Crystal structure, spectroscopy, and quantum chemical studies of (E)-2-[(2-chlorophenyl)iminomethyl]-4-trifluoromethoxyphenol

Article abstract of DOI:10.1021/jp205788b

The Schiff base compound (E)-2-[(2-chlorophenyl)iminomethyl]-4- trifluoromethoxyphenol has been synthesized and characterized by IR, UV-vis, and X-ray single-crystal determination. The molecular geometry from X-ray experiment in the ground state has been compared using the density functional theory (DFT) with the 6-311++G(d,p) basis set. The calculated results show that the DFT can well reproduce the structure of the title compound. Using the TD-DFT method, electronic absorption spectra of the title compound have been predicted, and a good agreement is determined with the experimental ones. To investigate the tautomeric stability, optimization calculations at the B3LYP/6-311++G(d,p) level were performed for the enol and keto forms of the title compound. Calculated results reveal that its enol form is more stable than its keto form. The predicted nonlinear optical properties of the title compound are much greater than those of urea. The changes of thermodynamic properties for the formation of the title compound with the temperature ranging from 200 to 500 K have been obtained using the statistical thermodynamic method. At 298.15 K, the change of Gibbs free energy for the formation reaction of the title compound is -824.841 kJ/mol. The title compound can spontaneously be produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is also computed as 3.85 × 10^-4 at 298.15 K for enol?keto tautomerization of the title compound. In addition, a molecular electrostatic potential map of the title compound was performed using the B3LYP/6-311++G(d,p) method.

Full text of DOI:10.1021/jp205788b

ARTICLE
pubs.acs.org/JPCA
Crystal Structure, Spectroscopy, and Quantum Chemical Studies of
(E)-2-[(2-Chlorophenyl)iminomethyl]-4-trifluoromethoxyphenol
Hasan Tanak*
Department of Physics, Faculty of Arts and Sciences, Amasya University, 05100 Amasya, Turkey
ABSTRACT: The Schiff base compound (E)-2-[(2-chlorophenyl)iminomethyl]-4-tri-
fluoromethoxyphenol has been synthesized and characterized by IR, UVꢀvis, and X-ray
single-crystal determination. The molecular geometry from X-ray experiment in the ground
state has been compared using the density functional theory (DFT) with the 6-311++G-
(d,p) basis set. The calculated results show that the DFT can well reproduce the structure of
the title compound. Using the TD-DFT method, electronic absorption spectra of the title
compound have been predicted, and a good agreement is determined with the experimental
ones. To investigate the tautomeric stability, optimization calculations at the B3LYP/6-311
++G(d,p) level were performed for the enol and keto forms of the title compound.
Calculated results reveal that its enol form is more stable than its keto form. The predicted
nonlinear optical properties of the title compound are much greater than those of urea. The changes of thermodynamic properties for the
formation of the title compound with the temperature ranging from 200 to 500 K have been obtained using the statistical thermodynamic
method. At 298.15 K, the change of Gibbs free energy for the formation reaction of the title compound is ꢀ824.841 kJ/mol. The title
compound can spontaneously be produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is also
computed as 3.85 ꢁ 10ꢀ4 at 298.15 K for enolTketo tautomerization of the title compound. In addition, a molecular electrostatic
potential map of the title compound was performed using the B3LYP/6-311++G(d,p) method.
1. INTRODUCTION
Azomethines (known as Schiff bases), having imine groups
(CdN) and benzene rings in the main chain alternately, and being
π-conjugated, peak interest as materials for a wide spectrum of
applications, particularly as corrosion inhibitors,1 catalyst carriers,2,3
thermo-stable materials,4ꢀ6 metal ion complexing agents,7 and in
biological systems.8ꢀ10 Schiff base compounds display interesting
photochromic and thermochromic features in the solid state and can
Figure 1. Tautomeric forms of the title compound.
be classified in terms of these properties.11 Photo- and thermo-
chromism arise via H-atom transfer from the hydroxy O atom to the
and dynamic image processing.27,28 Because of their high molecular
imine N atom.12,13 Such proton-exchanging materials can be utilized
hyperpolarizabilities, organic materials display a number of signifi-
for the design of various molecular electronic devices.14ꢀ16 In
cant nonlinear optical properties. NLO materials were categorized
general, o-hydroxy Schiff bases display two possible tautomeric
as multilayered semiconductor structures, molecular-based macro-
forms, the enol-imine (OH) and the keto-amine (NH) forms (see
scopic assemblies, and traditional inorganic solids. A variety of in-
Figure 1). Depending on the tautomers, two types of intramolecular
organic, organic, and organometallic molecular systems have been
hydrogen bonds are observed in Schiff bases: OꢀH
N in
studied for NLO activity.27,29
3 3 3
enol-imine17,18 and NꢀH
O in keto-amine19,20 tautomers.
3 3 3
Currently, fluoro-containing organic compounds play an impor-
tant role for the pharmaceutical as well as agrochemical industries,
because of their often unique biological properties, such as the
increased membrane permeability, enhanced hydrophobic binding,
and stability against metabolic oxidation.30ꢀ35 Among them, the
trifluoromethylated molecules are of great significance and attract
continuous attention from various fields.36ꢀ38
Different methods were used to show the presence of the enol-imine
and keto-amine forms, among them UVꢀvis, IR, MS, 1H, 13CNMR
spectroscopy, and X-ray crystallography techniques.21ꢀ24
Schiff base ligands consist of a variety of substituents with
different electron-donating and electron-withdrawing groups and
therefore may exhibit interesting electro-chemical properties. The
Schiff base compounds have been also under investigation during
past years because of their potential applicability in optical commu-
nications, and many of them have NLO behavior.25,26 NLO
materials have been attractive in recent years with respect to their
future potential applications in the field of optoelectronic such
as optical communication, optical computing, optical switching,
Received: June 20, 2011
Revised:
October 13, 2011
Published: October 13, 2011
r
2011 American Chemical Society
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Table 1. Crystallographic Data for the Title Compound
Scheme 1. Chemical Diagram of (E)-2-[(2-Chlorophenyl)-
iminomethyl]-4-trifluoromethoxyphenol
chemical formula
crystal shape/color
formula weight
crystal system
C14H9ClF3NO2
needle/light yellow
315.67
orthorhombic
Pbca
space group
unit cell parameters
a = 13.5727 (7) Å
b = 7.5342 (4) Å
c = 26.6045 (17) Å
2720.6 (3) Å3
8
volume
Z
In recent years, density functional theory (DFT) has been the
shooting star in theoretical modeling. The development of ever
better exchange-correlation functionals has made it possible to
calculate many molecular properties with accuracies comparable
to those of traditional correlated ab initio methods, at more
favorable computational costs.39 Literature surveys have
revealed the high degree of accuracy of DFT methods in
reproducing the experimental values in terms of geometry,
dipole moment, vibrational frequency, etc.40ꢀ43
The aim of this study is to investigate the energetic and
structural properties of the Schiff base compound, (E)-2-[(2-
chlorophenyl)iminomethyl]-4-trifluoromethoxyphenol (Scheme 1),
using density functional theory calculations. In this study, the
optimized geometry, vibrational spectra and assignments, statis-
tical thermodynamic parameters, electronic absorption spectra,
and nonlinear optical properties of the title compound have been
studied. These calculations are valuable for providing insight into
molecular properties of Schiff base compounds.
temperature
Dx (Mg cmꢀ3
296 K
)
1.541
μ (mmꢀ1
)
0.32
F000
1280
crystal size (mm3)
0.04 ꢁ 0.18 ꢁ 0.45
data collection
diffractometer/meas. meth
absorption correction
Tmin
STOE IPDS II/w-scan
integration
0.8924
Tmax
0.9894
no. of measured, independent, and
observed reflections
criterion for observed reflections
Rint
20 483, 2823, 1080
I > 2σ(I)
0.179
θmin, θmax
1.5°, 26.5°
refinement
refinement on
F2
R[F2 > 2σ (F2)], wR, S
no. of reflection
no. of parameters
weighting scheme
0.073, 0.069, 0.96
2823
2. COMPUTATIONAL AND EXPERIMENTAL METHODS
Physical Measurements. The melting point was determined
using a Gallenkamp melting point apparatus. The IR spectrum of
the title compound was recorded in the range 4000ꢀ400 cmꢀ1
using a Schmadzu FTIR-8900 spectrophotometer with KBr pellets.
Electronic absorption spectra were measured on a Unicam UVꢀvis
spectrophotometer in EtOH.
Synthesis. The compound (E)-2-[(2-chlorophenyl)imin
omethyl]-4-trifluoromethoxyphenol was prepared by reflux-
ing a mixture of a solution containing 2-hydroxy-5-(trifluoro
methoxy) benzaldehyde (0.027 g, 0.131 mmol) in 20 mL of
ethanol and a solution containing 2-chloroaniline (0.017 g,
0.131 mmol) in 20 mL of ethanol. The reaction mixture was
stirred for 1 h under reflux. The crystals of (E)-2-[(2-chloro
phenyl)iminomethyl]-4-trifluoromethoxyphenol suitable for
X-ray analysis were obtained from ethylalcohol by slow
evaporation (yield: 73%; mp, 368 K).
Crystal Structure Determination. A light yellow crystal of the
compound with dimensions of 0.04 ꢁ 0.18 ꢁ 0.45 mm was
mounted on a goniometer of a STOE IPDS II diffractometer.
Measurements were performed at room temperature (296 K)
using graphite monochromated Mo Kα radiation (λ = 0.71073
Å). The systematic absences and intensity symmetries indicated
the monoclinic Pbca space group. A total of 20 483 reflection
(1080 unique) within the θ range of [1.5° < θ < 26.5°] were
collected in the w scan mode. Cell parameters were determined
by using X-AREA software.44 The intensities collected were
corrected for Lorentz and polarization factors, absorption correc-
tion (μ = 0.32 mmꢀ1), by the integration method via X-RED32
software.44 The structure was solved by direct methods using
194
w = 1/[σ2(Fo
2
)
+ (0.006P)2]
2
P = (Fo2 + 2Fc )/3
(Δ/σ)max
0.001
ΔFmax, ΔFmin (e Åꢀ3
)
0.19, ꢀ0.15
SHELXS-97.45 The maximum peaks and deepest hole observed
in the final ΔF map were 0.19 and ꢀ0.15 e Åꢀ3, respectively. The
scattering factors were taken from SHELXL-97.45 The molecular
graphics were done using ORTEP-3 for Windows.46 All non-
hydrogen atoms were refined anisotropically. The refinement
was carried out using the full matrix least-squares method on the
positional and anisotropic temperature parameters of non-hy-
drogen atoms corresponding to 194 crystallographic parameters.
Atom H1 was located in a difference Fourier map and refined
isotropically, and the other H atoms were positioned geometri-
cally and treated using a riding model, fixing the bond lengths at
0.93 Å for aromatic CH. The displacement parameters of the H
atoms were fixed at Uiso(H) = 1.2Ueq(C) of their parent atoms.
Details of the data collection conditions and the parameters of
the refinement process are given in Table 1.
Computational Methods. The DFT calculations with a
hybrid functional B3LYP (Becke’s three-parameter hybrid func-
tional using the LYP correlation functional) at 6-311++G(d,p)
basis set by the Berny method47,48 were performed with the
Gaussian 03W software package.49 Geometry optimization was
started from the X-ray experimental atomic position. The
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molecular structure of the title compound in the ground state
was optimized. For all of the calculations in this study,
optimized structural parameters were used. The harmonic
vibrational frequencies were calculated at the same level of
theory for the optimized structure, and the obtained frequen-
cies were scaled by 0.96. Vibrational band assignments were
made using the Gauss-View molecular visualization program.50
The vibrational frequency calculations at the same level of
theory revealed no imaginary frequencies, indicating that an
optimal geometry at this level of approximation was found for
the title compound. The electronic absorption spectra were
calculated using the time-dependent density functional theory
(TD-DFT) method.51ꢀ54 Also, it is calculated in ethanol so-
lution using the Polarizable Continuum Model (PCM).55ꢀ58
To investigate the tautomeric stability, some properties such as
total energy, HOMO and LUMO energies, and the chemical
hardness59 for the OH and NH forms of the title compound
were obtained at the 6-311++G(d,p) level in the gas phase.
These properties were also examined for the title compound
in solvent media with three kinds of solvent (ε = 78.39, H2O;
ε = 24.55, C2H5OH; ε = 4.9, CHCl3) using the PCM method.
To investigate the reactive sites of the title compound, the
MEP were evaluated using the B3LYP/6-31++G(d,p) method.
MEP, V(r), at a given point r(x,y,z) in the vicinity of a molecule, is
defined in terms of the interaction energy between the electrical
charge generated from the molecule’s electrons and nuclei and a
positive test charge (a proton) located at r. For the system
studied, the V(r) values were calculated as described previously
using the equation:60
Figure 2. (a) Ortep-3 diagram of the title compound. Displacement
ellipsoids are drawn at the 30% probability level, and H atoms are shown
as small spheres of arbitrary radii. (b) The theoretical geometric
structure of the title compound (B3LYP/6-311++G(d,p) level).
Z
Fðr0Þ
Table 2. Hydrogen-Bond Geometry (Å, deg)
ZA
VðrÞ ¼
ꢀ
dr0
ð1Þ
∑
jRA ꢀ rj
jr0 ꢀ rj
D—H
A
D—H
H
A
D
A
D—H
A
A
3 3 3
3 3 3
3 3 3
3 3 3
where ZA is the charge of nucleus A, located at RA, F(r0) is the
electronic density function of the molecule, and r0 is the dummy
integration variable. The first hyperpolarizability of the title com-
pound was obtained by molecular polarizabilities based on theore-
tical calculations.
X-ray
O1ꢀH1 N1
B3LYP/6-311++G(d,p)
0.90 (3) 1.76 (4) 2.589 (4)
0.99 1.75 2.639
153 (4)
145.93
3 3 3
O1ꢀH1 N1
3 3 3
CCDC-810324 contains the supplementary crystallographic
data for the compound reported in this Article. These data can
be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK;
fax, +44(0)1223ꢀ336033; e-mail, deposit@ccdc.cam.ac.uk].
the title compound, the enol-imine form is favored over the keto-
amine form, as indicated by C9ꢀO1 [1.343(5) Å] and C7ꢀN1
[1.269(4) Å] bonds. The C9ꢀO1 and C7ꢀN1 bonds indicate
single-bond and a high degree of double-bond characters, respec-
tively. Similar results were observed for (E)-2-[(3-fluorophenyl)-
iminomethyl]-4-(trifluoromethoxy) phenol [CꢀO = 1.343(3) and
CꢀN = 1.269(5) Å].63
3. RESULTS AND DISCUSSION
The title compound is stabilized by OꢀH
N type hydro-
3 3 3
gen bond. There is a strong intramolecular O1ꢀH1
N1
Description of the Crystal Structure. The title compound,
an Ortep-3 view of which is shown in Figure 2a, crystallizes in the
orthorhombic space group Pbca with Z = 8 in the unit cell. The
asymmetric unit in the crystal structure contains only one
molecule. The dihedral angle between the aromatic ring systems
is 2.9(2)°. The imino group is coplanar with the hydroxyphenyl
ring, as shown by the C9ꢀC8ꢀC7ꢀN1 torsion angle of ꢀ
2.5(7)°. It is also known that Schiff bases may exhibit photochro-
mism depending on the planarity or nonplanarity, respectively.61
The C7ꢀN1 and C9ꢀO1 bonds of the title compound are the
most important indicators of the tautomeric type. The C9ꢀO1
bond is of a double bond for the keto-amine tautomer, whereas this
bond displays single bond character in the enol-imine tautomer. In
addition, the C7ꢀN1 bond is also a double bond in the enol-imine
tautomer and of single bond length in the keto-amine tautomer.62 In
3 3 3
hydrogen bond involving hydroxyl atom O1 and imine atom
N1, O1ꢀH1
N1, producing an S(6) ring motif64 resulting in
3 3 3
approximate planarity of the molecular skeleton [O
N =
3 3 3
2.589(4) Å]. The details of the hydrogen bond are summarized
in Table 2. In addition, there is also πꢀπ interaction between
the Cg2 rings in a neighboring molecule of 3.768(3) Å at (1/2 ꢀ x,
1
ꢀ /2 + y, z) [Cg2 is the ring centroid of the C8/C13 ring]. A
packing diagram of the title compound is shown in Figure 3.
Optimized Geometry. The optimized parameters (bond
lengths, bond angles, and dihedral angles) of the title compound
have been obtained using the B3LYP/6-311++G(d,p) method. The
atomic numbering scheme and the theoretical geometric structure of
the title compound are shown in Figure 2b. Calculated geometric
parameters are listed in Table 3 along with the experimental data.
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When the X-ray structure of the title compound is compared to its
optimized counterparts (see Figure 4), slight conformational dis-
crepancies are observed between them. The most remarkable
discrepancy exists in the orientation of the chlorophenyl ring in
the compound. The orientation of the chlorophenyl ring is defined
by the torsion angles C2ꢀC1ꢀN1ꢀC7 [179.8(4)°] and C6ꢀ
C1ꢀN1ꢀC7 [0.1(6)°]. These torsion angles have been calculated
at ꢀ142.30° and 40.99° for the B3LYP/6-311++G(d,p) level,
respectively. Another difference in the optimized structure is
observed in the relative orientation of the trifluoromethoxy group.
The methoxy group is almost perpendicular with the attached
ring with C11ꢀC12ꢀO2ꢀC14 and C13ꢀC12ꢀO2ꢀC14 tor-
sion angles of 95.0(5)° and ꢀ90.3(5)° for X-ray, while the corre-
sponding values are calculated as 95.18° and ꢀ89.12° for B3LYP,
respectively. According to X-ray studies, the dihedral angle between
the C1/C6 and C8/C13 rings is 2.9(2)°, whereas the dihedral angle
has been calculated at 41.24° for B3LYP. The geometry of hydrogen
bond in the optimized structure is examined, and it is seen that for
the O1ꢀH1 N1 hydrogen bond that exists between the phenol
3 3 3
O1 atom and imine N1 atom, DꢀH, H A, and DꢀH
A
3 3 3
3 3 3
values are 0.90(3) Å, 1.76(4) Å, and 153(4)° for X-ray and 0.99 Å,
1.75 Å, and 145.93° for B3LYP, respectively. The presence of the
H-bond appears as an important property of the molecule, stabilizing
its conformation in the crystal; as shown in the molecular modeling
part, this is also visible in the model obtained for the molecule
discussed.
A logical method for globally comparing the structures ob-
tained with the theoretical calculations is by superimposing the
molecular skeleton with that obtained from X-ray diffraction, giving
a RMSE of 0.161 Å for B3LYP/6-311++G(d,p) (Figure 4). This
RMSE value is quite large. This magnitude of RMSE can be
explained by the fact that the intermolecular Coulombic interaction
with the neighboring molecules is absent in gas phase, whereas the
experimental result corresponds to interacting molecules in the
crystal lattice.
IR Spectroscopy. Harmonic vibrational frequencies of the
title compound were calculated using the DFT/B3LYP method
with the 6-311++G(d,p) basis set. The vibrational band assign-
ments were made using the Gauss-View molecular visualization
program. To facilitate assignment of the observed peaks, we have
analyzed vibrational frequencies and compared our calculation
results to the experimental ones and showed in Table 4. The
agreement between the experimental and calculated frequencies
is quite good in general.
Figure 3. Packing diagram of the title compound.
The strong bands at 1392ꢀ1491 cmꢀ1 in the FT-IR spectra
correspond to in-plane bending vibration of the OH group.
The OH out-of-plane deformation vibration in phenols, in
general, lies in the region 517ꢀ710 cmꢀ1 67
.
The band at
829 cmꢀ1 in the FT-IR spectra corresponds to out-of-plane
bending mode of hydroxyl vibration. The calculated values of
OH group vibrations show good agreement with the experi-
mental results.
The aromatic CꢀH stretching, CꢀH in-plane bending, and
CꢀH out-of-plane bending vibrations appear in 2900ꢀ3150,
1100ꢀ1500, and 750ꢀ1000 cmꢀ1 frequency ranges, respec-
tively.68 The CꢀH aromatic stretching mode was observed at
3068 cmꢀ1 experimentally and was calculated at 3065 cmꢀ1. In
addition, the aliphatic CꢀH stretching mode was observed at
2922 cmꢀ1. The CꢀH in-plane bending vibrations assigned in
the region 1590ꢀ1150 cmꢀ1 are usually weak. The bands
observed at 965, 944, 889, 867, 757, and 737 cmꢀ1 in FT-IR
spectra are assigned to the CꢀH out-of-plane bending vibration
for the title compound.
The FT-IR spectrum of the title compound was recorded in the
4000ꢀ400 cmꢀ1 region using KBr pellets on a Schmadzu FT-IR
8900 spectrophotometer and is given in Figure 5. The spectra
contain some characteristic bands of the stretching vibrations of the
OꢀH, CꢀH, CdN, CꢀO, CꢀCl, and CꢀF groups. The OꢀH
group gives rise to three vibrations as stretching, in-plane bending,
and out-of-plane bending vibrations. The OH group vibrations are
likely to be most sensitive to the environment, so they show
pronounced shifts in the spectra of the hydrogen-bonded species.
The non-hydrogen-bonded or a free hydroxyl group absorbs
strongly in the 3550ꢀ3700 cmꢀ1 region.65 Intramolecular hydro-
gen bonding if present in the six-membered ring system would
reduce the OꢀH stretching band to the 3550ꢀ3200 cmꢀ1
region.66 The IR spectrum of the title compound shows a band
at 3422 cmꢀ1 due to OꢀH stretching vibration. This band has been
calculated at 3116 cmꢀ1 for B3LYP. Generally, the OH in-plane
The characteristic region of 1500ꢀ1700 cmꢀ1 can be used to
identify the proton transfer of Schiff bases. Azomethine (CdN)
bond stretching vibration was observed to be 1617 cmꢀ1 experi-
mentally, while it was calculated at 1602 cmꢀ1 for B3LYP. The
bands observed at 1617, 1590, 1571, and 1542 cmꢀ1, which can
be attributed to the CdC stretching vibrations, were calculated
at 1602, 1560, 1543, and 1538 cmꢀ1 for B3LYP, respectively.
Another characteristic region of the Schiff bases derivative
bending vibration in phenols lies in the region 1300ꢀ1400 cmꢀ1
.
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Table 3. Continued
Table 3. Selected Molecular Structure Parameters
parameters
experimental
B3LYP/6-311++G(d,p)
parameters
experimental
B3LYP/6-311++G(d,p)
C7ꢀN1ꢀC1
123.7 (3)
115.9 (4)
121.11
117.74
Bond Lengths (Å)
1.379 (4)
1.384 (5)
1.405 (4)
1.382 (5)
1.733 (3)
1.376 (5)
1.372 (5)
1.377 (5)
1.269 (4)
1.446 (5)
1.401 (5)
1.408 (5)
1.343 (5)
1.395 (5)
1.367 (6)
1.369 (6)
1.351 (5)
1.420 (5)
1.304 (5)
1.308 (6)
1.311 (6)
1.316 (6)
C14ꢀO2ꢀC12
C1ꢀC2
1.405
1.403
1.402
1.389
1.752
1.392
1.393
1.390
1.286
1.450
1.420
1.406
1.337
1.402
1.384
1.395
1.378
1.411
1.333
1.350
1.352
1.348
Torsion Angles (deg)
ꢀ0.3 (7)
C1ꢀC6
C1ꢀN1
C2ꢀC3
C2ꢀCl1
C3ꢀC4
C4ꢀC5
C5ꢀC6
C7ꢀN1
C7ꢀC8
C8ꢀC9
C8ꢀC13
C9ꢀO1
C9ꢀC10
C10ꢀC11
C11ꢀC12
C12ꢀC13
C12ꢀO2
C14ꢀF2
C14ꢀF1
C14ꢀF3
C14ꢀO2
C7ꢀC8ꢀC9ꢀO1
C8ꢀC7ꢀN1ꢀC1
N1ꢀC1ꢀC2ꢀCl1
F2ꢀC14ꢀO2ꢀC12
F1ꢀC14ꢀO2ꢀC12
F3ꢀC14ꢀO2ꢀC12
C11ꢀC12ꢀO2ꢀC14
C13ꢀC12ꢀO2ꢀC14
C2ꢀC1ꢀN1ꢀC7
C6ꢀC1ꢀN1ꢀC7
ꢀ0.29
ꢀ176.44
2.85
179.9 (4)
ꢀ1.5 (6)
ꢀ179.9 (4)
ꢀ59.4 (6)
60.3 (6)
ꢀ178.54
ꢀ58.81
62.06
95.0 (5)
95.18
ꢀ90.3 (5)
179.8 (4)
0.1 (6)
ꢀ89.12
ꢀ142.30
40.99
Bond Angles (deg)
118.1 (3)
116.2 (3)
125.7 (4)
121.7 (3)
120.7 (3)
117.6 (3)
119.2 (4)
119.8 (4)
120.7 (4)
120.4 (4)
122.0 (4)
118.2 (4)
121.0 (4)
120.9 (4)
118.5 (4)
121.9 (4)
119.7 (4)
120.1 (4)
120.4 (4)
121.0 (4)
120.2 (4)
118.6 (4)
120.7 (4)
107.4 (4)
107.2 (5)
104.7 (6)
109.6 (5)
114.0 (5)
113.4 (4)
C2ꢀC1ꢀC6
C2ꢀC1ꢀN1
C6ꢀC1ꢀN1
C1ꢀC2ꢀC3
C1ꢀC2ꢀCl1
C3ꢀC2ꢀCl1
C4ꢀC3ꢀC2
C5ꢀC4ꢀC3
C4ꢀC5ꢀC6
C5ꢀC6ꢀC1
N1ꢀC7ꢀC8
C9ꢀC8ꢀC13
C9ꢀC8ꢀC7
C13ꢀC8ꢀC7
O1ꢀC9ꢀC10
O1ꢀC9ꢀC8
C10ꢀC9ꢀC8
C11ꢀC10ꢀC9
C10ꢀC11ꢀC12
C13ꢀC12ꢀC11
C13ꢀC12ꢀO2
C11ꢀC12ꢀO2
C12ꢀC13ꢀC8
F2ꢀC14ꢀF1
F2ꢀC14ꢀF3
F1ꢀC14ꢀF3
F2ꢀC14ꢀO2
F1ꢀC14ꢀO2
F3ꢀC14ꢀO2
117.85
119.64
122.42
121.20
119.84
118.94
119.90
119.88
120.01
121.13
122.10
119.14
121.51
119.34
118.36
122.21
119.41
120.54
119.69
121.14
119.68
119.02
119.14
108.32
108.11
106.57
107.97
112.88
112.78
Figure 4. Atom-by-atom superimposition of the structures calculated
(blue) over the X-ray structure (black) for the title compound.
spectrum is 1100ꢀ1400 cmꢀ1, which is attributed to phenolic
CarꢀO stretching vibrations. The CarꢀO stretching vibration
was observed at 1293 cmꢀ1, which confirms the presence of
phenol group in the compound.
Three strong infrared bands at 1258, 1172, and 1150 cmꢀ1 are
assigned to the CꢀF stretching modes. According to the calcula-
tions, the first two bands are caused by symmetric CF3 stretching
modes, and the latter is due to the asymmetric CF3 stretching mode.
Furthermore, the medium bands at 573 and 468 cmꢀ1 in the IR
spectrum of the title compund are assigned to CF3 deformation
modes. For simple organic chlorine compounds, CꢀCl absorptions
are in the region 1129ꢀ480 cm
.
ꢀ1 69,70 In the FT-IR spectrum of the
title compound, a strong band at 684 cmꢀ1 is assigned to the CꢀCl
stretching vibration. The theoretical calculation by the B3LYP
method at 690 cmꢀ1 exactly correlates with experimental observa-
tion. The above conclusions are in good agreement with the similar
Shiff base compounds.71,72 The other vibrational frequencies can be
seen in Table 4.
As can be seen in Figure 1, the enol-imine form should have
phenol OꢀH, azomethine CdN, and CarꢀO bonds. The presence
of OꢀH, CdN, and CꢀO stretching vibrations in the FT-IR
spectrum of the title compound strongly suggests that the title
compound has the enol-imine form in the solid state.
Electronic Absorption Spectra. The UVꢀvisible spectrum
of ortho-hydroxylated Schiff bases that exist mainly as enol-imine
structure indicates the presence of a band at <400 nm, while
compounds that exist as keto-amine form show a new band,
especially in polar and nonpolar solvents at >400 nm.73,74
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Table 4. Comparison of the Experimental and Calculated
Vibrational Frequencies (cmꢀ1
)
experimental IR
with KBr
assignmentsa
calculated
ν(OꢀH)
3422
3068
2922
1617
1590
1571
1542
1491
1477
1444
1419
1392
1354
1293
1258
1203
1172
1150
965
3116
3065
2919
1602
1560
1543
1538
1457
1445
1424
1409
1374
1330
1265
1197
1174
1150
1084
963
νring(CꢀH) as
ν(CꢀH) aliphatic
ν(CdN) + ν(CdC)
ν(CdC) + γring(CꢀH)
ν(CdC) + γring(CꢀH)
ν(CdC) + γring(CꢀH)
γ(OH) + γring(CꢀH)
γring(CꢀH) + γ (CꢀH) aliphatic
γ(OH) + γring(CꢀH)
γring(CꢀH)
Figure 5. FT-IR spectrum of the title compound.
γ(OH) + γring(CꢀH) + γ(CH) aliphatic
γring(CꢀH) + γ(CH) aliphatic
ν(CꢀO) phenol + γring(CꢀH)
ν(CꢀF3) s + γring(CꢀH)
ν(CꢀOCF3) + γring(CꢀH)
ν(OꢀCF3) + γring(CꢀH) + ν(CꢀF3) s
γring(CꢀH) + ν(CꢀF3) as
ωring(CꢀH) + ω(CꢀH) aliphatic
δring(CꢀH)
944
931
ωring(CꢀH)
889
872
ωring(CꢀH)
867
864
ω (OꢀH)
829
800
ωring(CꢀH)
757
734
ωring(CꢀH)
737
715
ν(CꢀCl)
684
690
ω(FCF) β(CCC)
573
584
ω (FCF) + β(OCC)
468
478
Figure 6. UVꢀvis spectra of the title compound in ethanol solvent and
the comparison of calculated transitions in the gas phase and ethanol
solvent with the experimental spectra.
a ν, stretching; α, scissoring; γ, rocking; ω, wagging; δ, twisting; β,
bending; s, symmetric; as, asymmetric.
The electronic absorption spectra of the title compound in
ethanol solventwererecorded withinthe 200ꢀ600 nm range, and a
representative spectrum is shown in Figure 6. The experimentally
observed spectra showed three bands at 348, 272, and 226 nm.
These values are similar to those found in related Schiff base
compounds.71,75 The maximum absorption wavelength at 348 nm
is assigned to intramolecular charge transfer band of the azo-
methineCdN group, which corresponds to the enol-imineform of
the title compund. In this study, any band belonging to keto-amine
form was not observed with a value greater than 400 nm, which
indicates that the title compound is in the enol form and not in keto
in ethanol solvent.
Electronic absorption spectra of the title compound were
calculated by the TD-DFT method based on the B3LYP/6-311+
+G(d,p) level optimized structure in the gas phase. The calculated
results are listed in Table 5 along with the experimental absorption
spectra data. For TD-DFT calculations, the theoretical absorption
bands are predicted at 350, 272, and 227 nm, and it can easily be
seen that they correspond to the experimental absorption ones. In
addition to the calculations in the gas phase, TD-DFT calculations
of the title compound in ethanol solvent were performed by using
the PCM model. The PCM calculations reveal that the calculated
absorption bands have slight blue-shifts (see Figure 6) with values
of 349, 271, and 225 nm as compared to the gas-phase calculations
of the TD-DFT method.
According to the investigation on the frontier molecular orbital
(FMO) energy levels of the title compound, we can find that the
corresponding electronic transfers happened between the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO), HOMOꢀ3 and LUMO, HOMO and
LUMO+3 orbitals, respectively. Figure 7 shows the distributions
and energy levels of the FMOs computed at the B3LYP/6-311+
+G(d,p) level for the title compound. As was seen from Figure 7, in
the HOMOꢀ3 and LUMO, electrons are mainly delocalized on
the chlorobenzene ring, trifluoromethoxyphenol fragment, and the
atoms of imine group; in the HOMO and LUMO+3, electrons are
delocalized on the whole structure. Molecular orbital coefficients
analyses based on optimized geometry indicate that, for the title
compound, the frontier molecular orbitals are mainly composed of
p-atomic orbitals, so aforementioned electronic transitions are
mainly derived from the contribution of bands πfπ*.
Energetics and Stability. The NH and OHtautomerism of the
title compound is given in Figure 1. To investigate the tautomeric
stability, optimization calculations at B3LYP/6-311++G(d,p) level
were performed for the NH and OH forms of the title compound.
Some physicochemical properties such as total, HOMO, and
13870
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The Journal of Physical Chemistry A
ARTICLE
LUMO energies, dipole moment, and chemical hardness (η)
were also calculated with the same level of theory, and the results are
given in Table 6. The chemical hardness is quite useful to ration-
alize the chemical stability and reactivity of chemical species. Also,
the chemical hardness is quite useful to rationalize the relative stability
and reactivity of chemical species. Hard species having a large
HOMOꢀLUMO gap will be more stable and less reactive than
soft species having a small HOMOꢀLUMO gap.76 The hardness
value of a molecule is formulated by eq 2:77
OH form of the title compound is more stable than its NH form
in the gas phase.
To evaluate the solvent effect to the aforementioned proper-
ties of the title compound, calculations in three kinds of solvent
(ε = 78.39, H2O; ε = 24.55, C2H5OH; ε = 4.9, CHCl3) were
carried out with the B3LYP/6-311++G(d,p) level using the PCM
model, and the results are given in Table 6. From Table 6, we can
conclude that the total molecular energies obtained by PCM
method decrease with the increasing polarity of the solvent, while
the dipole moments and chemical hardness will increase with the
increase of the polarity of the solvent.
Molecular Electrostatic Potential. Molecular electrostatic
potential (MEP) is related to the electronic density and is a very
useful descriptor in understanding sites for electrophilic attack
and nucleophilic reactions as well as hydrogen-bonding inter-
actions.78 The electrostatic potential V(r) is also well suited for
analyzing processes based on the “recognition” of one mole-
cule by another, as in drugꢀreceptor, and enzymeꢀsubstrate
interactions, because it is through their potentials that the two
species first “see” each other.79,80
ð ꢀεHOMO þ εLUMO
Þ
η ¼
ð2Þ
2
where εHOMO and εLUMO are the energies of the HOMO and
LUMO orbitals. As seen from Table 6, the total energy of the OH
form is lower than the NH form, while chemical hardness of the
OH form is greater than the NH one, which indicates that the
Table 5. Experimental and Theoretical Electronic Absorp-
tion Spectra Values
To predict reactive sites for the electrophilic and nucleophilic
process for the studied molecule, MEP surface was obtained at
the B3LYP/6-311++G(d,p) optimized geometry. Following the
approach reported previously,60 we used those values of the MEP
that correspond to the surface determined from points with
electronic density F(r) = 0.001 au. The negative (red and yellow)
regions of MEP were related to electrophilic reactivity and the
positive (blue) ones to nucleophilic reactivity shown in Figure 8.
As can be seen in Figure 8, this molecule has several possible sites
for electrophilic attack. Negative regions in the studied molecule
were found around the phenol O1 atom, O2, Cl1, and fluor
atoms of the CF3 group. Also, a negative electrostatic potential
region is observed around the N1 atom. The negative V(r) values
are ꢀ0.041 au for O1, which is the most negative region, ꢀ0.025
au for O2, ꢀ0.015 au for Cl1, ꢀ0.016, ꢀ0.015, and ꢀ0.014 au for
F1, F2, and F3 atoms, respectively, and ꢀ0.006 au for N1 atom,
which is the least negative region. However, a maximum positive
region is localized on the C7ꢀH7 bond with a value of +0.044 au,
indicating a possible site for nucleophilic attack. According to
these calculated results, the MEP map shows that the negative
potential sites are on electronegative as well as the positive
potential sites are around the hydrogen atoms. These sites give
information about the region from where the compound can
have noncovalent interactions.
experimental
TD-DFT
TD-PCM
wavelength
wavelength oscillator wavelength
oscillator
strength
(nm)
abs
(nm)
strength
(nm)
348
272
226
0.258
0.278
0.480
350
272
227
0.351
0.271
0.084
349
271
225
0.487
0.282
0.081
Nonlinear Optical Effects. Nonlinear optical (NLO) effects
arise from the interactions of electromagnetic fields in various
media to produce new fields altered in phase, frequency, amplitude,
Figure 7. Molecular orbital surfaces and energy levels given in par-
entheses for the HOMOꢀ3, HOMOꢀ3, HOMO, LUMO, and LUMO
+3 of the title compound computed at the B3LYP/6-311++G(d,p) level.
Table 6. Calculated Energies, Dipole Moments, Frontier Orbital Energies, and Harnesses
gas phase (ε = 1)
chloroform (ε = 4.9)
ethanol (ε = 24.55)
water (ε = 78.39)
enol (OH) form
keto (NH) form
ETOTAL (hartree)
EHOMO (eV)
ELUMO (eV)
η (eV)
ꢀ1504.1621443
ꢀ6.626
ꢀ1504.1732592
ꢀ6.522
ꢀ1504.1777292
ꢀ6.500
ꢀ1504.1788921
ꢀ6.493
ꢀ2.545
ꢀ2.381
ꢀ2.346
ꢀ2.336
2.040
2.070
2.077
2.078
μ (D)
3.116
4.000
4.419
4.513
ETOTAL (hartree)
EHOMO (eV)
ELUMO (eV)
η (eV)
ꢀ1504.1538606
ꢀ6.108
ꢀ2.839
1.634
ꢀ1504.1685945
ꢀ6.065
ꢀ2.709
1.678
ꢀ1504.1747781
ꢀ6.076
ꢀ2.688
1.694
ꢀ1504.1765508
ꢀ6.076
ꢀ2.677
1.699
μ (D)
5.371
7.318
8.143
8.346
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The Journal of Physical Chemistry A
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providing the key functions of frequency shifting, optical modula-
tion, optical switching, optical logic, and optical memory for the
emerging technologies in areas such as telecommunications, signal
processing, and optical interconnections.82ꢀ85
The nonlinear optical response of an isolated molecule in an
electric field Ei(ω) can be presented as a Taylor series expansion
of the total dipole moment, μtot, induced by the field:
μtot ¼ μ0 þ αijEj þ βijkEjEk þ :::
ð3Þ
where α is the linear polarizability, μ0 is the permanent dipole
moment and βijk are the first hyperpolarizability tensor components.
The isotropic (or average) linear polarizability is defined as:86
αxx þ αyy þ αzz
αtot
¼
ð4Þ
3
First hyperpolarizability is a third rank tensor that can be
described by 3 ꢁ 3 ꢁ 3 matrix. The 27 components of 3D
matrix can be reduced to 10 components due to the Kleinman
symmetry87 (βxyy = βyxy = βyyx = βyyz = βyzy = βzyy; likewise
other permutations also take same value). The output from
Figure 8. Molecular electrostatic potential map calculated at the
B3LYP/6-311++G(d,p) level.
Gaussian 03 provides 10 components of this matrix as βxxx
,
βxxy, βxyy, βyyy, βxxz, βxyz, βyyz, βxzz, βyzz, and βzzz, respectively.
The components of the first hyperpolarizability can be calcu-
lated using the following equation:86
Table 7. Comparison of the Calculated βtot Values with the
Experimental βtot Values in Units of 10ꢀ30 esu
6-31
6-31
6-311
6-311
1
molecule experimentala 6-31G(d) +G(d) ++G(d,p) +G(d) ++G(d,p)
βi ¼ βiii
þ
ðβ þ βjij þ βjjiÞ
ð5Þ
∑
ijj
3
i¼j
urea
0.45
16
16.2
0.37
11.54
11.43
0.49
14.99
14.86
0.75
14.33
14.66
0.62
14.60
14.60
0.77
13.93
14.39
MNAb
PNAc
Using the x, y, and z components of β, the magnitude of the
first hyperpolarizability tensor can be calculated by:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
a Experimental data from refs 91 and 92. b 2-Methyl-4-nitroaniline.
c p-Nitroaniline.
βtot
¼
ðβ2x þ β2y þ β2zÞ
ð6Þ
or other propagation characteristics from the incident fields.81 NLO
is at the forefront of current research because of its importance in
The complete equation for calculating the magnitude of
β from Gaussian 03W output is given as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
βtot
¼
ðβxxx þ βxyy þ βxzzÞ þ ðβyyy þ βyzz þ βyxxÞ þ ðβzzz þ βzxx þ βzyy
Þ
ð7Þ
The calculations of the total molecular dipole moment (μ), linear
polarizability (α), and first-order hyperpolarizability (β) from the
Gaussian output have been explained in detail previously.88 It is well-
known that from the literature, the B3LYP approach provides fairly
reliable values in electric hyperpolarizability calculations when
compared to accuracies of traditional ab initio methods, and this
predictive capability of widely used B3LYP method is of interest to a
wide audience of computational scientists.89,90 To investigate the
effects of basis sets on the NLO properties, the βtot values of urea,
p-nitroaniline, and 2-methyl-4-nitroaniline were calculated by
the B3LYP method with 6-31G(d), 6-31+G(d), 6-31++G(d,p),
6-311+G(d), and 6-311++G(d,p) basis sets, and the obtained
results were compared to the experimental ones and given in
Table 7. From this table, we see that calculated values of the βtot
slightly depend on the size of basis sets. Obtained values of the
βtot with 6-31G(d) basis set are smaller than those obtained with
a large size of basis sets. It is found that the calculated results for
the basis sets from 6 to 31+G(d) to 6-311++G(d,p) have minor
differences from each other. The 6-31+G(d) basis set seems to be
adequate for reproducing the hyperpolarizabilities. Taking into
account reliability and the computational time required, the basis
set 6-31+G(d) was chosen for the calculations of the hyperpo-
larizabilities in this study.
The βtot of the title compound was calculated at the B3LYP/
6-31+G(d) level using the Gaussian 03W program package. The
calculated value of βtot is 3.985 ꢁ 10ꢀ30 esu, which is greater than
that of urea (the βtot of urea is 0.49 ꢁ 10ꢀ30 esu obtained by the
B3LYP/6-31+G(d) method). When it is compared to the similar
Schiff base compound in the literature, the calculated value of βtot
of the title compound is bigger than that of 3-[(1E)-N-ethyle-
thanimidoyl]-4-hydroxy-6-methyl-2H-pyran-2-one (βtot = 1.002 ꢁ
10ꢀ30 esu).29 These results indicate that the title compound is a
good candidate of NLO material.
To understand this phenomenon in the context of molecular
orbital theory, we examined the molecular HOMOs and mole-
cular LUMOs of the title compound. The HOMOꢀLUMO
energy gaps were calculated as 4.024 eV for the title compound,
4.666 eV for 3-[(1E)-N-ethylethanimidoyl]-4-hydroxy-6-meth-
yl-2H-pyran-2-one,29 and 6.891 eV for urea by the B3LYP/
6-31+G(d) method.
13872
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The Journal of Physical Chemistry A
ARTICLE
Table 8. Total Energies (kJ molꢀ1), Zero-Point Energies
(kJ molꢀ1), and Thermodynamic Properties of the Reactants
and Title Compound at Different Temperatures
Table 9. Enthalpy, Entropy, and Free Energy Changes of the
Formation Reaction of the Title Compound at the Different
Temperatures
a
temperature
(K)
C0p,m (J molꢀ1 S0m (J molꢀ1
Kꢀ1 Kꢀ1
temperature (K) ΔS (kJ molꢀ1 Kꢀ1
)
ΔH (kJ molꢀ1 b ΔG (kJ molꢀ1 c
) )
structure
H0m (kJ molꢀ1
)
)
)
200
ꢀ0.0172
ꢀ0.0138
ꢀ0.0116
ꢀ0.0115
ꢀ0.0098
ꢀ0.0087
ꢀ0.0078
ꢀ0.0070
ꢀ829.850
ꢀ829.123
ꢀ828.487
ꢀ828.485
ꢀ827.961
ꢀ827.491
ꢀ827.100
ꢀ826.776
ꢀ826.410
ꢀ825.673
ꢀ825.028
ꢀ825.035
ꢀ824.531
ꢀ824.011
ꢀ823.590
ꢀ823.276
250
Reactants
16.884
298.15
300
TBAa
200
250
131.796
160.284
186.472
187.447
212.597
235.341
255.575
273.420
390.944
425.286
457.244
458.453
490.544
521.556
551.451
580.195
24.606
350
298.15 33.359
400
300
350
400
450
500
ZPE
E
33.721
44.147
55.773
68.473
82.123
309.346
450
500
a
b
ΔS ¼ ðS0mÞtitle compound þ ðSm0 ÞH O ꢀ ðSm0 ÞTBA ꢀ ðSm0 ÞCA
2
ΔH ¼ ðHm0 þ E þ ZPEÞ
þ ðHm0 þ E þ ZPEÞH
O ꢀ ðHm0 þ E þ tZitlPe EcoÞmTpBoAunꢀd ðHm0 þ E þ ZPEÞCA
2
c
ꢀ2 187 193.272
10.583
ΔG ¼ ΔH ꢀ TΔS
CAb
200
250
78.592
304.662
326.142
346.426
347.196
367.811
387.915
407.437
426.328
15.416
97.993
298.15 20.972
116.068
116.746
134.281
150.272
164.611
177.359
300
350
400
450
500
ZPE
E
21.203
27.901
35.436
43.732
52.704
270.533
ꢀ1 961 772.502
Products
6.652
H2O
200
250
24.953
24.999
25.129
25.137
25.384
25.735
26.166
26.660
175.552
182.978
188.857
189.062
194.238
198.760
202.794
206.451
8.284
298.15 9.916
300
350
400
450
500
ZPE
E
9.957
11.631
13.346
15.070
16.805
Figure 9. Plot of the enthalpy versus entropy values for the formation
reaction of the title compound.
53.426
ꢀ200 611.847
24.981
title compound 200
250
202.175
247.081
289.026
290.591
331.226
368.141
401.065
430.144
502.795
554.597
603.203
605.044
654.218
701.995
748.287
792.981
Thermodynamic Properties. To determine thermodynamical
properties of the title compound, the standard thermodynamic
functions, heat capacity (C0p,m), entropy (Sm0 ), and enthalpy (H0m)
based on the vibrational analysis at B3LYP/6-311++G(d,p) level
and statistical thermodynamics for each of the reactants and
products in the formation reaction of the title compound, were
obtained andare listed in Table 8. The scale factor for frequencies is
0.96, which is used for an accurate prediction in determining the
thermodynamic functions.
36.631
298.15 49.944
300
350
400
450
500
ZPE
E
50.498
66.472
84.388
104.051
125.262
518.987
ꢀ3 949 180.477
As can be seen from Table 8, the standard heat capacities,
entropies, and enthalpies increase at any temperature from
200.00 to 500.00 K, because the intensities of molecular
vibration increase with the increasing temperature. The en-
thalpy change (ΔH) and the entropy change (ΔS) for the
formation reaction of the title compound are also obtained and
listed in Table 9. In the course of formation of the title
compound, the calculated ΔS and the ΔH decrease at any
temperature from 200 to 500 K. Hence, the formation of title
compound is an exothermic process that is accompanied by a
decrease in the entropy. Using the ΔG = ΔH ꢀ TΔS equation,
a 2-Hydroxy-5-(trifluoromethoxy)benzaldehyde. b 2-Chloroaniline.
As can be seen from the βtot values for these compounds,
there is an inverse relationship between first hyperpolarizability
and HOMOꢀLUMO gap, allowing the molecular orbitals to
overlap to have a proper electronic communication conjugation,
which is a marker of the intramolecular charge transfer from the
electron-donating group through the π-conjugation system to
the electron-accepting group.93,94
13873
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the change of Gibbs free energy (ΔG) at various temperatures
was also obtained (see Table 9).
and X-ray single-crystal diffraction. The X-ray, IR, and UVꢀvis
spectral data for the title compound show that the compound
exists in the enol form, which is stabilized by the intramolecular
One of the ways of getting the most stable tautomer is to calculate
the tautomeric equilibrium constant, KT, which is readily calculated
from the Gibb’s free energy via KT = eꢀΔG/RT, where ΔG is the
change in Gibb’s free energy between products and reactants at
temperature T and R is the ideal gas constant.95 The ΔG value at the
B3LYP/6-311++G(d,p) level is predicted to be 19.475 kJ/mol
between enol-imine and keto-amine tautomers of the title com-
pound. The KT is computed as 3.85 ꢁ 10ꢀ4 at 298.15 K for
enolTketo tautomerization. This observation indicates that the
direction of equilibrium is in favor of the enol structure.
Moreover, the linear relationship between ΔH and ΔS values,
which is known as enthalpyꢀentropy compensation (EEC),96 was
also examined. Using the thermodynamical data from Table 9, the
correlation between ΔH and ΔS is presented in Figure 9. As shown
in Figure 9, an acceptable linear relationship between ΔH and ΔS
for the formation reaction of the title compound is observed.
The graph of enthalpy versus entropy values for this reaction
generates a quite linear trend and can be expressed by ΔH = TcΔS +
b (Tc = compensation temperature and b = compensation free
enthalpy), with Tc = 303.678 ( 18.373 K and b = ꢀ824.841 (
0.208 kJ/mol. At this temperature, any variation in the standard
enthalpy in different temperatures is balanced by a compensating
variation in the standard entropy,97,98 such that the ΔG of the
reaction (the y intercept of the plot) remains constant at b = ꢀ
824.841 kJ/mol.
To test the reality of EEC correlation, the Starikov and Norden
EEC model can be applied,99 suggesting that if the EEC is a
statistically significant and physically nontrivial correlation, the
hidden Carnot process may be represented by a set of microscopic
heat engines, depending on the relation between the experimental
temperature T, which is the temperature at which the phase
transition takes place, and the compensation temperature, Tc;
Tc > T(“heat pump”) and Tc <T (“refrigerator”). The experimental
temperature in the formation reaction of the title compound is about
350 K, which is bigger than the observed Tc (303.678 ( 18.373 K).
According to our interpretation, the formation reaction of the
title compound can then be described in terms of an artificial
refrigerator model.
According to the data in Table 8 for the title compound, the
correlations between the thermodynamic properties C0p,m, Sm0 ,
and H0m and temperatures T are described. On the basis of these
relationships, the values of C0p,m, Sm0 , and H0m can be obtained at
any other temperature. The correlation equations of the title
compound are as follows:
OꢀH N hydrogen bond. FT-IR spectra analyses show that
3 3 3
the predicted vibrational frequencies are in good agreement with
the experimental values. The TD-DFT calculations lead to a very
closer agreement with the experimental absorption spectra
both in the gas phase and in solvent media. Molecular orbital
coefficients analyses reveal that the electronic transitions are
mainly assigned to πfπ* electronic transitions. The MEP map
shows that the negative potential sites are on electronegative
atoms while the positive potential sites are around the hydrogen
atoms. These sites give information about the region from where
the compound can have noncovalent interactions. The nonlinear
optical properties are also addressed theoretically. The predicted
NLO properties of the title compound are much greater than
ones of urea. The title compound is a good candidate as second-
order NLO material. The values of ΔG of the title compound
formation became more positive as temperature increases. At
298.15 K, the change of ΔG for the formation reaction of the title
compound is ꢀ824.841 kJ/mol. The title compound can spon-
taneously be produced from the isolated monomers at room
temperature. The tautomeric equilibrium constant is computed
as 3.85 ꢁ 10ꢀ4 at 298.15 K for enolTketo tautomerization of the
title compound. It is shown that the enol form is strongly
preferred over the keto form of the title compound. The
enthalpyꢀentropy compensation effect has been observed for
the formation reaction of the title compound, and the compensa-
tion temperature is reported. Analysis performed within the
framework of the Starikov and Norden EEC model shows that
the Tc is smaller than the average experimental temperature.
Thus, the compensation behavior between ΔH and ΔS for the
formation reaction of the title compound may be described in
terms of an artificial refrigerator model. I hope this Article will be
helpful for the design and synthesis of new materials.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: +90 358 218 01 60. Fax: +90 358 218 01 04. E-mail:
hasantanak@gmail.com.
’ ACKNOWLEDGMENT
I thank Professor Orhan B€uy€ukg€ung€or for his help with the
data collection and Associate Professor Ays-en Aꢀgar for helping
with the synthesized crystal, and I acknowledge the Faculty of
Arts and Sciences, Ondokuz Mayıs University, Department of
Physics, Turkey, for access to the Gaussian 03W program
package.
C0p, m ¼ ꢀ 17:2272 þ 1:22489T ꢀ 6:58119
ꢁ10ꢀ5T2 ðR2 ¼ 0:99991Þ
ð8Þ
ð9Þ
S0m ¼ 281:11283 þ 1:16416T ꢀ 2:80468 ꢁ 10ꢀ4T2 ðR2 ¼ 1Þ
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