Probing the compound (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol mainly from the point of tautomerism in solvent media and the solid state by experimental and computational methods

Article abstract of DOI:10.1016/j.molstruc.2011.06.018

In this study, (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol compound was investigated by mainly focusing on stacking interactions assembling the supramolecular network of the compound and on tautomerism in solvent media and in the solid state. In doi

Full text of DOI:10.1016/j.molstruc.2011.06.018

Journal of Molecular Structure 1000 (2011) 162–170
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Probing the compound (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol
mainly from the point of tautomerism in solvent media and the solid state
by experimental and computational methods
b
c
b
Çig˘dem Albayrak a, , Gökhan Kasßtaßs , Mustafa Odabasßog˘lu , Orhan Büyükgüngör
⇑
a Faculty of Education, Department of Science Education, Sinop University, 57000 Sinop, Turkey
b Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, 55139 Kurupelit-Samsun, Turkey
c Chemical Technology Program, Pamukkale University, 20070 Kinikli-Denizli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol compound was investigated by
mainly focusing on stacking interactions assembling the supramolecular network of the compound
and on tautomerism in solvent media and in the solid state. In doing so, the molecular structure and spec-
troscopic properties of (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol were experimentally char-
acterized by X-ray diffraction, FT-IR, NMR and UV/Vis spectroscopic techniques and computationally by
DFT method. The X-ray diffraction and FT-IR analyses of the title compound reveal the existence of enol
Received 12 April 2011
Received in revised form 14 June 2011
Accepted 14 June 2011
Available online 5 July 2011
Keywords:
Schiff base
form in the solid state. The non-covalent CAHꢀ ꢀ ꢀp and inter-molecular hydrogen bonding interactions
assemble the supramolecular structure of the title compound by forming 4-connected (4,4)-net in Wells
nomenclature. The dependence of tautomerism on solvent types was studied on the basis of UV/Vis spec-
tra recorded in different organic solvents. The results showed that the title compound exists in enol form
in all solvents. Computational investigation of enol–keto tautomerism was carried out at B3LYP
(6-311G(d,p)) level for both enol and keto forms. The results obtained for enol form are more compatible
to the experimental results than those of keto form. TD-DFT calculations carried out in both gas and solu-
tion phases indicate that the title compound adopt only enol form in solution. The enol–keto tautomer-
ism was also investigated by evaluating the changes in thermodynamic properties (heat capacity,
entropy, enthalpy and Gibbs free energy) with varying temperatures, showing that the formation of tau-
tomerism in the title compound is non-spontaneous between 100 and 500 K and that the title compound
must exist in enol form.
Intra-molecular hydrogen bond
Inter-molecular hydrogen bond
DFT
TD-DFT
Tautomerism
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
membranes [3]. In addition, they have widespread usage as ligands
in the field of coordination chemistry [4] as well as in diverse fields
o-Hydroxy Schiff bases that have a strong intra-molecular
hydrogen bond derive from the condensation of primary amines
with carbonyl compounds. These compounds are of interest be-
cause of their thermochromism and photochromism in the solid
state, which can involve reversible intra-molecular proton transfer
from an oxygen atom to the neighboring nitrogen atom. Intra-
molecular proton transfer mechanism of both excited-state and
ground-state is still the subject of intensive research. It was pro-
posed on the basis of thermochromic and photochromic Schiff
bases that the molecules exhibiting thermochromism are planar
while the molecules exhibiting photochromism are non-planar
[1,2]. Photochromic compounds are used as optical switches and
optical memories, variable electrical current, ion transport through
of chemistry and biochemistry owing to their biological activities
[5]. o-Hydroxy Schiff bases can exist in two tautomeric structures
as enol and keto forms in the solid state [6,7]. o-Hydroxy Schiff
bases were studied in solvent media and found to be existed in
both enol and keto forms. The absorption band at wavelength
greater than 400 nm is observed in polar solvents while this band
is not observed in apolar solvents. The absorption band at greater
than 400 nm was found to be belong to the keto form of o-Hydroxy
Schiff bases [8,9].
In this study, the crystal structure of (E)-2-[(4-bromophenylimi-
no)methyl]-6-ethoxyphenol was determined by single crystal
X-ray diffraction technique. The structural properties were also
characterized by FT-IR, NMR and UV/Vis spectroscopic methods.
The computational studies at DFT level were carried out in order
to get more insight on tautomerism and its dependence on solvent
type. The excitation energies were obtained using TD-DFT calcula-
tions starting from optimized geometry.
⇑
Corresponding author. Tel.: +90 368 2715532; fax: +90 368 2715530.
E-mail address: calbayrak@sinop.edu.tr (Ç. Albayrak).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.06.018

Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
163
tion of the title molecule was performed by using DFT method with
Becke’s three-parameters hybrid exchange–correlation functional
(B3LYP) [13] employing 6-311G(d,p) basis set [14,15] as imple-
mented in Gaussian 03W. Crystallographically obtained geometri-
cal data of the molecule was used for the optimization. Total
molecular energy and dipole moment were obtained from the opti-
mization output. The ground state geometry optimization of the ti-
tle compound for gas phase was calculated by using DFT method
with B3LYP adding 6-311G(d,p). Solution phase geometry optimi-
zations were performed with the same level of theory by the polar-
izable continuum model (PCM). The polarizable continuum model
(PCM) family of solvation models is among the most widely used
[16,17]. TD-DFT calculations starting from optimized geometry
for gas phase and solution phase were carried out at the same the-
ory level in both gas and solution phases to calculate excitation
energies of title compound. In addition, gas-phase vibrational fre-
quencies, non-linear optical properties and thermodynamic prop-
erties were calculated based on optimized geometry by using the
same theory level.
2. Experimental and computational methods
2.1. Synthesis
The compound (E)-2-[(4-bromophenylimino)methyl]-6-eth-
oxyphenol was prepared by refluxing a mixture of a solution con-
taining 3-ethoxysalicylaldehyde (0.5 g, 3 mmol) in 20 mL ethanol
and a solution containing 4-bromoaniline (0.52 g, 3 mmol) in
20 mL ethanol. The reaction mixture was stirred for 1 h under
reflux. The crystals of (E)-2-[(4-bromophenylimino)methyl]-6-eth-
oxyphenol suitable for X-ray analysis were obtained by slow
evaporation from methyl alcohol (yield 82%; m.p. 375–377 K).
2.2. Instrumentation
The melting point was determined by StuartMP30 melting point
apparatus. FT-IR spectrum of the title compound was recorded on a
Bruker 2000 spectrometer in KBr disk. UV/Vis absorption spectra
were recorded on a Thermo scientific BioGenesis UV/Vis spectrom-
eter using a 1 cm path length of the cell. The 13C NMR spectrum
was taken on Bruker AC 200 MHz spectrometer. The 1H NMR spec-
trum was taken on Bruker Ultrashield 300 MHz spectrometer.
3. Results and discussion
3.1. Description of the structure
2.3. X-ray crystallography
The tautomerism appears in o-Hydroxy Schiff bases as a result
of intra-molecular proton transfer from oxygen atom to nitrogen
atom. This proton transfer is resulted in two tautomeric structures
as enol and keto forms in the solid state. The C@N double bond and
CAO single bond distances (Table 2) compare well with reported
values [18–20]. It is seen that the title compound adopts enol form
and E configuration the C@N double bond (Fig. 1). The strong intra-
molecular interaction between the phenolic atom O1 and nitrogen
atom N1 forms a six-membered ring defined as S(6) in Etter’s nota-
tion [21]. This strong hydrogen bond is characterized by O1ꢀꢀꢀN1
distance of 2.634(3) Å (Table 3), being shorter than the sum of
the van der Waals’ radii for N and O [22].
Diffraction measurements were performed at 296 K on a STOE
IPDS 2 diffractometer using graphite monochromated Mo Ka radi-
ation and reflections were collected in the rotation mode and cell
parameters were determined by using X-AREA software [10].
Absorption correction was achieved by the integration method
via X-RED software [10]. The structure was solved by direct meth-
ods using SHELXS-97 [11]. The refinement was carried out by full-
matrix least-squares method on the positional and anisotropic
temperature parameters of the non-hydrogen atoms, or equiva-
lently corresponding to 176 crystallographic parameters. All non-
hydrogen atom parameters were refined anisotropically and all H
atoms except for H1 were located in their idealized positions and
refined using a riding model with CAH distances in the range
0.93–0.97 Å. The data collection conditions and parameters of
refinement process are listed in Table 1.
The preference of enol form by the title compound can be seen
alternatively by calculating the harmonic oscillator model of aro-
maticity (HOMA) index for the rings [23,24]:
"
#
n
X
a
n
2
HOMA ¼ 1 ꢁ
ðRi ꢁ Ropt
Þ
ð1Þ
2.4. Computational procedure
i¼1
All computations were performed using Gaussian 03W program
package running under Windows XP [12]. Full geometry optimiza-
Table 1
Table 2
0
Crystal data and structure refinement for title compound.
The selected bond lengths, angles and torsion angles (ÅA, °).
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C15H14BrNO2
320.18
296 K
0.71073 Å
Monoclinic
P 21/c
X-ray
DFT/B3LYP enol form DFT/B3LYP keto form
C9AO1
N1AC7
C7AC8
C8AC9
1.349(3)
1.280(3)
1.448(4)
1.397(4)
1.403(4)
1.384(4)
1.389(5)
1.365(4)
1.404(4)
1.417(3)
1.363(4)
1.895(3)
119.8(2)
122.3(2)
118.0(2)
123.1(2)
122.9(3)
1.33599
1.28931
1.44781
1.41359
1.41861
1.38889
1.40528
1.37656
1.41265
1.40554
1.35799
1.91723
121.307
122.747
118.057
122.816
123.204
1.25152
1.33582
1.39085
1.46572
1.46627
1.36926
1.43147
1.35863
1.43439
1.40092
1.35476
1.91517
128.265
122.753
121.344
123.134
123.312
ꢁ179.414
9.668
Unit cell dimensions
a = 12.0563 (5) Å,
b = 9.1322 (3) Å, b = 117.024 (3)°
a
= 90°
C9AC10
C10AC11
C11AC12
C12AC13
C8AC13
C1AN1
C10AO2
C4ABr1
C7AN1AC1
O1AC9AC8
O1AC9AC10
N1AC7AC8
N1AC1AC2
c = 14.3352 (5) Å,
1405.99 (9) Å3
4
c = 90°
Volume
Z
Density (calculated)
F(0 0 0)
Crystal size
1.513 Mg mꢁ3
648
0.69 ꢂ 0.51 ꢂ 0.15 mm3
Theta range for data collection
Index ranges
1.6–28.0°
ꢁ15 6 h 6 15, ꢁ11 6 k 6 11, ꢁ17 6 l 6 17
Number of measured
Independent reflections, Rint
Goodness-of-fit on F2
19020
2908, 0.037
1.10
0.039
R[F2 > 2
r
(F2)]
C8AC7AN1AC1 ꢁ176.1(2) ꢁ177.247
C7AN1AC1AC2 30.7(4) 37.449
C7AN1AC1AC6 ꢁ152.7(3) ꢁ144.989
wR(F2)
0.092
D
qmax
,
D
qmin
0.64, ꢁ0.49 e Åꢁ3
ꢁ170.734

164
Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
C7AC8, C8AC9, C9AC10, C10AC11, C11AC12, C12AC13 distances
for enol form are 1.44781, 1.41359, 1.41861, 1.38889, 1.40528,
1.37656 Å, the corresponding distances in keto form are 1.39085,
1.46572, 1.46627, 1.36926, 1.43147, 1.35863 Å, respectively. Con-
sidering these results, the optimization for enol form can be said
to be more compatible to the experimental results. The conforma-
tional differences between the energy minimized molecule (blue)
and the crystallographically observed geometry (red) were ana-
lyzed quantitatively by r.m.s. overlay (Fig. 3). The r.m.s. fit of the
atomic positions of experimental and calculated geometry is
0.122 Å, indicating the agreement between the two geometries.
However, the calculated values slightly deviate from experimental
values since the theoretical calculations are performed for gas
phase while experimental results are belong to solid phase.
Fig. 1. A view of the title compound, with the atom numbering scheme.
Table 3
Hydrogen bonding geometry (ÅA,°).
0
3.2. Natural bond orbital (NBO) analysis
DꢁHꢀ ꢀ ꢀA
DꢁH (Å)
0.77(3)
0.96
Hꢀ ꢀ ꢀA (Å)
1.94(4)
2.83
DꢀꢀꢀA (Å)
2.634(3)
3.688(4)
DꢁHꢀ ꢀ ꢀA (°)
150(4)
149
Natural bond orbital analysis provides an efficient method for
studying intra- and inter-molecular bonding and interaction
among bonds, and also provides a convenient basis for investigat-
ing charge transfer or conjugative interaction in molecular systems
[28]. Delocalization of electron density between occupied Lewis-
type (bond or lone pair) NBO and formally unoccupied (antibond
or Rydgberg) non-Lewis NBO correspond to a stabilizing donor–
acceptor interaction. The second order Fock matrix was carried
out to evaluate the donor–acceptor interactions in the NBO analy-
sis. For each donor NBO(i) and acceptor NBO(j), the stabilization
energy E(2) associated with electron delocalization between donor
and acceptor is estimated as
O1AH1ꢀ ꢀ ꢀN1
C15AH15Cꢀ ꢀ ꢀBr1i
XAH
C14AH14A
C15AH15A
Cg
Hꢀ ꢀ ꢀCg (Å)
3.264
2.874
XAHꢀ ꢀ ꢀCg (°)
148
141
Xꢀ ꢀ ꢀCg (Å)
4.117(3)
3.670(4)
Cg1ii
Cg2iii
Symmetry codes: (i) x + 1, y + 1, z + 1; (ii) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z; (iii) 1 ꢁ x, 2 ꢁ y, 1 ꢁ z.
where n is the number of bonds in the ring,
a is a constant equal to
257.7 and Ropt is equal to 1.388 Å for CAC bonds. For purely aro-
matic compounds, HOMA index is equal to1 while it is equal to 0
for non-aromatic compounds. The HOMA indexes in the range of
0.900–0.990 or 0.500–0.800 correspond to aromatic or the non-
aromatic rings, respectively [25,26]. In this study, the calculated
HOMA indexes for C1AC6 and C8AC13 rings are 0.979 and 0.952,
respectively. These results correspond to aromatic character for
the rings. In other words, the enol form is adopted by the title
compound.
2
ðFijÞ
Eð2Þ
¼
D
Eij ¼ ꢁq
ð2Þ
i ej
ꢁ
ei
where qi is the donor orbital occupancy, ei
,
e
j are diagonal elements
(orbital energies) and Fij is the off-diagonal NBO Fock matrix ele-
ment [29,30]. The larger the E(2) value, the more intensive is the
interaction between electron donors and electron acceptors, i.e.,
the more donating tendency from electron donors to electron
acceptors and the greater the extent of conjugation of the whole
system. NBO analysis of the title compound was performed by
applying the same level of theory as in the case of enol form in dif-
ferent solvents and gas phase. The interactions between donor N1
lone pair and acceptor H1AO1 bond for the title compound in var-
ious solvents and gas phase are given in Table 4. NBO analysis for
the title compound reveals that there is a strong interaction
The supramolecular network of the title compound is con-
structed by inter-molecular hydrogen bonds and CAYꢀ ꢀ ꢀ
p interac-
tions as non-covalent interactions (Table 3). The inter-molecular
C15ꢁH15AꢀꢀꢀBr1i (i: x + 1, y + 1, z + 1) interaction occurs between
the methyl group and bromide atom. The repetition of this interac-
tion along [1 1 1] is resulted in C(14) chain. Further analysis of non-
covalent interactions indicates the presence of complementary
CAYꢀ ꢀ ꢀ
rings (Cg) with hydroxyl groups. The geometry of the interaction
is characterized by Hꢀ ꢀ ꢀCg distance of 2.874 Å with
p interactions between the methyl groups and the phenyl
ðnN1
!
rꢃO1—H1Þ which increases with the increase in the polarity
a
C15AH15Aꢀ ꢀ ꢀCg2iii angle of 141°. The discrete molecules of the ti-
tle compound are connected by these interactions. Thus, the C(14)
polymeric chains are linked to each other, generating fused R22(12)
and R44(24) synthons located at (1/2 + n, 1 + n, 1/2 + n; n = 0 or inte-
ger) and (n, 1/2 + n, n) positions. This arrangement of ring motifs is
resulted in a tape structure in the supramolecular network. It is
also seen that each tape is connected to each other by
C14AH14Aꢀ ꢀ ꢀCg1ii interactions, giving rise to R00344(14) and
R22(20) synthons (Fig. 2). From topologic point of view, each discrete
molecule can be considered as a node. In this sense, each node is
connected to four nodes. As a result, a two-dimensional network
sheet (a square grid) is formed, which is defined as (4,4)-net in
Wells nomenclature or (4462) in Schläfli notation [27].
of solvent. As a result, the O1AH1 bond weakens and lengthens.
Table 4 shows the calculated delocalization energies for the title
compound in various solvents. It is seen that the delocalization en-
ergy of the title compound increases with an increase in the polarity
of solvent. The increase in the delocalization energy indicates that
the title compound is more stable in polar solvent. In addition,
Nꢀ ꢀ ꢀO distance shortens upon an increase in the polarity of solvent.
The increase of ðnN1
!
rꢃO1—H1Þ delocalization causes the intra-
molecular hydrogen bond to strengthen, weaken and elongation
of the OAH sigma bond.
3.3. FT-IR spectrum
In order to investigate the enol–keto tautomerism, the title
compound was optimized at B3LYP/6-311G(d,p) theory level for
both enol and keto forms. The calculated parameters and X-ray
geometry of the title compound are comparatively given in Table
2. While C9AO1 and C7AN1 distances for optimized enol form of
the compound are 1.336 Å and 1.289 Å, these distances for keto
form are 1.25152 and 1.33582 Å, respectively. In addition, while
FT-IR spectrum of the title compound is given in Fig. 4. The OH
stretching vibration is very sensitive to inter- and intra-molecular
hydrogen bonds and lies in the region 3000–3500 cmꢁ1 in FT-IR
spectrum. The OH group gives rise to three vibrations as stretching,
in-plane bending and out-of-plane bending vibrations. In 2500–
3500 cmꢁ1 region, the absorption band is attributed to the
t(OAH) stretching vibration which appears as a broad band owing

Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
165
Fig. 2. A partial packing diagram for the title compound, with CAHꢀ ꢀ ꢀp and CAHꢀ ꢀ ꢀBr interactions shown as dashed lines.
and out-of-plane CAH vibrations were observed at 1118 cmꢁ1
and 835 cmꢁ1
respectively. The asymmetric and symmetric
,
stretching vibrations of the aliphatic CH2 and CH3 groups were ob-
served at 2974, 2926, and 2882 cmꢁ1. The deformation mode of
these groups was observed at 1414 cmꢁ1 while umbrella modes
and rocking modes were observed at 1395 cmꢁ1 and (1014,
705 cmꢁ1), respectively. The CAO stretching vibration in phenols
appears as a strong band in 1300–1100 cmꢁ1 frequency ranges
[31,32]. The absorption band observed at 1196 cmꢁ1 for the title
compound was assigned to CAO stretching vibration. The absorp-
tion bands at 1600–1400 cmꢁ1 are due to CAC stretching vibra-
tions of the aromatic compounds [31,32]. The vibration modes
for the title compound were observed at 1574 and 1464 cmꢁ1
which agree well with the literature data.
,
Fig. 3. Superimposition of gas phase optimized (blue) and experimental geometry
(red) of the title compound. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
The vibrational frequencies of the title compound were also
investigated computationally by using B3LYP/6-311G(d,p) method.
The vibrational frequencies were scaled by 0.9682 [33]. The vibra-
tional bands assignments have been made by using Gaussview
molecular visualization program. The experimental and the calcu-
lated frequencies are given in Table 5. The calculated results show
slight deviations from experimental values due to the intra-molec-
ular hydrogen bond between N and O atoms.
to the formation of strong intra-molecular OAHꢀꢀꢀN hydrogen bond
in the structure. Generally, the in-plane bending and out-of- plane
bending vibrations of OH groups in phenols span the region
1300–1400 cmꢁ1 and 517–710 cmꢁ1, respectively [31]. The band
observed at 1340 cmꢁ1 was assigned to in-plane bending vibration
of OH group while the band at 778 cmꢁ1 was attributed to its out-
of-plane bending vibration. The absorption band at 1613 cmꢁ1 is
due to t(C@N) stretching vibration. The aromatic CAH stretching,
3.4. NMR spectra
CAH in-plane bending and CAH out-of-plane bending vibrations
appear in 3000–3100 cmꢁ1, 1100–1500 cmꢁ1 and 800–1000 cmꢁ1
frequency ranges, respectively [31,32]. The absorption bands at
3086 and 3061 cmꢁ1 correspond to the aromatic CAH stretching
vibrations of the title compound. In addition, in-plane bending
The 1H NMR spectrum of the title compound was recorded in
DMSO-d6 (Fig. 5). The resonance of hydroxyl proton is at
12.97 ppm which is typical for intra-molecular hydrogen bonding
(OAHꢀ ꢀ ꢀN) proton. The resonance of imino proton is at 8.98 ppm.
Table 4
Second order perturbation theory analysis of the Fock matrix in NBO basis and intra-molecular hydrogen bond lengths in different solvents.
Solvent
Donor
Acceptor
E2 (kcal/mol)
OAH (Å)
Nꢀ ꢀ ꢀH (Å)
Nꢀ ꢀ ꢀO (Å)
Gas phase
Benzene
EtOH
LP(1)N1
LP(1)N1
LP(1)N1
LP(1)N1
BDꢃ(1)O1AH1
BDꢃ(1)O1AH1
BDꢃ(1)O1AH1
BDꢃ(1)O1AH1
23.17
24.43
26.40
26.64
0.99151
0.99388
0.99628
0.99654
1.73902
1.72767
1.71187
1.70984
2.63085
2.62377
2.61416
2.61293
DMSO

166
Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
Fig. 4. FT-IR spectrum of the title compound.
In 1H NMR spectrum of the title compound, the absorption
and 123–121 ppm (C12). The C2, C6 and C3, C5 carbons of aromatic
ring are at 122.832 ppm and 132.573 ppm, respectively. The C11,
C12, C13 and C8 carbons are at 116.781 ppm, 124.016 ppm,
118.743 ppm and 119.164 ppm, respectively. The CH2 (C14) and
CH3 (C15) peaks of ethoxy group are seen at 64.787 ppm and
14.931 ppm, respectively (Fig. 6). The results agree well with the
reported values [34].
peaks of aromatic ring protons are between 6 and 8 ppm (Fig. 5).
The H2 and H6 protons are coupled to the H3 and H5 protons
and show doublet peaks at 7.39 ppm (J2,3 = 8.71 Hz). The H3 and
H5 protons are coupled to the H2 and H6 protons and show dou-
blet peaks (7.65 ppm, J3,2 = 8.71 Hz). The H12 proton is coupled
to the H11 and H13 protons and give the triplet at 6.9 ppm
(J12,11 = J12,13 = 7.9). The H11 proton coupled to H12 shows doublet
and gives another doublet by coupling H13 as being 7.13
(J11,12 = 7.9 Hz, J11,13 = 1.35 Hz). The H13 proton coupled to H12
shows doublet and gives another doublet by coupling H11 as being
7.24 ppm (J13,12 = 7.9 Hz, J13,11 = 1.35 Hz). H14 protons couple to
H15 protons, they give a quartet (4.07 ppm, J14,15 = 6.95 Hz) and a
triplet (1.35 ppm, J15,14 = 6.95 Hz) (Fig. 5), respectively. These re-
sults are in accordance with the literature values [34]. The absorp-
tion of ring protons are in the range of 6–8 ppm which corresponds
to aromatic character. This result reveals that the title compound
adopts only enol form in solution. If the title compound existed
in keto form, the absorption of ring protons would shift towards
in the range of 4–6 ppm because the aromatic character would de-
crease with the formation of keto form.
3.5. UV/Vis absorption spectra
The total energy, dipole moment and frontier molecular orbital
energies were calculated at B3LYP/6-311G(d,p) level using polariz-
able continuum model (PCM) in solvent media for enol and keto
forms. The calculated results for the solvents of different polarities
are comparatively given in Table 6. It is seen in Table 6 that the to-
tal energy decreases with an increase in the polarity of the solvent.
In other words, the stability of molecule increases. The energy gap
(DE) between the highest occupied molecular orbital (HOMO) and
the lowest-lying unoccupied molecular orbital (LUMO) and the di-
pole moment increase in enol and keto forms when an increase oc-
curs in the polarity of the solvent. The estimated dipole moments
of 3.3355 D and 2.3464 D for the enol and the keto forms in gas
phase respectively, show that the enol form is more polar than
the keto form both gas phase and solution (Table 6). HOMO and
LUMO called Frontier molecular orbitals (FMO) are the most
important ones in terms of their roles in reactions between mole-
cules and in electronic spectra of a molecule. The frontier molecu-
lar orbitals for both enol and keto forms of the title molecule are
shown in Fig. 7 for EtOH.
In the 13C NMR spectrum of the title compound recorded in
CDCl3 (Fig. 6), the imino carbon (C7) was observed at
163.021 ppm, and the phenolic carbon (C9) appeared at
147.823 ppm. The 13C NMR spectrum of the title compound shows
peaks at 151.757 ppm (C1), 147.313 ppm (C10), 120.504 ppm (C4)
Table 5
The experimental and the calculated vibrational frequencies (cmꢁ1).
In addition, the first 10 spin-allowed singlet–singlet excitations
for both enol and keto forms were calculated by TD-DFT approach.
TD-DFT calculations were started from optimized geometry using
the same level of theory and were performed for gas phase to cal-
culate excitation energies. The percentage contributions of molec-
ular orbitals to formation of the bands were obtained by using
SWizard Program [35]. For both enol and keto forms of the title
compound, wavelength (k), oscillator strength (f), major contribu-
tions of calculated transitions are given in Table 7. Considering
TD-DFT calculations (Table 7) it can be said that the excitation en-
ergy values at 324 nm and 376 nm arise from HOMO-1 ? LUMO
and HOMO ? LUMO transitions for enol form, respectively. The
excitation energy above 400 nm arises from HOMO ? LUMO of
Assignments
Experimental
DFT/B3LYP
OAH str.
2500–3500
3086, 3061
2926, 2882
2974
1613
1574
1464
1118
835
1414
3131
CAH str. (aromatic)
CAH (CH2) str
CAH (CH3) str
C@N str.
C@C str. (aromatic)
C@C str. (aromatic)
CAH bend. (aromatic)
CAH bend. (aromatic)
CH2, CH3 bend.
CH2, CH3 bend.
OAH bend.
3105, 3102, 3086, 3065
2929, 2894
3009, 2943
1613
1570, 1549
1481
1153
842
1477, 1456
1386, 1384
1328
1395
1340
1196
1014, 705
CAO str.
CH2, CH3 bend.
1239
1143, 1107, 813, 737
the keto form. These transitions are attributed to
shown in Fig. 7. Referring to TD-DFT calculations it can be
p ?
pꢃ transition
Str.: stretching; bend.: bending.

Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
167
Fig. 5. 1H NMR spectrum of the title compound.
Fig. 6. 13C NMR spectrum of the title compound.
concluded that the absorption band in the range greater than
400 nm is belong to the keto form.
However, the experimental spectrum shows no absorption peak in
this region. On the other hand, TD-DFT calculations for enol form
show two absorption bands at 324 nm and 376 nm which arise
from HOMO-1 ? LUMO and HOMO ? LUMO transitions, respec-
tively. The calculated results for enol form are very similar to
experimental results as seen in Table 7. Considering the absence
of peak above 400 nm and the agreement between calculated
It is noteworthy that there are two absorption bands in experi-
mental UV–Vis spectra of the title compound in all solvents. These
are observed as an absorption band at 318 nm and a shoulder at
366 nm (Fig. 8). The HOMO ? LUMO transition for the keto form
occurs at 462 nm with respect to the calculated excitation energy.

168
Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
Fig. 7. The molecular orbital surfaces of the title compound for enol and keto forms in EtOH.
Table 6
Calculated total energies, frontier orbital energies and dipol moments of title compound for enol and keto form.
Gas phase
Benzene
EtOH
DMSO
Enol form
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢁ3359.564465
ꢁ2.022
ꢁ5.821
3.819
ꢁ3359.570393
ꢁ1.993
ꢁ5.829
3.836
ꢁ3359.579185
ꢁ2.000
ꢁ5.886
3.886
ꢁ3359.579681
ꢁ2.004
ꢁ5.892
3.888
D
E (eV)
l
D
(D)
3.3355
9.548
3.8271
5.828
4.6647
0.311
4.7269
0.000
Ea (kcal/mol)
Keto form
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢁ3359.557681
ꢁ2.284
ꢁ3359.565319
ꢁ2.267
ꢁ5.320
3.053
ꢁ3359.577078
ꢁ2.290
ꢁ5.393
3.103
ꢁ3359.577547
ꢁ2.303
ꢁ5.398
3.095
ꢁ5.304
D
E (eV)
3.020
l
D
(D)
2.3464
13.805
2.7911
9.012
3.8645
1.633
3.9566
1.339
Ea (kcal/mol)
a
Relative energies as enol form in DMSO.
values when the change is from enol to keto form of the title com-
pound are given in Table 9. It is clearly seen that the change in en-
tropy increases with the increase of temperature. This result may
evoke that the reaction tends to keto form from enol form. How-
ever, the increase in entropy change is not greater than that in en-
and experimental spectra for the enol form, it can be said that the
title compound adopts only enol form in solution.
3.6. Thermodynamic properties
thalpy. As
temperature decreases
a
result,
D
D
G
becomes positive. The increase of
The standard thermodynamic functions, namely, the heat
capacity ðC0p;mÞ, entropy ðSm0 Þ and enthalpy ðH0mÞ, were calculated
at B3LYP/6-311G(d,p) level for varying temperatures from 100 to
500 K. The correlation equations between heat capacity, entropy,
enthalpy and temperature are given in Table 8. The results reveal
that an increase in temperature causes an increase in heat capaci-
ties, entropies and enthalpies due to increasing intensities of
molecular vibrations.
G, however, it is not negative between
100 and 500 K. Tautomerism for the title compound is typically
endothermic with positive entropy and enthalpy changes. This
means that the tautomerism for the title compound is non-sponta-
neous (disfavoured reaction) within these temperatures. Consider-
ing these results, it can be said that the title compound must
exist in enol form.
The enol–keto tautomerism of the title compound can also be
investigated by using the following equation which gives the
3.7. Non-linear optical (NLO) properties
change in Gibbs free energy (
DG) in terms of changes in entropy
(
D
S) and enthalpy ( H) between enol and keto forms:
D
Having the knowledge of non-linear optical properties is of cru-
cial importance in the design of materials in communication tech-
nology, signal processing, optical switches and optical memory
devices [36]. The non-linear optical properties are associated with
D
G ¼ H ꢁ T
D
D
S
ð3Þ
The reaction is spontaneous for
D
G < 0, disfavoured reaction (non-
spontaneous) for
D
G > 0 or equilibrium reaction for
D
G = 0.
D
G
delocalized p electrons of a molecule. That is, the increase of the

Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
169
Table 7
For enol and keto forms wavelength, oscillator strength, major contributions of calculated transitions.
Experimental
Calculated
Enol form
Major contribution
Keto form
Major contribution
k (nm)
k (nm) (f)
k (nm) (f)
Gas phase
–
278.47(0.2132)
H-2 ? L (61%)
277.9(0.1146)
H ? L + 3(80%)
H-3 ? L (13%)
H-4 ? L (15%)
H-1 ? L (81%)
H ? L (87%)
–
–
324.39(0.5729)
376.68(0.1079)
336.9 (0.6444)
462.3 (0.2087)
H-1 ? L (83%)
H ? L (77%)
Benzene
EtOH
282
280.2(0.1968)
H-2 ? L(66%)
H-4 ? L(16%)
H-1 ? L (85%)
H ? L (89%)
320
366
330.2(0.7144)
377.0(0.1492)
344.5 (0.7484)
468.9 (0.2755)
H-1 ? L (86%)
H ? L (81%)
282
279.4 (0.1928)
H-2 ? L (64%)
H-4 ? L (18%)
H-1 ? L (84%)
H ? L (89%)
318
364
327.4 (0.7105)
371.3 (0.1267)
341.4 (0.7392)
459.7 (0.2479)
H-1 ? L (84%)
H ? L (81%)
DMSO
286
279.8 (0.1905)
H-2 ? L (65%)
H-4 ? L (18%)
H-1 ? L (85%)
H ? L (89%)
318
366
328.7 (0.7371)
371.8 (0.1340)
344.7 (0.7783)
463.1 (0.2621)
H-1 ? L (88%)
H ? L (81%)
Table 9
The changes in Gibbs free energy, enthalpy and entropy of title compound with
increasing temperature.
T (K)
D
HT (kcal/mol)
D
ST (cal/mol K)
DGT (kcal/mol)
100
200
298.15
300
400
4.116
4.139
4.162
4.163
4.181
4.194
1.707
1.865
1.962
1.964
2.018
2.045
3.945
3.766
3.577
3.574
3.374
3.172
500
The title compound includes delocalized
p
electron system. In
this study, the title compound was computationally studied at
DFT (B3LYP) level in order to investigate the effect of
system.
p electron
The total static dipole moment
l
, the average linear polarizabil-
ity a, and the first hyperpolarizability b can be calculated by using
the following equations [36]:
Fig. 8. The solvent effect on UV–Vis spectra of the title compound in (- - - -) DMSO,
(––) EtOH, (– ꢀ ꢀ –) benzene.
1=2
l
a
¼ ðl2x
¼ 1=3ðaxx
þ
ly2
þ
lz2
ayy
Þ
ð4Þ
ð5Þ
þ
þ
azz
Þ
Table 8
2
2
b ¼ ½ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ bxxy þ byzz
Þ
Thermodynamic properties of title compound at different temperatures.
2
1=2
H0m (kcal/mol)
S0m (cal/mol K)
C0p;m (cal/mol K)
þ bzzz þ bxxz þ byyzÞ ꢄ
ð6Þ
T (K)
100
200
298.15
300
400
1.881
5.741
11.406
11.531
19.267
28.753
89.739
115.744
138.532
138.950
161.076
182.177
27.167
46.095
65.436
65.800
84.589
100.640
The dipole moment, polarizability and the first hyperpolarizability
were calculated using polar = ENONLY at B3LYP/6-311G(d,p) level.
The results are given in Table 10.
500
C0p;m = 5.90822 + 0.21384T ꢁ 4.73368 ꢂ 10ꢁ5T2, R2 = 0.9994.
S0m = 63.1027 + 0.2761T ꢁ 7.64785 ꢂ 10ꢁ5T2, R2 = 0.99988.
H0m = ꢁ0.13722 + 0.0106T + 9.44545 ꢂ 10ꢁ5T2, R2 = 0.99999.
Table 10
Calculated dipole moments (D), polarizability and first hyperpolarizability compo-
nents (a.u.) for the title compound.
lx
ly
lz
ꢁ1.0727042
0.7538113
ꢁ0.0554935
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
bxzz
byzz
bzzz
ꢁ586.169051
365.7455037
45.3084432
246.4135493
ꢁ99.5239317
10.2456604
ꢁ44.5872231
ꢁ51.7564499
ꢁ0.3705271
9.1749768
conjugation on molecule is resulted in an increase in the non-linear
optical properties. The addition of donor and acceptor groups to
conjugated systems also affects non-linear optical properties in
axx
axy
ayy
axz
ayz
azz
393.1448879
ꢁ15.2703594
191.3129894
2.0760258
11.2549907
103.6088697
increasing way. The delocalization of
p electron cloud on organic
molecules increases in the case of powerful donor and acceptor
groups. This is resulted in an increase in the polarizability and first
hyperpolarizability of organic molecules [37].

170
Ç. Albayrak et al. / Journal of Molecular Structure 1000 (2011) 162–170
[2] E. Hadjoudis, M. Vitterakis, I. Moustakali-Mavridis, Tetrahedron 43 (1987)
The calculated polarizability
a and first hyperpolarizability b for
1345.
the title compound are 33.953 Å3 and 7.45 ꢂ 10ꢁ30 cm5/esu,
respectively. It is found that the first hyperpolarizability and polar-
izability of the title compound are greater than those of some re-
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p electron cloud leads
to an increase in the conjugation and, consequently, to an increase
in its non-linear optical properties. These results show that the title
compound can be used as a good non-linear optical material.
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4. Conclusion
In this study, the enol–keto tautomerism of the title compound
was investigated by experimental and computational techniques.
X-ray diffraction and FT-IR analyses reveal that the title compound
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exists in enol form in the solid state. The non-covalent CAHꢀ ꢀ ꢀ
p
and inter-molecular hydrogen bonding interactions form 4-con-
nected (4,4)-net in the supramolecular structure of the title com-
pound. The dependence of tautomerism on solvent types was
investigated by UV/Vis experiments performed for different organ-
ic solvents. The results show that the title compound exists in enol
form in all solvents. Computational investigation of enol–keto tau-
tomerism was carried out at B3LYP (6-311G(d,p)) theory level for
both enol and keto forms. The results obtained for enol form are
more compatible to the experimental results. Considering
TD-DFT calculations, it can be said that the experimentally ob-
served excitation energies at 318 nm and 366 nm correspond to
HOMO-1 ? LUMO and HOMO ? LUMO transitions for enol form,
respectively. TD-DFT calculations also give an absorption band
above 400 nm which belong to the keto form. Considering the ab-
sence of a peak above 400 nm in experimental spectra and the
agreement between calculated and experimental spectra for the
enol form, the title compound was concluded to adopt only enol
form in solution. The enol–keto tautomerism in the title compound
was also investigated by considering the changes in thermody-
namic properties (heat capacity, entropy, enthalpy and Gibbs free
energy) with varying temperatures from 100 and 500 K. The calcu-
lated results for the change in Gibbs free energy show that the for-
mation of tautomerism in the title compound is non-spontaneous
between 100 and 500 K and that the title compound must exist
in enol form. The investigation of non-linear optical properties of
title compound was carried out as a complementary study. The re-
sults show that the title compound can be used as a good non-lin-
ear optical material.
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Supplementary data
Crystallographic data for the structure in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
the supplementary publication no. CCDC 819312. Copies of the
data can be obtained, free of charge, on application to CCDC, 12 Un-
ion Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk).
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