Spectroscopic, molecular structure characterizations and quantum chemical computational studies of (E)-5-(diethylamino)-2-[(2-fluorophenylimino)methyl] phenol

Article abstract of DOI:10.1016/j.molstruc.2010.09.030

The molecular structure and spectroscopic properties of (E)-5-(diethylamino)-2-[(2-fluorophenylimino)methyl]phenol were characterized by X-ray diffraction, IR and UV/Vis spectroscopy. These properties of title compound were also investigated from calculat

Full text of DOI:10.1016/j.molstruc.2010.09.030

Journal of Molecular Structure 984 (2010) 214–220
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic, molecular structure characterizations
and quantum chemical computational studies of
(E)-5-(diethylamino)-2-[(2-fluorophenylimino)methyl]phenol
b
Çig˘dem Albayrak a, , Rene Frank
⇑
a Faculty of Education, Department of Science Education, Sinop University, 57000 Sinop, Turkey
b Faculty of Chemistry and Mineralogy, Leipzig University, D-04103 Leipzig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The molecular structure and spectroscopic properties of (E)-5-(diethylamino)-2-[(2-fluorophenylimino)-
methyl]phenol were characterized by X-ray diffraction, IR and UV/Vis spectroscopy. These properties of
title compound were also investigated from calculative point of view. Geometry optimization in gas
phase was performed using DFT method with B3LYP applying 6-311G(d,p) basis set and geometry opti-
mizations in solvent media were performed with the same level of theory by the polarizable continuum
model (PCM). TD-DFT calculations starting from optimized geometry were carried out in both gas and
solution phase to calculate excitation energies of title compound. In addition, while the non-linear optical
properties were computed, thermodynamic properties were obtained at the optimized geometry with the
same level of theory. The intramolecular proton transfer process from enol form to keto form was inves-
tigated using DFT method with B3LYP applying 6-311G(d,p) basis set. Transition state structure in EtOH
was performed with the same level of theory by the polarizable continuum model (PCM).
Ó 2010 Elsevier B.V. All rights reserved.
Received 4 August 2010
Received in revised form 15 September
2010
Accepted 15 September 2010
Available online 25 September 2010
Keywords:
Schiff base
DFT
TD-DFT
Computational study
Non-linear optical properties
Intramolecular proton transfer
1. Introduction
vents. The absorption band at greater than 400 nm was found to
belong to the keto form of o-hydroxy Schiff bases [8,9].
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 the intramolecular proton transfer
from an oxygen atom to the neighboring nitrogen atom. Intramo-
lecular proton transfer mechanism of both excited state and
ground state is still the subject of intensive research. On the basis
of thermochromic and photochromic Schiff bases, it was proposed
that molecules exhibiting thermochromism are planar, while mol-
ecules exhibiting photochromism are non-planar [1,2]. Photochro-
mic 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-
In recent years, among the computational methods to calculate
the electronic structure, excitation energies of molecular systems,
DFT and TD-DFT are of particular interest steadily owing to give
satisfactory results with experiment by costing low computational
demands among the computational methods calculating [10,11].
A new (E)-5-(diethylamino)-2-[(2-fluorophenylimino)methyl]-
phenol compound was synthesized and its spectroscopic properties
were studied both experimental and theoretical insight. The crystal
structure of title Schiff base compound was determined by single
crystal X-ray diffractometry. The structure of (E)-5-(diethyl-
amino)-2-[(2-fluorophenylimino)methyl]phenol was experimen-
tally characterized by IR, UV/Vis spectroscopy, investigated by
using density functional theory DFT and excitation energies were
carried out using TD-DFT calculations starting from optimized
geometry. Also, intramolecular proton transfer process of (E)-
5-(diethylamino)-2-[(2-fluorophenylimino)methyl]phenol was in-
vestigated for ground state in gas phase and EtOH.
2. Experimental and computational methods
2.1. Synthesis
⇑
The compound (E)-5-(diethylamino)-2-[(2-fluorophenylimino)
methyl]phenol was prepared by refluxing a mixture of a solution
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 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.09.030

Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
215
containing 5-(diethylamino)-2-hydroxybenzaldehyde (Alfa Aesar,
3. Results and discussion
99%, 0.5 g, 2.59 mmol,) in 20 mL ethanol (Kimetsan, 96%) and a
solution containing 2-fluoroaniline (Alfa Aesar, 99%, 0.29 g,
2.59 mmol) in 20 mL ethanol. The reaction mixture was stirred
for 1 h under reflux. The crystals of (E)-5-(diethylamino)-2-[(2-flu-
orophenylimino)methyl]phenol suitable for X-ray analysis were
obtained by slow evaporation from acetone (Alfa Aesar, 99.5+%,
yield 76%; m.p. 367–369 K).
3.1. Structure determination
The crystal data and refinement details of (E)-5-(diethylamino)-
2-[(2-fluorophenylimino)methyl]phenol compound are given in
Supplementary data. Selected bond lengths and angles are given
in Table 1. The molecular structure of title compound is shown
in Fig. 1 with the atom numbering scheme. As it can be seen in
Fig. 1, the molecule adopts E configuration about the C@N double
bond.
2.2. Instrumentation
o-Hydroxy Schiff bases show tautomerism by intramolecular
proton transfer from oxygen atom to nitrogen atom. As a result
of this, o-hydroxy Schiff bases can exist in two tautomeric struc-
tures as enol and keto forms in the solid state. As shown Fig. 2,
depending on their forms, two types of intramolecular hydrogen
bonds are possible (a) OAHꢁ ꢁ ꢁN in enol form and (b) NAHꢁ ꢁ ꢁO in
keto form. For title compound, enol form is favoured over the keto
form in the solid state. The C7AN1 bond length of 1.2939(17) Å and
C2AO1 bond length of 1.3542(16) Å are consistent with the dis-
tances of the CAN double bond and the CAO single bond as pre-
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 solvents used for UV/
Vis studies are benzene (Carlo Erbaa, 99.8+%), CHCl3 (Alfa Aesar,
99.5+%) DMSO (Alfa Aesar, 99.9+%).
2.3. X-ray crystallography
A suitable sample of 0.2 ꢀ 0.2 ꢀ 0.2 mm3 size was selected for
sented in related compounds previously studied [20–22].
A
significant intramolecular interaction is noted involving phenolic
atom O1 and nitrogen atom N1 and constitutes a six-membered
ring S(6) [23]. The O1ꢁ ꢁ ꢁN1 distance of 2.5778(14) Å is indicative
of strong intramolecular hydrogen bonding (Table 2). This length
is clearly shorter than the sum of the van der Waals’ radii for N
and O [24].
While N2AC4 bond length is 1.3689(17) Å, N1AC8 bond length
is 1.4117(16) Å. N2 atom has two unpaired electrons. The contrac-
tion of N2AC4 bond length shows that the unpaired electrons of
nitrogen atom do not reside entirely on nitrogen atom but are
the crystallographic study. All diffraction measurements were per-
formed at 130 K using graphite monochromated Mo K
a radiation
and a Xcalibur diffractometer. Absorption correction was achieved
by Semi-empirical from equivalents. The structure was solved by
direct methods using SHELXS-97 [12]. The refinement was carried
out by full-matrix least-squares method on the positional and
anisotropic temperature parameters of the non-hydrogen atoms.
All non-hydrogen atom parameters were refined anisotropically
and all H atoms were located in a difference Fourier map, and their
coordinates and Uiso(H) values were refined freely. The data col-
lection conditions and parameters of refinement process are listed
in Supplementary data.
spread over the aromatic ring, overlap with
ring.
p electron of aromatic
The intermolecular hydrogen bonded geometry and details of
CAHꢁ ꢁ ꢁO interactions for the title compound are listed in Table 2.
The molecules are linked by intermolecular CAHꢁ ꢁ ꢁO interactions
2.4. Computational procedure
and CAHꢁ ꢁ ꢁ
p interactions between p electrons of aromatic ring
and aromatic hydrogen as shown in Fig. 3.
All computations were performed using Gaussian 03W pro-
gram package running under Windows XP [13]. Full geometry
optimization of the title molecule was performed by using DFT
method with Becke’s three-parameters hybrid exchange–correla-
tion functional (B3LYP) [14] employing 6-311G(d,p) basis set
[15–17] as implemented in Gaussian 03W. Crystallographically
obtained geometrical data of the molecule was used for the
optimization. Total molecular energy, and dipole moment were
obtained from the optimization output. The ground state geom-
etry optimization of the title compound for gas phase was calcu-
lated at using DFT method with B3LYP adding 6-311G(d,p).
Solution phase geometry optimizations were performed with
the same level of theory by the polarizable continuum model
(PCM). The polarizable continuum model (PCM) family of solva-
tion models is among the most widely used [18,19]. TD-DFT cal-
culations starting from gas phase and solution phase optimized
geometry using same level of theory were carried out in both
gas and solution phase to calculate excitation energies of title
compound. Calculated electronic transitions of the title com-
pound were obtained by TD-DFT excited state calculation. Also
gas-phase vibrational frequencies for title compound were per-
formed, the non-linear optical properties were computed and
thermodynamic properties were obtained at the optimized
geometry with the same level of theory. The ground state geom-
etry optimizations of the title compound in gas phase and solu-
tion phase for TS structure were performed with the same level
of theory.
The optimized parameters for title compound were obtained by
using the B3LYP/6-311G(d,p). Selected bond lengths and angles for
the optimized structure and X-ray geometry of the molecule are
listed in Table 1. Superimposition of X-ray and optimized struc-
tures of title compound is shown in Supplementary data. As it
can be seen from Table 1, C2AO1 and C7AN1 distances for enol
form at the optimized geometry are 1.338 Å and 1.295 Å. N2AC4
distance for optimized structure is 1.379 Å. The results obtained
from the optimization for enol form are in agreement with the
experimental ones. But the experimental bond lengths are slightly
different from optimization ones. These differences are because the
theoretical calculations are performed for gas phase while experi-
mental results belong to solid phase.
3.2. Non-linear optical (NLO) properties
The non-linear optical properties play an important role for de-
sign of materials in modern communication technology, signal pro-
cessing and optical switchs and optical memory devices [25]. The
organic molecules that contain
p electron cloud movement from
donor group to acceptor group are known as non-linear optical
materials. The donor and acceptor groups have an important role
in the polarizability and first hyperpolarizability. If the donor and
acceptor groups are powerful, delocalization of
p electron cloud
of molecules increases and as a result of this, the polarizability

216
Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
Table 1
The selected bond lengths, angles and torsion angles (Å, °).
X-ray
DFT/B3LYP enol form
DFT/B3LYP keto form
N1AC7
O1AC2
N1AC8
C1AC7
C1AC2
C2AC3
C3AC4
C4AC5
C5AC6
C1AC6
N2AC4
F1AC13
C7AN1AC8
N1AC7AC1
N1AC8AC9
N1AC8AC13
O1AC2AC1
O1AC2AC3
C8AN1AC7AC1
C7AN1AC8AC9
C7AN1AC8AC13
C7AC1AC2AO1
C6AC1AC2AO1
C14AN2AC4AC5
1.2939(17)
1.3542(16)
1.4117(16)
1.4301(18)
1.4150(18)
1.3795(18)
1.4078(19)
1.4195(19)
1.3667(19)
1.4101(18)
1.3689(17)
1.3629(16)
121.66(11)
121.52(12)
126.25(12)
117.52(12)
120.14(11)
118.56(12)
178.35(11)
ꢂ2.4(2)
1.295
1.338
1.397
1.435
1.425
1.392
1.409
1.426
1.376
1.408
1.379
1.349
121.59
122.37
124.78
118.20
121.51
118.11
177.10
ꢂ33.42
149.62
0.15
1.341
1.258
1.395
1.383
1.476
1.429
1.390
1.451
1.358
1.428
1.379
1.351
127.35
122.86
124.88
117.88
121.05
122.25
ꢂ179.29
4.06
ꢂ176.12
ꢂ0.29
179.46
ꢂ178.77
177.96(12)
1.80(19)
ꢂ179.27(12)
ꢂ179.83
173.46(12)
179.84
Table 2
Hydrogen bonding geometry (Å,°).
DAHꢁ ꢁ ꢁA
DAH
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
hDAHꢁ ꢁ ꢁA
O1AH18ꢁ ꢁ ꢁN1
0.950(19) 1.692(18) 2.5778(14) 153.6(13)
0.964(15) 2.509(15) 3.4590(18) 168.5(12)
0.940(15) 2.971(15) 3.8018(16) 148.0(12)
C11AH11ꢁ ꢁ ꢁO1i
C10AH10ꢁ ꢁ ꢁCg1ii
Calculated O1AH18ꢁ ꢁ ꢁN1 0.994
1.735
2.629
147.49
Symmetry code: (i) 1/2 ꢂ x, ꢂ1/2 + y, 3/2 ꢂ z; (ii) 3/2 ꢂ x, ꢂ1/2 + y, 3/2 ꢂ z.
Cg1 is the centroid of C1–C6 ring.
The total static dipole moment
l, the average linear polarizabil-
Fig. 1. A view of title compound, with the atom numbering scheme.
ity
a, and the first hyperpolarizability b can be calculated by using
the Eqs. (1), (2), (3), respectively [25].
1=2
¼ ðl2x
þ
ly2
þ
lz2
Þ
ð1Þ
ð2Þ
and first hyperpolarizability of organic molecules increase [26].
Quantum chemical calculations can be used to describe the rela-
tionship between the electronic structure of molecules and their
l
a
¼ 1=3ðaxx
þ
ayy
þ
azz
Þ
non-linear optical properties. The increase of
p electron delocaliza-
tion of molecules and the strengthen of donor and acceptor groups
increase polarizability and first hyperpolarizability. The energy gap
between HOMO and LUMO is important in getting polarizability of
a molecule [27]. The molecules having a small energy gap are more
polarizable than molecules having a large energy gap. Also, UV–Vis
spectra can be used to correlate with polarizability. A small
HOMO–LUMO gap means small excitation energy. Absorption
bands of molecules having a small energy gap are shifted toward
the visible.
h
2
2
b ¼ ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ bxxy þ byzz
Þ
ꢀ
i
1=2
2
þ
bzzz þ bxxz þ byyz
Þ
ð3Þ
The dipole moment, polarizability and the first hyperpolariz-
ability were calculated using polar = ENONLY at the level of
B3LYP/6-311G(d,p) and the results obtained from calculation were
given in Table 3.
Fig. 2. Enol and keto tautomeric forms of title compound.

Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
217
Fig. 3. A partial packing diagram for title compound, with CAHꢁ ꢁ ꢁ
(ii) 3/2 ꢂ x, ꢂ1/2 + y, 3/2 ꢂ z].
p
and CAHꢁ ꢁ ꢁO hydrogen bonds shown as dashed lines [symmetry codes: (i) 1/2 ꢂ x, ꢂ1/2 + y, 3/2 ꢂ z;
C0p;m ¼ 5:49806 þ 0:24645T ꢂ 4:18785 ꢀ 10ꢂ5T2; R2 ¼ 0:99935
Table 3
Calculated dipole moments (D), polarizability and first hyperpolarizability
components (a.u.) for the title compound.
S0m ¼ 62:6245 þ 0:30408T ꢂ 6:86279 ꢀ 10ꢂ5T2; R2 ¼ 0:99992
H0m ¼ ꢂ0:04742 þ 0:00948T þ 1:12845 ꢀ 10ꢂ4T2; R2 ¼ 0:99999
lx
ly
lz
ꢂ3.24
ꢂ2.92
0.02
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
bxzz
byzz
bzzz
5397.4
ꢂ673.5
ꢂ365.2
ꢂ82.7
46.9
axx
axy
ayy
axz
ayz
azz
454.2
ꢂ4.2
198.2
10.5
ꢂ113.3
11.1
3.4. IR spectroscopy
ꢂ112.2
ꢂ28.7
18.8
ꢂ3.5
IR spectrum of the title compound was shown in Supplemen-
tary data. The strong intramolecular hydrogen bonding occurs in
title compound. The absorption band in 2000–3000 cmꢂ1 region
116.5
is attributed to the t(OAH) stretching frequencies which broaden
The calculated dipole moment
l
tot, polarizability
a
and first
owing to the formation of strong intramolecular hydrogen bonding
hyperpolarizability b for title compound are 4.36 D, 37.94 Å3 and
43.06 ꢀ 10ꢂ30 cm5/esu respectively. The first hyperpolarizability
of title compound is greater than those of 2-methyl-6-[2-[trifluo-
rormethyl]phenyliminomethyl]phenol [28], 4-(2,3,4-trihydroxyb-
enzylidene amino)antipyrine [29]. The energy gap between
HOMO and LUMO of title compound is 3.75 eV for gas phase.
Absorption band of title compound is at 380 nm. The decrease of
the energy gap between HOMO and LUMO shows that title com-
pound has a smaller energy gap than related compounds [28,30]
and as a result of this, absorption band is shifted toward the visible.
These results show that title compound can be used as a good non-
linear optical material.
OAHꢁ ꢁ ꢁN. The absorption band at 1629 cmꢂ1 is attributed to
t
(C@N) stretching. The IR spectrum of the molecule shows also
the presence of t
(CAF) stretching at 1132 cmꢂ1. These values are
in accordance with the literature [31].
The vibrational frequencies of the title compound were calcu-
lated by using the same level of theory. The scale factor of
0.9682 was applied to vibrational frequencies [32]. Vibrational
bands have been made by using Gaussview. The experimental
and the calculated vibrational frequencies are given in Supplemen-
tary data. The results obtained by the calculation for title
compound have shown a satisfactory consistency with the experi-
mental data.
3.3. Thermodynamic properties
3.5. UV/Vis absorption spectra
The heat capacity (Cp0;m), entropy (Sm0 ) and enthalpy (H0m) that are
the standard thermodynamic functions were performed using DFT/
B3LYP method with 6-311G(d,p). The results obtained from the ba-
sis of vibrational analysis are shown in Supplementary data. The
heat capacities, entropies and enthalpies were obtained by increas-
ing temperature from 100 K to 500 K. As results, increase of tem-
perature increases heat capacities, entropies and enthalpies due
to increasing intensities of molecular vibration.
We have calculated the total energy, dipole moment and fron-
tier molecular orbital energies by using DFT/B3LYP method with
6-311G(d,p) basis set by adding the polarizable continuum model
(PCM) in solvent media. Results obtained from used solvents hav-
ing different polarities are shown in Table 4. As it is shown in Table
4, the total energies decrease with increase of the polarity of the
solvent, in other words stability of molecule increases. While the
energy gap (DE) between the highest occupied molecular orbital
The correlation equations between heat capacities, entropies,
enthalpies and temperature are shown below and can be used
for analyzing heat capacities, entropies and enthalpies in different
temperature.
(HOMO) and the lowest-lying unoccupied molecular orbital
(LUMO) decreases, dipole moment increases with increase of the
polarity of the solvent. HOMO and LUMO called Frontier molecular
orbitals (FMO) are the most important orbitals because they play

218
Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
Table 4
Calculated total energies, frontier orbital energies and dipol moments of title compound for enol and keto form.
Gas phase
Benzene
CHCl3
EtOH
DMSO
Enol form
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢂ944.064280
ꢂ1.60
ꢂ944.070248
ꢂ1.64
ꢂ944.074691
ꢂ1.68
ꢂ944.079151
ꢂ1.72
ꢂ944.079682
ꢂ1.74
ꢂ5.35
ꢂ5.36
ꢂ5.38
ꢂ5.40
ꢂ5.41
D
E (eV)
3.75
3.72
3.70
3.68
3.67
l
(D)
4.36
5.10
5.66
6.24
6.32
Keto form
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢂ944.057234
ꢂ1.36
ꢂ944.065392
ꢂ1.93
ꢂ944.071571
ꢂ1.97
ꢂ944.077830
ꢂ2.01
ꢂ944.078580
ꢂ2.02
ꢂ3.42
ꢂ5.41
ꢂ5.49
ꢂ5.55
ꢂ5.56
D
E (eV)
2.06
3.48
3.52
3.54
3.54
l
(D)
4.28
5.15
5.88
6.68
6.78
important roles in reactions between molecules and in UV/Vis
spectra of a molecule. The energy gap between HOMO and LUMO
is important to influence the stability of a molecule and determines
the chemical reactivity, kinetic stability, polarizability and chemi-
cal hardness–softness of a molecule [27]. The molecules having a
large energy gap are stable molecules as chemical. Hard molecules
have a large energy gap and they are more stable than soft mole-
cules having a small energy gap. Because soft molecules have a
small energy gap, they are more polarizable and more reactive
than hard molecules. The energy gap between HOMO and LUMO
of title compound is 3.75 eV for gas phase. The energy gap between
HOMO and LUMO of title compound is smaller than that of related
compounds [28,30]. This result indicates that title compound is
more polarizable, more reactive and more soft molecule than re-
lated compound. The frontier molecular orbitals for enol and keto
forms of the title molecule are shown in Supplementary data.
Also, the first 10 spin-allowed singlet–singlet excitations for ti-
tle compound were calculated by TD-DFT approach. TD-DFT calcu-
lations were started from gas phase and solution phase optimized
geometry using same level of theory and carried out in both gas
and solution phase to calculate excitation energies. The percentage
contributions of molecular orbitals to formation of the bands were
extracted from output by using SWizard Program [33]. For title
compound, wavelength (k), oscillator strength (f) larger than 0.5,
major contributions of calculated transitions are given in Table 5.
The UV/Vis spectra of title compound in various organic sol-
vents (EtOH, DMSO, benzene and CHCl3) were recorded within
200–500 nm range. The characteristic UV/Vis absorption bands
of the molecule in EtOH, DMSO, benzene and CHCl3 are given in
Table 5. The UV/Vis spectra are shown in Supplementary data.
The absorption band is observed at 380 nm in DMSO, CHCl3 and
absorption bands are not observed in DMSO, CHCl3 and benzene.
The theoretical calculations and experimental studies show that
the new absorption bands belong to the keto form of o-hydroxy
Schiff bases [8,30]. In EtOH, title compound exists both enol
and keto form. This can not be explained as the polarity of sol-
vent. If it depended on the polarity of solvent, keto form would
have observed 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. EtOH is a polar and protic solvent
and this property causes both enol and keto forms to be stable
in solvent.
3.6. Intramolecular proton transfer process
DFT calculations with B3LYP adding 6-311G(d,p) have been per-
formed to investigate the tautomeric equilibrium for both enol
form and keto form. This compound in the solid state exists enol
form. Selected geometric parameters for enol, keto and TS struc-
tures are given in Table 6. The tautomerization energies in gas
phase and EtOH were calculated as a difference in energies be-
tween enol form and TS structure. Fig. 4 is a schematic representa-
tion of the potential energy curve at ground state indicating enol,
keto and TS structure in gas phase and EtOH. For title compound
enol form is more stable than keto form in gas phase. The keto form
energy is found at 4.42 kcal/mol above enol form. The activation
energy needed to overcome the barrier of intramolecular proton
transfer reaction is 6.2 kcal/mol in gas phase. The geometric opti-
mizations of enol and keto forms and TS structure in EtOH were
performed with the same level of theory by the polarizable contin-
uum model (PCM). The energies of enol, keto and TS structures
were calculated to determine the effect of solvent on tautomeriza-
tion. As seen in Fig. 4, enol form in EtOH is also more stable than
benzene. This transition is attributed
p ? p transition. However,
the compound shows new absorption bands in EtOH but these
Table 5
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
Benzene
CHCl3
–
353.87 (1.0421)
369.59 (1.2361)
370.4 (1.2321)
H ? L (80%)
H ? L (84%)
H ? L (84%)
366.53 (1.102)
423.36 (0.016)
383.12 (1.2872)
421.54 (0.0296)
382.08 (1.2905)
415.32 (0.0245)
–
379.63 (1.2808)
408 (0.021)
382.61 (1.3084)
408.06 (0.0263)
H–1 ? L(74%), H ? L(+5%)
H ? L(+79%), H–1 ? L(8%)
H–1 ? L(79%)
H–L(83%) H–1 ? L(6%)
H–1 ? L(65%) H ? L(18%)
H ? L(67%) H–1 ? L(21%)
378
378
EtOH
358
388
430
386
369.37 (1.2161)
–
–
H ? L (83%)
–
–
H ? L(79%)
H–1 ? L(87%)
H ? L(80%)
DMSO
372.33 (1.2464)
H ? L (84%)
H–1 ? L(87%)

Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
219
Table 6
Selected bond lengths (Å) and angles (°), energies and dipole moments for enol, keto and TS structures.
Gas phase
Enol
EtOH
Enol
TS
Keto
TS
Keto
Etotal (a.u.)
(D)
ꢂ944.064280
4.36
ꢂ944.054402
4.01
ꢂ944.057234
4.28
ꢂ944.079151
6.24
ꢂ944.069245
6.03
ꢂ944.077830
6.68
l
N1AH18
O1AH18
N1ꢁ ꢁ ꢁO1
N1AC7
1.735
0.994
2.629
1.295
1.195
1.300
2.421
1.322
1.043
1.711
2.592
1.341
1.709
0.998
2.615
1.298
1.229
1.256
2.418
1.318
1.036
1.755
2.622
1.338
C1AC7
1.435
1.402
1.383
1.434
1.409
1.388
O1AC2
1.338
1.289
1.258
1.348
1.308
1.273
C1AC2
1.425
1.454
1.476
1.425
1.447
1.470
C2AC3
1.391
1.411
1.429
1.389
1.403
1.422
C3AC4
1.409
1.399
1.390
1.415
1.409
1.399
C4AC5
1.426
1.441
1.451
1.428
1.439
1.449
C5AC6
1.376
1.366
1.358
1.376
1.369
1.360
N1AH18AO1
H18AO1AC2
H18AN1AC8
147.49
107.47
99.31
151.96
104.15
105.41
139.14
104.62
111.59
148.84
106.79
99.39
153.38
103.76
104.77
138.40
103.90
112.17
does not have an important role during reaction. But if there are
significant changes in charge distribution during the reaction the
solvent plays a more important role. If the TS is more polar than
reactants it will be stabilized by polar solvents [34]. As seen in
Table 6, the results obtained from calculation show that enol form
is more polar than TS. Because enol form is more polar than TS, it
will be stabilized by polar solvents. Energies of both enol form and
TS structure decrease with the increase of polarity of solvent but
decrease of energy for enol form is more than that for TS. So energy
needed to overcome the barrier between enol form and TS slightly
increase with increase of polarity of solvent. Because energy of
enol form in gas phase and all solvents is less than that of keto
form, enol form is favored. UV/Vis spectra of title compound
showed the tautomeric equilibrium favored the enol form in ben-
zene, CHCl3 and DMSO. But in EtOH title compound tends to shift
toward the keto form. Experimentally, while in EtOH new absorp-
tion bands 358 nm and 430 nm are observed, in benzen, CHCl3 and
DMSO it is not. This result clearly shows that not only the polarity
of solvent but also hydrogen donor–acceptor properties of solvent
influence the equilibrium. The results obtained from theoretical
calculations show that the strong hydrogen bond interaction de-
creases activation energy between enol form and TS [35,36]. Ben-
zene and CHCl3 are solvents with low polarity and DMSO is polar
and aprotic solvent but EtOH is polar and protic solvent acting as
both hydrogen donor and acceptor. Because intramolecular proton
transfer from enol form to keto form occurs in Transition state,
protic solvents as EtOH can facilitate proton transfer. Therefore,
molecules are strongly solvated by EtOH. TS structure of title com-
pound will be stabilized by hydrogen bond involving EtOH and
activation energy needed to overcome the barrier of intramolecular
proton transfer between enol form and TS structure will decrease.
The decrease of activation energy experimentally causes the for-
mation of keto form in EtOH.
Fig. 4. Schematic energy profile of intramolecular proton transfer in gas phase and
EtOH for ground state. Energies relative to enol form in EtOH are given in kcal/mol.
4. Conclusions
keto form. The energy gap between enol form and keto form
decreases from gas phase to solution phase. This shows that tauto-
merization in polar solvent occurs more easily than in gas phase.
The dipole moment of keto form is less than that of enol form in
gas phase but the dipole moment of keto form is more than that
of enol form in benzene, CHCl3 EtOH and DMSO (Table 4). Keto
form is easily stabilized by solvent and as a result of this, the en-
ergy of keto form decreases. The stability of TS structure and the
energy gap between enol form and TS structure slightly increases
from gas phase to EtOH due to dielectric constant of solvent. If
nonpolar reactant generates a nonpolar transition state, solvation
The compound (E)-5-(diethylamino)-2-[(2-fluorophenylimino)-
methyl]phenol was experimentally characterized by means of IR
and UV/Vis spectroscopy techniques. It has been determined that
this compound is in enol form in the solid state both on the basis
of X-ray and IR spectroscopic data. UV/Vis spectra of the title com-
pound were recorded in different organic solvents. The results
show that molecule exists only enol form in DMSO, CHCl3 and
benzene but in EtOH molecule exists as both enol and keto form.
Also, DFT/B3LYP optimization was performed based on X-ray
Geometry. TD-DFT calculations starting from optimized geometry

220
Ç. Albayrak, R. Frank / Journal of Molecular Structure 984 (2010) 214–220
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uum model (PCM). Solvent effects on intramolecular proton trans-
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Appendix A. Supplementary material
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Crystallographic data for the structure in this paper have been
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the supplementary publication no. CCDC 776183. 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). Supplementary data associated
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