0.46
(0.03)
0.12
(0.31)
22.41
22.26
21.99
18.83
5.72
6.15
6.04
4.03
In light of these results, the energy barrier of AII (1-AII → TS-AII, Table 2) was calculated
using the MP4(SDTQ)/6-31G(d,p)//B3LYP protocol. As a consequence, the activation energy
of 1-AII (MP4 = 22.41 kcal/mol compared to MP4 = 9.15 kcal/mol in malonaldimine) indicates
that the transformation from 1-AII to 2-AII is not spontaneous, yet kinetically possible. On the
other hand, going in the opposite direction (2-AII to TS-AII) requires lower energy (5.72 kcal/mol)
and the value indicates that the absence of the first external stimuli (stimuli-1) allows retrieving
the first tautomer (1-AII) by a smaller energy (stimuli-2). The nature of stimuli-2 depends on
the operating temperature. A support to the values of the proton transfer activation energies
comes from the CCSD(T)/D95(d,p)//B3LYP/6-31G(d,p) and QCISD(T)/D95(d,p)//B3LYP/6-31G(d,p)
protocols, as recommended earlier [14,18]. In a previous work, we demonstrated that, for a specific
molecular system, tautomerism is a controllable process [32]. Therefore, fine-tuning of the barriers
(increasing/decreasing) and the relative energies of tautomers may be achieved by a suitable substituent.
In order to obtain a qualitative idea about the contribution of the aromaticity and the N . . . N
distance in increasing the transition state energy the tautomerization reaction of 2-aminobenzaldimine
(2-NH2 -PhCH=NH, Figure 2; 1-ABA and 2-ABA) is compared, in addition to malonaldimine. The N
. . . N distance of 1-ABA is equal to 2.7122 Å, which is a negligible increase (0.0034 Å) compared
to that of malonaldimine. The transition state energy, with respect to 1-ABA, was calculated using
the MP4(SDTQ) protocol and found to be equal to 16.51 kcal/mol. This increase, with respect to
malonaldimine (7.36 kcal/mol), is attributed mainly to the loss of the aromatic stabilization energy.
On the other hand, the N . . . N distance in 1-AII is equal to 2.9326 Å; a 0.2238 Å increase with respect
to malonaldimine, and accompanied by an increase in the transition state energy to 22.41 kcal/mol.
This indicates 13.26 kcal/mol more than malonaldimine. As a rough estimation (1) 7.36 kcal/mol is
due to the loss of the benzene ring aromaticity in AII as a result of the tautomerization reaction; and
(2) 5.9 kcal/mol is due to the larger N . . . N distance (2.9326 Å in 1-AII). Therefore, these observations
indicate that, in addition to aromaticity, the N . . . N distance is an important factor in determining the
proton transfer energy barrier.
3.2. The Frontier Orbitals of the Two Tautomers
Converting one tautomer to another can be useful only if it is accompanied with a significant
change in a physical property. Focusing on the optical property of the tautomers, the initial evaluation
of the frontier orbitals—the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO)—illustrated that the HOMO-LUMO energy difference changes from 1-AII
Molecules 2017, 22, 720
6 of 11
to 2-AII. In order to obtain a more quantitative idea about the gaps the TD-DFT theoretical approach
was adopted to evaluate the first vertical π → π* excitation energies using the CAM-B3LYP functional
along with the 6-31G(d) basis set.
The first vertical π → π* excitation energies of 1-AII and 2-AII were calculated using the
TD-(CAM-B3LYP/6-31G(d))//B3LYP/6-31G(d,p) protocol (Table 3) in vacuum, acetonitrile, and
water. The results show that the dielectric constant of the medium has only a negligible effect on the
energies of the frontier orbitals. This is consistent with the small dipole moment values of the two
tautomers (MP4(SDTQ)/6-31G(d,p): 1-AII = 1.255, 2-AII = 2.509 Debye). The calculations in vacuum
illustrate that using a larger basis set than 6-31G(d,p) (compared with the triple-zeta TZVP basis set)
with the CAM-B3LYP functional is not necessary to obtain reasonable values.
Table 3. The estimated vertical π → π* excitation energies (eV) and wavelength (nm) of 1-AII and 2-AII
using the TD-(CAM-B3LYP/6-31G(d)) protocol in different media (ε is the dielectric constant). The
values in parentheses were obtained using the TZVP basis set.
1-AII
Medium
ε
Vacuum
1
Acetonitrile
Water
36.64
78.39
∆E
3.39
(3.37)
3.40
3.40
2-AII
λ
365
(368)
365
364
∆E
2.40
(2.39)
2.43
2.43
λ
518
(518)
510
510
|∆∆E|
0.99
(0.98)
0.97
0.97
∆λ
153
(150)
145
146
The ∆λ values (λ(2-AII)-λ(1-AII); Table 3) illustrate that the excitation energies of the two tautomers
differ by more than 140 nm and this difference varies slightly by the medium’s dielectric constant.
The ∆E values indicate that in 2-AII the valence electron density is less tightly bound to the nuclear
charges and can move or transfer easier than in 1-AII. Consequently, and at the molecular level,
the molecular conductivity of 2-AII is better. It is obvious (Table 3) that the two tautomers exhibit
valuable physical variations (∆∆E ≥ 0.97 (eV) and ∆λ > 140 (nm)) and differ significantly in their
relative energies and, as a consequence, supports that derivatives of AII are promising building blocks
for molecular switching and signaling applications.
Anthracene, tetracene, and pentacene are known organic semiconductors [33]. The estimated
vertical π → π* excitation energy based on TD-(CAM-B3LYP/6-31G(d)) calculations (Table 4) are
equal to 3.63 eV, 2.85 eV, and 2.37 eV, which are very close to the experimental values (3.60, 2.88,
and 2.37 eV, respectively) [34]. The data gives a clearer qualitative look at the difference in the
conductivity at the molecular level and indicate that both tautomers may conduct current. Taking
into consideration the frontier orbitals of the two tautomers (Figure 6; 1-AII and 2-AII), they distribute
over the entire molecular skeleton (Figure 6) which allows efficient inter-molecular π-π interaction.
Relatively, this supports that AII (1-AII ↔ 2-AII) is a new molecular semiconductor candidate.
Table 4. The TD-(CAM-B3LYP/6-31G(d)) frontier orbitals—the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)—HOMO-LUMO gap (eV), the
frontier orbitals energies (eV), and the first vertical excitation wavelength of AII tautomers and
selected polyacenes.
Compound
Energy (eV)
HOMO
LUMO
λ (nm)
1-AII
2-AII
AII-BH2
3.39
2.40
2.04
3.63
(experimental: 3.60)
2.85
(experimental: 2.88)
2.31
(experimental: 2.37)
–6.70
–6.07
–6.12
–0.65
–1.12
–1.44
365
518
609
–6.44
–0.51
342
–5.99
–1.03
435
–5.67
–1.40
537
Anthracene
Tetracene
Pentacene
Anthracene
Tetracene
Molecules 2017,Pentacene
22, 720
(experimental: 3.60)
2.85
(experimental: 2.88)
2.31
(experimental: 2.37)
–6.44
–0.51
342
–5.99
–1.03
435
–5.67
–1.40
537
7 of 11
Figure 6. The frontier orbitals—the highest occupied molecular orbital (HOMO) and the lowest
Figure
6. The
frontierorbital
orbitals—the
highest occupied
molecular
orbital (HOMO)
and the
lowest
unoccupied
molecular
(LUMO)—spreading
over the
entire molecular
skeleton allow
an efficient
unoccupied
molecular
orbital
(LUMO)—spreading
over
the
entire
molecular
skeleton
allow
an
π-π interaction and intra-molecular electrons flow in the two tautomerism states.
efficient π-π interaction and intra-molecular electrons flow in the two tautomerism states.
The literature shows that both tetracene [35] and pentacene [36] are efficient electron donors when
The literature shows that both tetracene [35] and pentacene [36] are efficient electron donors
coupled with C60 (as an acceptor) and can be used to fabricate organic solar cells. It is noticed (Table 4)
when coupled with C60 (as an acceptor) and can be used to fabricate organic solar cells. It is noticed
that the frontier orbitals’ energies of 1-AII are close to that of tetracene. The energy of the HOMO
(Table 4) that the frontier orbitals’ energies of 1-AII are close to that of tetracene. The energy of the
orbital (of 1-AII) may be increased by incorporating an electron donating group. However, relatively,
HOMO orbital (of 1-AII) may be increased by incorporating an electron donating group. However,
the energy of the HOMO orbital at this level, being lower than the HOMO orbital of tetracene, indicates
relatively, the energy of the HOMO orbital at this level, being lower than the HOMO orbital of
that it is more resistant to the environmental chemical factors and, therefore, gives it the advantage of
easier handling and processing.
3.3. Effect of N-H/BH2 Substitution
The advantage of the resonance-assisted hydrogen bond (RAHB) skeleton of 1-AII, compared to
other possible positional isomers, allows using it as a chelate by replacing the transferrable proton
with metal ions and some metalloids. For example, the boron derivative (BH2 -AII) of AII, was
optimized using the B3LYP/6-31G(d,p) method. The structure shows that boron is bonded to the
two nitrogen atoms and participates in a total of four bonds. For simplicity, the two hydrogen
atoms (of BH2 ) were proposed but they may be replaced by halogen, oxygen, or nitrogen ligands.
The TD-Cam-B3LYP/6-31G(d) calculation indicates that the first vertical HOMO-LUMO transition
of this derivative occurs at a higher wavelength (compared to 1-AII and 2-AII tautomers) equal to
609 nm (∆E = 2.036 eV). This is a significant redshift with respect to the two tautomers. Among the
other observations, the structure of BH2 -AII is planar and the frontier orbitals distribute over the entire
skeleton with a lower gap than the other two tautomers (Figure 7).
BH2) were proposed but they may be replaced by halogen, oxygen, or nitrogen ligands. The
TD-Cam-B3LYP/6-31G(d) calculation indicates that the first vertical HOMO-LUMO transition of this
derivative occurs at a higher wavelength (compared to 1-AII and 2-AII tautomers) equal to 609 nm
(∆E = 2.036 eV). This is a significant redshift with respect to the two tautomers. Among the other
observations, the structure of BH2-AII is planar and the frontier orbitals distribute over the entire
Molecules 2017, 22, 720
8 of 11
skeleton with a lower gap than the other two tautomers (Figure 7).
Figure 7. A pictorial representation of the frontier orbitals of BH2 -AII.
Figure 7. A pictorial representation of the frontier orbitals of BH2-AII.
4.
Conclusions
4. Conclusions
Being
of scientists
in the
Being inspired
inspired by
by the
the outstanding
outstanding pioneering
pioneering achievements
achievements of
scientists in
the various
various related
related
fields,
and
after
carrying
out
the
presented
calculations,
there
are
several
points
to
highlight:
fields, and after carrying out the presented calculations, there are several points to highlight:
1.
To calculate
calculatethe
theproton
protontransfer
transferenergy
energybarrier
barrierusing
using
high-level
MP4(SDTQ)
theory
1. To
thethe
high-level
MP4(SDTQ)
theory
to
to
search
many
basis
sets
identified
the
popular
6-31G(d,p)
basis
set
as
a
feasible
description
of
search many basis sets identified the popular 6-31G(d,p) basis set as a feasible description of atomic
atomic
(reasonably
balanced
between
accuracy
computationaldemands).
demands). Testing
Testing the
orbitalsorbitals
(reasonably
balanced
between
accuracy
andand
computational
the
MP4(SDTQ)/6-31G(d,p)//B3LYP/6-31G(d,p)
protocol
confirmedthat
thatitit can
can produce
produce satisfactory
MP4(SDTQ)/6-31G(d,p)//B3LYP/6-31G(d,p) protocol
confirmed
satisfactory
values
and FP-CCSD(T)
values compared
compared to
to the
the heavier
heavier reference
referenceCCSD(T)/CBS
CCSD(T)/CBS and
FP-CCSD(T) calculations
calculations and
and excellent
excellent
consistency
with
the
previously-recommended
more-demanding
protocols
(CCSD(T)
and
QCISD(T),
consistency with the previously-recommended more-demanding protocols (CCSD(T) and
along
with along
the D95(d,p)
basis
set). The
identified
MP4(SDTQ)
protocol can protocol
be used to
calculate
QCISD(T),
with the
D95(d,p)
basis
set). The
identified MP4(SDTQ)
can
be usedthe
to
proton
transfer
energy
barrier
for
a
large
number
of
known
and
unknown
molecules.
Additionally,
calculate the proton transfer energy barrier for a large number of known and unknown molecules.
it
can be takenitascan
a reference
evaluate
the performance
other
computational
methods.
Additionally,
be takento as
a reference
to evaluateofthe
performance
of other
computational
2.
In
addition
to
the
importance
of
aromaticity,
the
MP4(SDTQ)
calculations
demonstrated that
methods.
increasing
the distance
the of
two
nitrogen atoms
(in the HNCCCN
tautomerism
segment)
2. In addition
to thebetween
importance
aromaticity,
the MP4(SDTQ)
calculations
demonstrated
that
increases
value
of thebetween
activationthe
energy
of the proton
transfer
process
(N1 ↔tautomerism
H ↔ N2), as segment)
expected.
increasingthe
the
distance
two nitrogen
atoms
(in the
HNCCCN
The
MP4(SDTQ)
calculations
indicate energy
that theofbarrier
heighttransfer
of 4-amino-3-iminoindene
to
increases
the value
of the activation
the proton
process (N1 ↔ H is
↔equal
N2), as
22
kcal/mol
(compared
to 9 calculations
kcal/mol in malonaldimine).
value
means
that 1-AII is the dominant
expected.
The
MP4(SDTQ)
indicate that the This
barrier
height
of 4-amino-3-iminoindene
is
structure,
but
generating
the
second
tautomer
to
an
appreciable
amount
using
the that
suitable
equal to 22 kcal/mol (compared to 9 kcal/mol in malonaldimine). This value means
1-AIIenergy
is the
(stimuli-1)
is kinetically
Thethe
small
increase
in the N1
↔appreciable
N2 distanceamount
in 4-amino-3-iminoindene
dominant structure,
butpossible.
generating
second
tautomer
to an
using the suitable
(compared to malonaldimine) proves the theory that the N1 . . . H . . . N2 path length is a critical
geometrical parameter in controlling the tautomerism process.
3. Based on reference benchmark data, it appears that the TD-(CAM-B3LYP/6-31G(d)) calculation
is suitable for estimating the first valence vertical excitation energy of these systems. These calculations
illustrate that 2-AII is a better conductor than 1-AII at the molecular level, which fulfills the basic
requirement of molecular switching and signaling applications.
4. The effect of replacing the transferring proton by BH2 was examined. This substitution prevents
the tautomerism phenomenon from happening and leads to a decrease in the energy difference
between the frontier orbitals, which causes a significant redshift (244 nm) with respect to the more
stable tautomer. The resulting structure is fully conjugated and planar.
5. In addition to this, the HOMO-LUMO gap of 4-amino-3-iminoindene tautomers and a boron
derivative are similar to that of known molecular semiconductors (based on identical calculations).
In addition to these two features, the relative position of the frontier orbitals may qualify the skeleton
as an electron donor in conjunction with C60 or C70 receptors in organic solar cells.
difference between the frontier orbitals, which causes a significant redshift (244 nm) with respect to
the more stable tautomer. The resulting structure is fully conjugated and planar.
5. In addition to this, the HOMO-LUMO gap of 4-amino-3-iminoindene tautomers and a boron
derivative are similar to that of known molecular semiconductors (based on identical calculations).
Molecules
2017, 22,
11
In addition
to 720
these two features, the relative position of the frontier orbitals may qualify9 ofthe
skeleton as an electron donor in conjunction with C60 or C70 receptors in organic solar cells.
These results and insights encourage further investigations toward novel organic molecular
These results and insights encourage further investigations toward novel organic molecular
materials. Figure 8 shows suggested similar molecular skeletons that may be investigated
materials. Figure 8 shows suggested similar molecular skeletons that may be investigated theoretically
theoretically and experimentally. We are also interested in finding new factors that may affect
and experimentally. We are also interested in finding new factors that may affect tautomerism in
tautomerism in interesting structures.
interesting structures.
NHR1
NR2
R3
Y
X
X, Y: CR4, N, O, S
Figure 8. Suggested indene-like skeletons that may be suitable for molecular technology applications.
Figure 8. Suggested indene-like skeletons that may be suitable for molecular technology
applications.
Acknowledgments: Sincere thanks are due to James Herndon at New Mexico State University and Haobin Wang
at
the University of Colorado,
Denver,
We thank
Al-Hussein
Bin Talal
University
for the
Acknowledgments:
Sincere thanks
are for
dueprevious
to Jameshelp.
Herndon
at New
Mexico State
University
and Haobin
financial
through
number
78/2008.
Wang at support
the University
ofproject
Colorado,
Denver,
for previous help. We thank Al-Hussein Bin Talal University for
Conflicts
of Interest:
The author
declares
no conflict
of interest.
the financial
support through
project
number
78/2008.
Conflicts of Interest: The author declares no conflict of interest.
References
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Sample Availability: the cartisian coordinates of the optimized structures are available from the author.
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