Survey of conformational isomerism in (E)-2-[(4-bromophenylimino)methyl]-5- (diethylamino)phenol compound from structural and thermochemical points of view

Article abstract of DOI:10.1016/j.saa.2012.04.074

In this study, (E)-2-[(4-bromophenylimino)methyl]-5-(diethylamino)phenol compound was investigated by mainly focusing on conformational isomerism. For this purpose, molecular structure and spectroscopic properties of the compound were experimentally characterized by X-ray diffraction, FT-IR and UV-Vis spectroscopic techniques, and computationally by DFT method. The X-ray diffraction analysis of the compound shows the formation of two conformers (anti and eclipsed) related to the ethyl groups of the compound. The two conformers are connected to each other by non-covalent C-H?Br and C-H?π interactions. The combination of these interactions is resulted in fused R22(10) and R24(20) synthons which are responsible for the tape structure of crystal packing arrangement. The X-ray diffraction and FT-IR analyses also reveal the existence of enol form in the solid state. From thermochemical point of view, the computational investigation of isomerism includes three studies: the calculation of (a) the rate constants for transmission from anti or eclipsed conformations to transition state by using Eyring equation, (b) the activation energy needed for isomerism by using Arrhenius equation, (c) the equilibrium constant from anti conformer to eclipsed conformer by using the equation including the change in Gibbs free energy. The dependence of tautomerism on solvent types was studied on the basis of UV-Vis spectra recorded in different organic solvents. The results showed that the compound exists in enol form in all solvents except ethyl alcohol.

Full text of DOI:10.1016/j.saa.2012.04.074

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Survey of conformational isomerism in
(E)-2-[(4-bromophenylimino)methyl]-5-(diethylamino)phenol compound
from structural and thermochemical points of view
b
c
d
Çig˘dem Albayrak a, , Gökhan Kasßtaßs , Mustafa Odabasßog˘lu , René Frank
⇑
a Sinop University, Faculty of Education, Department of Science Education, 57000 Sinop, Turkey
b Ondokuz Mayıs University, Faculty of Arts and Sciences, Department of Physics, 55139 Samsun, Turkey
c Pamukkale University, Chemical Technology Program, 20070 Kınıklı-Denizli, Turkey
d Leipzig University, Faculty of Chemistry and Mineralogy, D-04103 Leipzig, Germany
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Isomerism and tautomerism are
investigated.
Isomerism is evaluated from
thermochemical point of view.
Tautomerism is investigated
depending on solute–solvent
interactions.
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]-5-(diethylamino)phenol compound was investigated
by mainly focusing on conformational isomerism. For this purpose, molecular structure and spectroscopic
properties of the compound were experimentally characterized by X-ray diffraction, FT-IR and UV–Vis
spectroscopic techniques, and computationally by DFT method. The X-ray diffraction analysis of the com-
pound shows the formation of two conformers (anti and eclipsed) related to the ethyl groups of the com-
Received 15 November 2011
Received in revised form 14 April 2012
Accepted 18 April 2012
Available online 25 April 2012
pound. The two conformers are connected to each other by non-covalent C–Hꢀ ꢀ ꢀBr and C–Hꢀ ꢀ ꢀ
p
Keywords:
Schiff base
Intramolecular hydrogen bond
DFT
Conformational analysis
Isomerism
interactions. The combination of these interactions is resulted in fused R22ð10Þ and R24ð20Þ synthons which
are responsible for the tape structure of crystal packing arrangement. The X-ray diffraction and FT-IR
analyses also reveal the existence of enol form in the solid state. From thermochemical point of view,
the computational investigation of isomerism includes three studies: the calculation of (a) the rate con-
stants for transmission from anti or eclipsed conformations to transition state by using Eyring equation,
(b) the activation energy needed for isomerism by using Arrhenius equation, (c) the equilibrium constant
from anti conformer to eclipsed conformer by using the equation including the change in Gibbs free
energy. The dependence of tautomerism on solvent types was studied on the basis of UV–Vis spectra
recorded in different organic solvents. The results showed that the compound exists in enol form in all
solvents except ethyl alcohol.
IR and UV–Vis spectroscopies
Ó 2012 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +90 368 2715532; fax: +90 368 2715530.
E-mail address: calbayrak@sinop.edu.tr (Ç. Albayrak).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.074

Ç. Albayrak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
665
(0.5 g, 2.59 mmol) in 20 mL ethanol and a solution containing 4-
bromoaniline (0.29 g, 2.59 mmol) in 20 mL ethanol. The reaction
mixture was stirred for 1 h under reflux. The crystals of (E)-2-[(4-
bromophenylimino)methyl]-5-(diethylamino)phenol suitable for
X-ray analysis were obtained by slow evaporation from acetone
(yield 82%; m.p. 380–381 K).
Introduction
o-Hydroxy Schiff bases derived from the condensation of pri-
mary amines with carbonyl compounds have a strong intramolec-
ular hydrogen bond. These compounds are of interest because of
their thermochromism and photochromism in the solid state,
which can involve reversible intramolecular proton transfer from
an oxygen atom to the neighboring nitrogen atom. Intramolecular
proton transfer mechanism of both excited-state and ground-state
is still the subject of intensive research. On the basis of thermo-
chromic and photochromic Schiff bases, it was proposed that mol-
ecules exhibiting thermochromism are planar, while molecules
exhibiting photochromism are non-planar [1,2]. Photochromic
compounds are used as optical switches and optical memories,
variable electrical current, ion transport through membranes [3].
In addition, they have widespread usage as ligands in the field of
coordination chemistry [4] as well as in diverse fields 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 exist as both enol and keto form.
While the absorption band at wavelength greater than 400 nm is
observed in polar solvents, this band is not observed in apolar sol-
vents. The absorption band at greater than 400 nm was found to
belong to the keto form of o-Hydroxy Schiff bases [8,9].
Instrumentation
The melting point was determined by StuartMP30 melting point
apparatus. FT-IR spectrum of the compound was recorded in the
4000–400 cmꢁ1 region with a Bruker 2000 spectrometer using
KBr pellet. UV–Vis absorption spectra were recorded on a Thermo
scientific BioGenesis UV–Vis spectrometer using a 1 cm path
length of the cell. Diffraction experiment was carried out at
130 K with an Xcalibur diffractometer using graphite monochro-
mated MoK radiation. The structure was solved by direct methods
a
using SHELXS-97 [10]. The refinement was carried out by full-matrix
least-squares method. All non-hydrogen atoms were refined aniso-
tropically. The H atoms except those bonded to O atoms were
placed in geometrically idealized positions and refined by a riding
model with Uiso(H) = 1.2 Ueq(C) and Uiso(Hmethyl) = 1.5 Ueq(C). The
relevant crystal data, experimental conditions and final refinement
parameters can be obtained from Cambridge Structural Database
(CCDC: 801848).
In this study, (E)-2-[(4-bromophenylimino)methyl]-5-(diethyl-
amino)phenol compound was studied mainly from the point of
conformational isomerism. For this purpose, molecular structure
and spectroscopic properties of the compound were experimen-
tally characterized by X-ray diffraction, FT-IR and UV–Vis spectro-
scopic techniques, and computationally by DFT method.
Computational procedure
All computations were performed using GAUSSIAN 03W [11]. Full
geometry optimization of the title molecule was performed by
using DFT method with Becke’s three-parameters hybrid
exchange-correlation functional (B3LYP) [12] employing
6-311G(d,p) basis set [13–15]. The ground state geometry optimi-
zation of the title compound for gas phase was calculated using
DFT method with B3LYP adding 6-311G(d,p). The ground state
geometry optimization of the title compound in gas phase for TS
structure was performed with the same level of theory. A compu-
tational study was carried out from global minimum through a
transition state to local minimum for the title compound. Vibra-
Experimental and computational methods
Synthesis
The
compound
(E)-2-[(4-bromophenylimino)methyl]-5-
(diethylamino)phenol (I) was prepared by refluxing a mixture of
a solution containing 5-(diethylamino)-2-hydroxybenzaldehyde
Fig. 1. A view of the title compound, showing anti and eclipsed conformers. H atoms not involved in non-covalent interactions were not labeled for clarity.

666
Ç. Albayrak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
Table 1
Bond distances (Å), angles (°) and non-covalent interactions in anti and eclipsed conformers of the title compound.
Bond distance
Anti
Bond angle
Anti
Torsion angle
Anti
Eclipsed
Eclipsed
Eclipsed
N2–C4
N2–C16
N2–C14
C14–C15
C16–C17
C2–O1
N1–C7
N1–C8
1.360(2) N4–C21 1.366(2) N2–C14–C15 111.1(2) N4–C33–C34 113.9(1) C4–N2–C16–C17 89.6(2)
1.360(2) N4–C31 1.461(2) N2–C16–C17 112.9(2) N4–C31–C32 114.1(2) C4–N2–C14–C15 96.8(2)
C21–N4–C31–C32 ꢁ88.8(2)
C21–N4–C33–C34 76.8(2)
C18–C24–N3–C25 178.6 (2)
C20–C21–N4–C33 ꢁ175.1(2)
C20–C21–N4–C31 ꢁ0.9(2)
O2–C19–C18–C24 ꢁ3.5(3)
1.487(2) N4–C33 1.467(2) C14–N2–C16 116.4(1) C31– N4–C33 116.9(1) C1–C7–N1–C8
174.0(2)
166.2(2)
ꢁ9.4(3)
1.1(3)
1.494(3) C31–C32 1.516(2) C4–N2–C14
121.4(1) C21–N4–C33 120.6(1) C3–C4–N2–C14
122.0(2) C21–N4–C31 122.2(1) C3–C4–N2–C16
121.7(2) C20–C21–N4 122.1(2) O1–C2–C1–C7
121.0(2) C22–C21–N4 120.8(2) N1–C7–C1–C6
120.8(1) C24–N3–C25 119.6(1) C7–N1–C8–C9
120.3(2) C18–C19–O2 120.2(2) C7–N1–C8–C13
1.508(3) C33–C34 1.518(2) C4–N2–C16
1.361(2) C19–O2 1.361(2) C3–C4–N2
1.296(2) N3–C24 1.294(2) C5–C4–N2
1.411(2) N3–C25 1.413(2) C7–N1–C8
1.898(2) C28–Br2 1.896(2) C1–C2–O1
ꢁ177.9(2) N3–C24–C18–C23 ꢁ179.7(2)
ꢁ28.5(2)
C24–N3–C25–C26 ꢁ33.6(2)
C11–Br1
156.4(2)
C24–N3–C25–C30 148.8(2)
DꢁHꢀ ꢀ ꢀA
d(DꢁH)
0.839
0.765
0.990
0.989
Cg
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
<DHA
153.3
151.4
143.5
137.7
<XHCg
120.4
122.0
O1–H1ꢀ ꢀ ꢀN1
O1–H2ꢀ ꢀ ꢀN3
aC14–H14Aꢀ ꢀ ꢀBr2i
aC33–H33Aꢀ ꢀ ꢀBr1ii
X–H
1.821
2.598
1.933
2.882
3.077
2.630
3.724
3.867
d(Hꢀ ꢀ ꢀCg)
d(Xꢀ ꢀ ꢀCg)
aC14–H14A
aC33–H33A
Cg1iii
Cg2iii
3.183
3.782
2.970
3.587
a
Symmetry codes: (i) ꢁ1 + x, ꢁ1 + y, z; (ii) ꢁ1 + x, ꢁ1 + y, ꢁ1 + z; (iii) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z. Cg represents ring centroid. There are no s.u values in geometric parameters of non-
covalent interactions since most of H atoms were placed in geometrically idealized positions.
The C@N double bond and C–O single bond distances compare well
with previously reported values for similar compounds [18,19].
Both conformers adopt E configuration about the C@N double bond
(Fig. 1). The strong intramolecular interactions between the pheno-
lic atoms (O1, O2) and nitrogen atoms (N1, N3) constitute six-
membered rings defined as S(6) in Etter’s notation [20]. These
hydrogen bonds are characterized by O1ꢀ ꢀ ꢀN1 and O2ꢀ ꢀ ꢀN3 dis-
tances of 2.598(2) and 2.630(2) Å (Table 1), being shorter than
the sum of the van der Waals’ radii for N and O [21].
The crystal packing of the compound is mainly stabilized by
C–Hꢀ ꢀ ꢀBr interactions between methylene groups and bromide
atoms (Table 1). The repetition of intermolecular C14–H14Aꢀ ꢀ ꢀBr2i
(i: ꢁ1 + x, ꢁ1 + y, z) and C33–H33Aꢀ ꢀ ꢀBr1ii (ii: ꢁ1 + x, ꢁ1 + y,
ꢁ1 + z) interactions is resulted in C22ð28Þ chains (Fig. S1). Further
analysis of non-covalent interactions shows that the crystal pack-
ing are also supported by C14–H14Aꢀ ꢀ ꢀCg1iii (iii: 1 ꢁ x, 1 ꢁ y,
1 ꢁ z) and C33–H33Aꢀ ꢀ ꢀCg2iii interactions which involve the same
methylene groups (Fig. S1). The geometry of these interactions are
characterized by the Hꢀ ꢀ ꢀCg distances of 2.970 and 3.183 Å involv-
ing methylene groups and centroid of phenyl rings. It is seen that
the two conformers are connected by these interactions. Thus,
the C22ð28Þ polymeric chains are linked to each other, generating
fused R22ð10Þ and R42ð20Þ synthons in (010) plane. This arrangement
of ring motifs is resulted in a tape structure in the crystal packing.
In order to determine the conformational energy profiles for dif-
ferent arrangements, the energy values as a function of the torsion
angle h1(C4–N2–C16–C17) were calculated by DFT method. The
calculated molecular energy profile as a function of h1 is given in
Fig. 2. The molecular energy profile shows two minima at 90°
and 280° which correspond to anti and eclipsed conformers,
respectively. The energies of two conformers are close to each
other. The eclipsed conformer is slightly higher than that of anti
conformer by 0.73 kcal/mol due to the steric hindrance. After
determining the both stable conformers, the geometry optimiza-
tions pertaining to both stable conformers of title compound were
performed again by DFT method with B3LYP adding 6-311G(d,p)
basis set in order to compare the results obtained for two conform-
ers. The rate of conformational isomerism was studied by calculat-
ing the rate constants for both anti and eclipsed conformers. In the
transition state theory, reactants are considered to pass through a
transition state to form the final product. The transition state
optimization was carried out in order to calculate the rotation rate
Fig. 2. Variations of the relative energies of the title compound against the torsional
angle h1(C16–C17–N2–C4).
tional frequencies were obtained for characterization at stationary
points called global and local minima for which all eigenvalues of
the Hessian are positive and transition state corresponding to only
one imaginary frequency [16,17]. The thermodynamic properties
were obtained in the range of 100–500 K at the corresponding opti-
mized geometries using the same theory level.
Results and discussion
Structure determination
Two independent molecules exist in the asymmetric unit,
which correspond to two conformers of the compound (Fig. 1).
The two conformers are resulted from rotation about C16–N2 sin-
gle bond of ethyl group. In conformer I (anti conformer), the ethyl
groups are positioned in the opposite directions. On the other
hand, the ethyl groups in conformer II (eclipsed conformer) are
in the same directions. The torsion angle C4–N2–C16–
C17 = 89.6(2)° in anti conformer while its corresponding value in
eclipsed conformer is ꢁ88.8(2)°. The characteristic bond distances
and angles related to two conformers are comparatively given in
Table 1. The compound prefers enol form in both conformers.

Ç. Albayrak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
667
Table 2
Table 3
The rate constants calculated at different temperatures.
The Gibbs free energy, enthalpy, entropy changes and equilibrium constants
calculated at different temperatures.
T (K)
Anti conformer
G– (kcal/mol)
Eclipsed conformer
G– (kcal/mol) k (sꢁ1
)
T(K)
D
H (kcal/mol)
D
S (cal/molK)
D
G (kcal/mol)
Keq
D
k (sꢁ1
)
D
100
200
298.15
300
400
0.805
0.800
0.794
0.794
0.789
0.786
0.754
0.705
0.681
0.682
0.669
0.661
0.730
0.659
0.591
0.589
0.521
0.455
0.025
0.190
0.369
0.372
0.519
0.633
100
200
298.15
300
400
7.498
7.824
8.246
8.256
8.756
9.308
8.48 ꢂ 10ꢁ5
1.17 ꢂ 104
5.56 ꢂ 106
6.04 ꢂ 106
1.37 ꢂ 108
8.90 ꢂ 108
6.766
7.166
7.656
7.666
8.234
8.852
3.40 ꢂ 10ꢁ3
6.16 ꢂ 104
1.51 ꢂ 107
1.60 ꢂ 107
2.64 ꢂ 108
1.41 ꢂ 109
500
500
0.681 kcal/mol, respectively. It is seen that
DG of the title com-
between the two conformers observed in the crystal structure of
the compound.
pound is positive with 0.591 kcal/mol for the room temperature.
The equilibrium constant from anti conformer to eclipsed con-
former can be calculated with following equation [23]:
anti conformer $ TS $ eclipsed conformer
ð1Þ
DG=RT
K ¼ eꢁ
ð5Þ
The rate constant for TS formation from the reactant can be calcu-
lated using Eyring equation [22,23]:
where K is the equilibrium constant from anti conformer to eclipsed
conformer. G is the free energy change calculated from anti con-
–
kBT
h
D
G
k ¼
eꢁ
ð2Þ
RT
D
former to eclipsed conformer. The calculated equilibrium constant
for isomerism between two conformers is 0.369 in room tempera-
ture. This means that two conformers can be converted between
them. These results explain why the title compound exists in both
anti and eclipsed conformers in the crystal structure.
where k is the reaction rate for TS formation, kB and h are Boltzmann
and Planck constants, respectively.
G– is the change in free energy
of activation between TS and reactant.
G– and k constant were cal-
D
D
culated to investigate the isomerism from anti conformer to TS in
different temperatures. The rate constant increases when the tem-
perature is increased from 100 to 500 K. The calculated results are
given in Table 2. As seen from Table 2, the transmission rate from
anti conformer to TS increases upon an increase in temperature.
The same procedure was repeated for TS formation from eclipsed
conformer and the results were compared. It was concluded that
the transmission rate to TS of eclipsed conformer is more than that
of anti conformer.
FT-IR spectrum
FT-IR spectrum of the title compound was given in Fig. 3. The OH
group has three vibrations as stretching, in-plane bending and out-
of-plane bending vibrations. The OH stretching vibration is very
sensitive to inter and intramolecular hydrogen bonding. In 1700–
3000 cmꢁ1 region, the absorption band is attributed to the
t(O–H)
The relationship between the rate constant and temperature is
given by Arrhenius equation [24] as follows:
stretching vibration which broaden owing to the formation of
strong intramolecular hydrogen bonding O–Hꢀ ꢀ ꢀN in the structure.
Generally, the OH in-plane bending and out-of-plane bending
vibrations of phenols lie in the region 1300–1400 and 517–
710 cmꢁ1, respectively [25]. The band observed at 1354 cmꢁ1 was
assigned to OH in-plane bending vibration and the band observed
at 786 cmꢁ1 was assigned to OH out-of-plane bending vibration
for the title compound. The absorption band located at 1625 cmꢁ1
k ¼ AeꢁE
ð3Þ
a=RT
where Ea and A are activation energy and integration constant,
respectively. It is seen that the activation energy (Ea) needed for
isomerism can be determined from the slope of the graph Ink-1/T.
Therefore, we plotted lnk-1/T graphs to calculate activation energies
for transmissions from anti or eclipsed conformers to TS (Fig. S2).
While the activation energy from anti conformer to TS is
7.45 kcal/mol, the activation energy from eclipsed conformer to TS
is 6.65 kcal/mol (Fig. S3). It is seen that the activation energies for
both conformers are relatively low. The activation energy needed
for transmission from eclipsed conformer to TS is smaller than that
required for anti conformer. In other words, when the title com-
pound is formed in eclipsed conformer, it is easily converted to
the anti conformer (thermodynamically stable form) through a
reversible reaction. The rate constants for anti and eclipsed con-
formers are 5.56 ꢂ 106 and 1.51 ꢂ 107 sꢁ1, respectively, showing
that the rotation between two conformers is very fast.
is attributed to
t(C@N) stretching vibration. The aromatic C–H
stretching, C–H in-plane bending and C–H out-of-plane bending
vibrations appear in 3000–3100, 1100–1500 and 800–1000 cmꢁ1
frequency ranges, respectively [25]. The absorption band at
3066 cmꢁ1 corresponds to the aromatic CH stretching vibrations
of the title compound. In addition, in plane bending and out-
of-plane CH vibrations were observed at 1128 and at 823 cmꢁ1
,
The calculation of Gibbs free energies, entropies and enthalpies
were carried out for the temperatures from 100 to 500 K by DFT/
B3LYP method with 6-311G(d,p) basis set [22,23]. The results ob-
tained on the basis of vibrational analysis are shown in Table 3.
The isomerism of the title compound can also be investigated by
using the following equation which gives the change in Gibbs free
energy (DG) in terms of entropy (DS) and enthalpy changes (DH)
between anti and eclipsed conformers:
D
G ¼ H ꢁ T
The reaction is spontaneous for
spontaneous) for
culated H and
D
D
S
ð4Þ
D
G < 0, disfavored reaction (non-
DG = 0. The cal-
S values are both positive with 0.794 and
D
D
G > 0 or equilibrium reaction for
D
Fig. 3. FT-IR spectrum of the title compound.

668
Ç. Albayrak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
form of o-Hydroxy Schiff bases [8,28]. While the compound prefers
enol form in the solid state, it exists in both enol and keto form in
EtOH. The keto form is important in the solution and stabilized by
the polar solvents through solute and solvent interactions. Fig. S5
shows the possible structures of compounds in the solution.
N,N-diethylamino group is an electron-donating group. Before
keto form occurs, zwitter ionic form arises. The electron-donating
N,N-diethylamino group stabilizes zwitter ionic form and eases
the formation of keto form with resonance. If it depended on the
electron-donating N,N-diethylamino group, keto form would be
observed in all solvents. This shows that the solvent is also effec-
tive in the formation of keto form in addition to the electron-
donating N,N-diethylamino group. In addition, the formation of
keto form cannot be explained only by the polarity of solvent. If
it depended on the polarity of solvent, keto form would be ob-
served in DMSO. The fact that keto structure did not formed in
DMSO shows that the solvent polarity is not effective on the proton
transfer and instead it depends on the H-donor-acceptor property
of solvent. The title compound adopts both enol and keto forms in
EtOH which is a polar and protic solvent. Previous computational
studies also proved that protic solvents decrease activation energy
and ease proton transfer [17,29].
Fig. 4. The solvent effect on UV–Vis spectra of the title compound in (–ꢀ–) DMSO,
(—) EtOH, (----) THF, (ꢀ ꢀ ꢀꢀ ꢀ ꢀ) CHCl3, (–ꢀꢀ–) CH3CN, (–––) benzene.
respectively. The asymmetric and symmetric stretching vibrations
of the aliphatic CH2 and CH3 group were observed at 2972, 2932,
2896 and 2872 cmꢁ1. The bending vibrations of these groups were
observed at 1450, 1423 and 1390 cmꢁ1 while rocking modes were
observed at 1014 and 707 cmꢁ1. The C–O stretching vibrations in
phenols are expected to appear as a strongest band in 1300–
1100 cmꢁ1 frequency ranges [25]. The absorption band observed
at 1191 cmꢁ1 for the title compound was assigned to C–O stretching
mode of vibration. The absorption bands observed at 1600–
1400 cmꢁ1 are due to CC stretching vibrations of the aromatic com-
pounds [25]. The CC stretching modes of aromatic rings of the title
Conclusion
The conformational isomerism in (E)-2-[(4-bromophenylimino)
methyl]-5-(diethylamino)phenol compound was experimentally
investigated by X-ray diffraction, FT-IR and UV–Vis spectroscopic
techniques, and computationally by DFT method. In the crystal
structure, the compound has two conformers (anti and eclipsed)
related to the ethyl groups. The two conformers are connected to
each other by non-covalent C–Hꢀ ꢀ ꢀBr and C–Hꢀ ꢀ ꢀ
p interactions.
The combination of these interactions generate fused R22ð10Þ and
R42ð20Þ synthons which form a tape structure in the crystal packing.
The X-ray diffraction and FT-IR analyses also reveal the existence of
enol form in the solid state. From thermochemical point of view,
the computational investigation of isomerism includes three stud-
ies: The rate constants for transmission from anti or eclipsed con-
formations to transition state were calculated using Eyring
equation. The results show that the transmission rate to TS of
eclipsed conformer is more than that of anti conformer. Applying
Arrhenius equation for the activation energies needed for isomer-
ism, it is found that the eclipsed conformer of the title compound
is easily converted to the anti conformer (thermodynamically sta-
ble form) through a reversible reaction. The calculated equilibrium
constant for isomerism between two conformers is 0.369 in room
temperature, supporting the previous conclusion related to the
conversion of two conformers. The dependence of tautomerism
on solvent types was studied on the basis of UV–Vis spectra re-
corded in different organic solvents. The results showed that the
compound exists in enol form in DMSO, THF, CH3CN, benzene
and CHCl3 while both enol and keto forms are observed in EtOH.
compound were observed at 1599, 1573, and 1517, 1482 cmꢁ1
which agree well with the literature data.
,
UV–Vis absorption spectra
The UV–Vis spectra of the title compound in various organic
solvents (EtOH, DMSO, THF, CH3CN, benzene and CHCl3) were re-
corded within 200–500 nm range (Fig. 4). The absorption bands
at 356 nm (in THF, CH3CN, benzene and CHCl3) and 364 nm (in
DMSO) are attributed to
p ?
p⁄ transition. However, a new absorp-
tion band at 410 nm was observed in the spectrum of the title com-
pound in EtOH (Fig. 4), which was not observed in case of DMSO,
THF, CH3CN, benzene and CHCl3.
HOMO and LUMO called Frontier molecular orbitals (FMO) are
the most important ones in terms of their roles in reactions be-
tween molecules and in electronic spectra of a molecule [26].
The frontier molecular orbitals for both conformers are shown in
Fig. S4 for gas phase. HOMO and LUMO of both conformers are sim-
ilar. Therefore, it is expected that absorption bands of two con-
formers have same values. To prove this conclusion, the first 10
spin-allowed singlet-singlet excitations for both conformers were
calculated by TD-DFT approach. TD-DFT calculations started from
optimized geometry were performed for gas phase to calculate
excitation energies so that we can see if two conformers show
two absorption bands in the spectrum. The percentage contribu-
tions of molecular orbitals to formation of the bands were obtained
by using SWizard Program [27]. Considering TD-DFT calculations it
can be said that the excitation energy value at 355.09 nm arises
from HOMO ? LUMO (81%) transitions for both conformers since
HOMO and LUMO of two conformers are same.
Appendix A. 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 801848. Copies of the
data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk). The crystal packing (Fig. S1), cor-
relation graphic (Fig. S2), energy profile (Fig. S3), molecular orbital
surfaces (Fig. S4) and the possible structures of the title compound
(Fig. S5) are also provided.
The previous computational and experimental studies show
that the new absorption band above 400 nm belongs to the keto

Ç. Albayrak et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 664–669
669
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