Molecular structure, spectroscopic investigations, second-order nonlinear optical properties and intramolecular proton transfer of (E)-5-(diethylamino)-2- [(4-propylphenylimino)methyl]phenol: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.saa.2012.03.004

This work presents a combined experimental and theoretical study on an ortho-hydroxy Schiff base compound, (E)-5-(diethylamino)-2-[(4- propylphenylimino)methyl]phenol. The crystal structure and spectroscopic properties of the compound have been determined by using X-ray diffraction, IR and UV-vis spectroscopy techniques. The electronic structure, vibrational frequencies and electronic absorption spectra have been investigated from the calculative point of view. A relaxed potential energy surface scan has been performed based on the optimized geometry of OH tautomeric form to describe the potential energy barrier belonging to intramolecular proton transfer and to observe the effects of transfer on the molecular geometry. The second-order nonlinear optical properties have been investigated based on the first static hyperpolarizability (β) by using the density functional theory.

Full text of DOI:10.1016/j.saa.2012.03.004

Spectrochimica Acta Part A 93 (2012) 1–9
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Molecular structure, spectroscopic investigations, second-order nonlinear optical
properties and intramolecular proton transfer of
(E)-5-(diethylamino)-2-[(4-propylphenylimino)methyl]phenol: A combined
experimental and theoretical study
Bas¸ ak Kos¸ ara,∗, C¸ ig˘dem Albayraka, Cem Cüneyt Ersanlıb, Mustafa Odabas¸ og˘luc, Orhan Büyükgüngörd
a Department of Science Education, Sinop University, Sinop 57000, Turkey
b Department of Physics, Sinop University, Sinop 57000, Turkey
c Chemical Technology Program, Pamukkale University, Denizli 20070, Turkey
d Department of Physics, Ondokuz Mayıs University, Samsun 55139, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
This work presents a combined experimental and theoretical study on an ortho-hydroxy Schiff base
compound, (E)-5-(diethylamino)-2-[(4-propylphenylimino)methyl]phenol. The crystal structure and
spectroscopic properties of the compound have been determined by using X-ray diffraction, IR and
UV–vis spectroscopy techniques. The electronic structure, vibrational frequencies and electronic absorp-
tion spectra have been investigated from the calculative point of view. A relaxed potential energy surface
scan has been performed based on the optimized geometry of OH tautomeric form to describe the poten-
tial energy barrier belonging to intramolecular proton transfer and to observe the effects of transfer on
the molecular geometry. The second-order nonlinear optical properties have been investigated based on
the first static hyperpolarizability (ˇ) by using the density functional theory.
Received 23 December 2011
Received in revised form 23 February 2012
Accepted 3 March 2012
Keywords:
Density functional theory
Crystal structure
Second-order non-linear optical properties
Intramolecular proton transfer
IR and UV–vis spectra
Schiff base
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
compounds and the interest on this class of compounds continues
for many years. They have wide application in many fields, e.g., as
Molecules containing C N double bond are known as Schiff
bases and formed by a reaction of a primary amine and an alde-
hyde. Especially ortho-hydroxy Schiff base derivatives are the most
commonly studied class of Schiff bases because of their interest-
ing photochromic and thermochromic features in the solid state
[1]. These features are caused by an intramolecular proton trans-
fer associated with a change in ␲-electron configuration from
the hydroxyl O atom to the imine N atom occurring under the
influence of light for photochromic and temperature for ther-
mochromic Schiff bases and the potential energy profile of this
transfer shapes the photochromic and thermochromic features of
the compound [2]. These features were investigated first time in
1909 by Senier and Shepheard [3,4]. It has been suggested that
the molecules showing photochromism are non-planar and show-
ing thermochromism are planar [5,6]. Many published research
studies concern the related features of ortho-hydroxy Schiff base
ligands in the field of coordination chemistry [7], as starting mate-
rials in synthesis of important drugs like antibiotics, antiallergics,
antitumors and antifungals in pharmacy [8], as new organic materi-
als in nanotechnology [9] and as elements for the design of various
electronic devices such as optical switches and optical data storage
devices through the agency of their non-linear optical properties
[10].
Ortho-hydroxy Schiff bases exist either as OH (enol-imine) or
NH (keto-amine) tautomeric forms in solid state and these forms
can be distinguished easily by using the X-ray diffraction technique
[11]. NH form is a resonance hybrid of two canonical structures,
the zwitterionic and cis-quinoid forms [12]. The zwitterionic and
cis-quinoid forms are not just resonance structures of each other,
they can also exist separately. The zwitterionics can differ easily
from cis-quinoids with their N+ H bond distances and aromatic-
ity of the rings [13]. The position of transferred proton determines
the tautomeric form of compounds and the type of intramolecular
hydrogen bonds. Three types of intramolecular hydrogen bonds are
possible in ortho-hydroxy Schiff base compounds: O H· · ·N in OH
forms, N H· · ·O in cis-quinoid NH forms and N+ H· · ·O− in zwit-
terionic NH forms as can be seen from Fig. 1.
∗
Corresponding author at: Department of Science Education, Sinop University,
Sinop 57010, Turkey. Tel.: +90 368 2715526; fax: +90 368 2715530.
E-mail addresses: basak.kosar@gmail.com, bkosar@sinop.edu.tr (B. Kos¸ ar).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.03.004

2
B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
Fig. 1. Possible tautomeric forms of (E)-5-(diethylamino)-2-[(4-propylphenylimino)methyl]phenol.
In conjunction with the development of technology, among
recorded on a Bruker 2000 spectrometer in KBr disk. Absorption
spectra were recorded on a Unicam UV-Vis spectrometer using
a 1 cm path length of the quartz cell. All diffraction measure-
ments were performed at room temperature (296 K) using graphite
monochromated Mo K␣ radiation (ꢀ = 0.71073 A) with a Stoe IPDS
2 diffractometer.
the computational methods calculating the electronic structure of
molecular systems, DFT has been a favorite one due to its great
accuracy in reproducing the experimental values of in molecule
geometry, vibrational frequencies, atomic charges, dipole moment,
etc. [14]. In recent years, besides the experimental studies, an
increase is seen in quantum chemical computational studies with
DFT on tautomerism and intramolecular proton transfer process in
ortho-hydroxy Schiff base compounds [15–18].
˚
2.3. Crystal structure determination
A suitable sample of size 0.62 mm × 0.53 mm × 0.46 mm was
chosen for the single crystal X-ray study. Reflections were collected
in the rotation mode (␻ scanning mode) and cell parameters were
determined by using X-AREA software [19]. Absorption correction
(ꢁ = 0.07 mm−1) was achieved by the integration method via X-
RED32 software [19]. The structure was solved by direct methods
using SHELXS-97 [20]. The refinement was carried out by full-
matrix least-squares method using SHELXL-97 on the positional
and anisotropic temperature parameters of the non-hydrogen
atoms, or equivalently corresponding to 424 crystallographic
parameters [20]. All non-hydrogen atom parameters were refined
anisotropically and all H atoms except for H1a and H1b were refined
using a riding model. The C H bond distances of these H atoms were
fixed to 0.93 A for CH, 0.96 A for CH3 and 0.97 A for CH2 groups. The
Uiso values of H atoms were also fixed to 1.2 times Ueq value of par-
ent atom for CH and CH2 groups and 1.5 times Ueq value of parent
atom for CH3 groups. Under the condition of I > 2ꢂ(I) threshold, the
structure was refined to Rint = 0.072 with 3860 observed reflections.
Other data collection conditions and parameters of refinement pro-
cess are listed in Table 1.
Concerning with the above mentioned phenomena, we present
here the synthesis, crystallographic, spectroscopic and quan-
tum chemical computational studies on the newly synthesized
ortho-hydroxy Schiff base compound, (E)-5-(diethylamino)-2-[(4-
propylphenylimino)methyl]phenol.
2. Materials and methods
2.1. Synthesis of the title compound
The
compound
(E)-5-(diethylamino)-2-[(4-propylpheny-
limino)methyl]phenol was prepared by refluxing a mixture of
a solution containing 5-(diethylamino)-2-hydroxybenzaldehyde
(0.5 g, 2.59 mmol) in 20 mL ethanol and a solution containing
4-propylaniline (0.35 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-[(4-propylphenylimino)methyl]phenol suitable
for X-ray analysis were obtained by slow evaporation from acetone
(yield 65%; m.p. 353–355 K).
˚
˚
˚
2.2. Instrumentation
2.4. Supplementary data
The melting point was determined by an electro-thermal melt-
ing point apparatus. FT-IR spectrum of the title compound was
CCDC 772023 contains the supplementary crystallographic
data for this paper. These data can be obtained free of

B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
3
Table 1
package [23]. The calculations for geometry optimizations were
started with the crystallographically obtained coordinates of OH
tautomer and no imaginary frequency modes were obtained at the
optimized geometries for the tautomers, so there are true minima
on the potential energy surfaces were found for both. The vibra-
tional frequencies were calculated at the same level of theory and
the obtained frequencies were scaled by 0.9648 depends on the
method and basis set used [24]. To investigate the energetic behav-
iors and changes of the geometric parameters of the tautomers
in various environments, we performed optimizations in CHCl3
(ε = 4.9), and DMSO (ε = 46.7), too by using polarizable continuum
model (PCM) [25]. If there is no specific interaction such as hydro-
gen bond formation between the solute and solvent, this model
is able to valid approximations of solvent effects on the molecule
[26,27]. The absorption spectra of the tautomers were calculated
using time-dependent density functional theory (TD-DFT) method
started from the solution phase optimized geometries. A relaxed
potential energy surface (PES) scan was performed based on the
optimized geometry of the OH tautomeric form by varying the
redundant internal coordinate (O1 H1 bond distance) from 1.00
to 1.70 A with 15 steps of 0.05 A in order to describe the potential
energy barrier belonging to the intramolecular proton transfer and
to observe the effects of transfer on the molecular geometry. In the
scan process, all the remaining internal coordinates were fully opti-
mized. With this kind of calculation it is possible to observe step by
step the changes in the molecular geometry. In order to show the
non-linear optical (NLO) activity of the molecule, the linear polar-
izability and the first static hyperpolarizability were obtained by
molecular polarizabilities with the same level of theory.
Crystal data and refinement parameters of (E)-5-(diethylamino)-2-[(4-
propylphenylimino) methyl]phenol.
CCDC
Formula
772023
C20H26N2O
310.43
Yellow
Orthorhombic
Pna21
Formula weight
Crystal color
Crystal system
Space group
Z
a, b, c
˛, ˇ, ꢃ
V
8
˚
24.1689 (8), 8.0549 (4), 18.7206 (6) A
90◦
3
˚
3644.5 (2) A
Radiation type
ꢁ
Tmax, Tmin
Mo K␣
0.07 mm−1
0.970, 0.855
23,624
7544
3860
Measured reflections
Independent reflections
Reflections with I > 2ꢂ(I)
Number of parameters
ꢄmax, ꢄmin
Scan range
R[F2 > 2ꢂ(F2)]
wR(F2)
424
26.5◦, 1.7◦
−30 < h < 30, −10 < k < 9, −23 < l < 23
0.047
0.113
0.880
˚
˚
S
−3
˚
ꢅꢆmax, ꢅꢆmin
0.10, −0.18e A
charge via www.ccdc.cam.ac.uk/data request/cif, by emailing
data request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336033.
2.5. Computational details
In computational procedure, the geometry optimizations of
tautomers of the title compound in gas phase and in different
solvents leading to energy minima were performed using the ana-
lytical gradient methods of DFT with Becke’s three parameters (B3)
exchange functional together with the Lee–Yang–Parr (LYP) non-
local correlation functional, symbolized B3LYP [21,22] by means
of 6-31+G(d,p) basis set implemented in Gaussian 03W software
3. Results and discussion
3.1. Crystal structure, optimized geometries of tautomers and
intramolecular proton transfer process
An ORTEP-3 [28] plot of the asymmetric unit of title compound
is shown in Fig. 2. Displacement ellipsoids are drawn at the 30%
Table 2
Selected geometric parameters for (E)-5-(diethylamino)-2-[(4-propylphenyl imino)methyl]phenol.
Parameters
X-ray
DFT/B3LYP/6-31+G(d,p)
OH
Molecule A
Molecule B
NH
Gas
CHCl3
DMSO
Gas
Bond lengths (Å)
C2 C7
C7 N1
C1 O1
N1 C8
N2 C5
C3 C4
C5 C6
O1 H1
1.419 (4)
1.295 (4)
1.365 (4)
1.415 (4)
1.352 (4)
1.352 (4)
1.404 (4)
0.930 (3)
–
1.442 (4)
1.276 (4)
1.350 (4)
1.411 (4)
1.353 (4)
1.357 (4)
1.407 (4)
0.930 (4)
–
1.438
1.298
1.345
1.406
1.379
1.381
1.412
1.000
–
1.438
1.301
1.351
1.408
1.374
1.380
1.417
1.005
–
1.438
1.392
1.337
1.273
1.403
1.379
1.365
1.396
–
1.303
1.355
1.409
1.372
1.381
1.418
1.007
–
N1 H1
1.049
Bond angles (◦)
C1 O1 H1
C6 C1 O1
C2 C7 N1
C7 N1 C8
107.0 (2)
117.2 (3)
123.7 (3)
123.1 (3)
122.8 (3)
121.4 (3)
108.0 (3)
119.6 (3)
123.6 (3)
119.7 (3)
121.3 (3)
122.6 (3)
107.2
106.8
106.6
–
118.2
122.5
121.3
122.0
121.8
118.4
122.2
121.4
122.1
121.9
118.5
122.1
121.3
122.1
122.0
122.1
122.3
128.3
122.8
121.3
C5 N2 C17
C5 N2 C19
Torsion angles (◦)
C18 C17 N2 C5
C20 C19 N2 C5
C11 C14 C15 C16
C3 C2 C7 N1
87.2 (4)
88.3 (4)
−177.8 (4)
178.7 (3)
177.8 (3)
−157.9 (3)
−89.6 (4)
−85.2 (4)
−177.1 (4)
−177.3 (3)
179.5 (3)
−89.3
−87.7
−89.0
−87.7
−88.9
−87.8
−90.1
−87.8
−179.8
−178.6
−176.9
−147.6
−179.9
−178.8
−176.9
−148.8
−179.4
−178.6
−176.9
−148.9
179.9
−179.9
−179.0
−172.8
C2 C7 N1 C8
C7 N1 C8 C9
−141.5 (3)

4
B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
Fig. 2. Atom numbering scheme of (E)-5-(diethylamino)-2-[(4-propylphenyl imino)methyl]phenol with the relative positions of the two independent molecules. Dashed
lines indicate the intramolecular hydrogen bonds.
probability level and hydrogen atoms are shown as small spheres
of arbitrary radii.
intramolecular hydrogen bonds can be seen in Table 3. The crys-
tal structure is stabilized by weak van der Waals interactions in
three-dimensional network.
The asymmetric unit of title compound contains two indepen-
dent identical molecules, labeled A and B. The imine frames and
the six-membered rings (C1/C6 and C8/C13) of the molecules can
be seen in Fig. 2. The molecular geometry of the title compound
is non-planar with the dihedral angle between the rings of 24.50◦
for molecule A and 42.13◦ for molecule B. According to the value of
dihedral angle, the title compound displays photochromic features
as mentioned in introduction. Selected geometrical parameters
both molecule A and B are listed in Table 2.
As can be seen from their values in Table 2, the C7 N1 double,
C2 C7 single and C1 O1 single bonds point out that the compound
adopts the OH tautomeric form rather than the NH form in the solid
state. These bond lengths are in agreement with the corresponding
values in the literature [29,30]. In the OH tautomeric form, both
rings of compound must be aromatic (see Fig. 1). In order to provide
the further verification of OH form, we calculated the harmonic
oscillator model of aromaticity (HOMA) indices for both rings by
using following equation [31,32]:
Besides observed geometrical parameters, B3LYP/6-31+G(d,p)
calculated geometrical parameters in gas phase for OH and NH
forms and in various solvents for OH form were listed in Table 2.
There are no significant differences between the experimental
and calculated geometrical parameters of OH form, except for
the torsion angles controlling the planarity of molecular geom-
etry. The dihedral angles between the rings are 36.59◦ for gas
phase optimized OH form and 8.12◦ for gas phase optimized NH
form according to the DFT calculations. The differences which
observed between the geometries of counterparts are caused by
underestimating the intermolecular interactions. DFT and similar
calculations cannot take the intermolecular hydrogen bond for-
mations into account, and consider molecules in the gas phase
(in vacuo), but in real, the experimental results belong to the solid
state consists of interacting molecules.
For a visual comparison, superimposition of the experimental
(black) and theoretical (gray) molecule skeletons with r.m.s. devi-
˚
ation of 0.782 A is shown in Fig. 3.
ꢀ
ꢂ
n
ꢁ
We have investigated the intramolecular proton transfer in the
gas phase for the title compound by performing a PES scan process
at the B3LYP/6-31+G(d,p) level so as to take a closer look to its
effects on the molecular geometry. Process was started from the
gas phase optimized OH geometry by assigning the O1 H1 bond
as redundant internal coordinate. The relative energy versus the
redundant coordinate O1 H1 bond distance in PES scan process
can be seen in Fig. 4. The relative energy values are calculated with
respect to the energy of stable OH tautomer.
1
n
2
HOMA = 1 −
˛i(Ri − Ropt
)
(1)
i=1
where n is the number of bonds in the molecular fragments inter-
ested (in our case n is equal to 6 for the six-membered rings), ˛i
˚
normalization constant is equal to 257.7 and Ropt is equal to 1.388 A
for C C bonds. The HOMA index is equal to 1 for the purely aromatic
systems and equal to 0 for non-aromatics [33,34]. It is expected
that the HOMA index of C1/C6 and C8/C13 rings should be in the
range of 0.800–0.990 if the rings are aromatic and 0.500–0.700 if
the rings are non-aromatic. While the calculated indexes of C1/C6
rings are 0.898 for molecule A and 0.782 for molecule B, indexes
of C8/C13 rings are 0.929 for molecule A and 0.915 for molecule
B. This result also indicates that the independent molecules of the
title compound show OH tautomerism in solid state.
Table 3
Hydrogen bonding geometry of (E)-5-(diethylamino)-2-[(4-propylphenylimino)
methyl]phenol.
D
H· · ·A
D
H (Å)
H· · ·A (Å)
D· · ·A (Å)
D
H· · ·A (◦)
The title compound displays strong intramolecular interac-
tions including the atoms O1 and N1 as a common feature
of o-hydroxysalicylidene systems [35,36]. The geometries of
O1a H1a· · ·N1a
O1b H1b· · ·N1b
0.93 (3)
0.93 (4)
1.75 (3)
1.78 (4)
2.595 (3)
2.603 (4)
149 (3)
145 (4)

B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
5
Fig. 3. Superimpositions of the experimental (black) and theoretical (gray) geometries of the title compound.
˚
˚
C2 C7 (1.438 A) and C1 O1 (1.345 A) belonging to stable OH tau-
˚
tomer were found as 1.337, 1.391 and 1.272 A at the fourteenth
step of scan where the stable NH tautomer was observed. The fig-
ure clearly indicates that the intramolecular proton transfer affects
the double and single characters of these indicative bonds.
The HOMA indexes of C1/C6 and C8/C13 rings were calculated
at every step of the scan process and Fig. 6 shows the changes of
HOMA indexes in terms of scan coordinate. It is seen from the figure
that the aromaticity level of C1/C6 ring is decreasing with the scan
˚
˚
coordinate going from 1.00 A to 1.70 A. On the other hand, HOMA
index of C8/C13 ring is nearly constant at 0.955 and the ring holds
its aromaticity during the process, as expected.
3.2. Vibrational frequencies
FT-IR spectrum of the title compound was given for comparison
with the calculated IR spectrum in Fig. 7.
The first remarkable absorption band in experimental spec-
Fig. 4. Relative energy versus the redundant coordinate in PES scan process.
trum is in 2000–3000 cm−1 region and can be attributed to the
O
H stretching. It is well known that the intramolecular and inter-
As seen in Fig. 4, there are two minima representing the stable
forms. While the first and deeper one is a global minimum and cor-
responds to stable OH form, second one is a local minimum and
corresponds to stable NH form of the title compound. The potential
energy barrier was determined as 4.957 kcal mol−1 for the transi-
tion from the OH tautomeric form to the NH tautomeric form.
The effects of the intramolecular proton transfer on the molec-
ular geometry can be seen better via examining the changes in
HOMA index of C1/C6 ring and indicative bond lengths for every
steps in the scan process. Fig. 5 reflects that the changes occurred
in the lengths of C7 N1 double, C2 C7 single and C1 O1 single
molecular hydrogen bond formations affect the O H stretching
vibration. Due to the formation of strong intramolecular hydrogen
bond between the O1 and N1 atoms, the experimental-based vibra-
tional frequency of O H stretching is shifted and widened to the
2000–3000 cm−1 range. At this point, a notable difference is seen
in the figure for the O H stretching. The corresponding theoretical
value (3012 cm−1) is very sharp because the intra- and intermolec-
ular hydrogen bonds formations have not taken into account by
the DFT calculations. The O H in plane and out of plane bending
vibrations are located at 1342 cm−1 and 790 cm−1, respectively. The
second one is located at 1623 cm−1 and can be attributed to C
N
˚
bonds during the transfer process. Bond lengths of C7 N1 (1.298 A),
Fig. 6. HOMA indexes of C1/C6 and C8/C13 rings versus the redundant coordinate
Fig. 5. Indicative bond lengths versus the redundant coordinate in PES scan process.
in PES scan process.

6
B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
Fig. 7. The comparison of the experimental IR spectrum (black) with theoretical IR spectrum (blue). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
stable than the NH form by 3.451 and 0.439 kcal mol−1, respec-
tively. It is not surprising because the OH form has two aromatic
rings correspond to a more delocalized ␲-electronic system and
generally o-hydroxy salicylideneanilines are observed as OH tau-
tomeric form. But the calculations also point out that the NH form
of compound is more stable than the OH form by 1.066 kcal mol−1
in DMSO by contrast with the situations in CHCl3 and gas phase. In
fact, according to the DFT the NH form is favored this time. So, keto
form is easily stabilized by the polar solvents and this result is in
accordance with the literature [40].
stretching. The spectrum shows also the presence of C O stretching
at 1243 cm−1 cm. The absorption bands located at 2971–2865 cm−1
region are belonging to aliphatic C H stretching. The other remark-
able vibrational frequencies and scaled computational vibrational
frequencies of compound can be seen and compared in Table 4.
These frequency values are in good agreement with the literature
[37]. The DFT-based IR results show a good agreement with the
experimental values except for the O H stretching.
3.3. Energetic behavior and electronic absorption spectra
The UV–vis electronic spectra of title compound in CHCl3 and
DMSO were recorded within 200–500 nm range at the room tem-
perature. The theoretical electronic excitation energies, oscillator
strengths and nature of the first 10 spin-allowed singlet–singlet
excitations were calculated by the TD-DFT method for the same
solvents. The major contributions of the transitions were desig-
nated with the aid of SWizard program [41]. The experimental
and calculated results of UV–vis spectral data are compared in
Table 6. In addition, the characteristic UV–vis absorption bands of
title molecule can be seen in Fig. 8.
The interactions between the solute and solvent molecules
influence the molecular geometry, vibrational frequencies, total
energy, electronic spectrum, etc. [38,39]. So, the solvents are good
helpers for the investigation of compounds showing tautomerism.
Table 5 reflects the total energies, frontier molecular orbital ener-
gies and dipole moments of both tautomeric forms of the title
compound in gas phase and various solvents.
It is clear from Table 4 that while the calculated total ener-
gies of tautomers decreases, the dipole moments of tautomers
increases with the increasing polarity of solvent. The energy gap
(ꢅE) between the highest occupied molecular orbital (HOMO) and
the lowest-lying unoccupied molecular orbital decreases for the
OH form and increases for the NH form when an increase occurs
in polarity of solvent. In CHCl3 and gas phase OH form is more
In the UV–vis spectra of ortho-hydroxy Schiff base compounds,
the presence of an absorption band at <400 nm indicates the OH
Table 5
Calculated energies, frontier orbital energies and dipole moments for (E)-5-
(diethylamino)-2-[(4-propylphenylimino) methyl]phenol in gas phase and various
solvents.
Table 4
The experimental and calculated vibrational frequencies of IR spectra (cm−1).
Gas
CHCl3
DMSO
Assignmentsa
Experimental
DFT/B3LYP/6-31+G(d,p)
OH
ꢇ
aromatic(C H)
3062–3021
3000–2000
2971–2865
1623
1581–1520
1449–1427
1342
1243
1132
826
790
3087–3055
3012
2991–2914
1607
1551–1504
1452–1443
1376
1289
1142
910
858
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢅE (eV)
ꢁ (D)
−962.6041
−1.4988
−5.2673
3.7685
−962.6141
−1.5916
−5.2991
3.7075
−962.6189
−1.6471
−5.3351
3.6880
ꢇ(O H)
aliphatic(C H)
ꢇ(HC N)
aromatic(C C)
aliphatic(C H)
ꢇ
ꢇ
␥
3.7517
4.9356
5.5424
␥(O H)
ꢇ(C O)
NH
Etotal (a.u.)
ELUMO (eV)
EHOMO (eV)
ꢅE (eV)
ꢁ (D)
−962.5986
−1.8291
−5.2904
3.4613
−962.6134
−1.9211
−5.4238
3.5026
−962.6206
−1.9815
−5.5064
3.5249
␥
aromatic(C H)
␨
aromatic(C H)
␨(O H)
3.3589
4.9209
5.8578
a
ꢇ: stretching, ␥: bending and ␨: out of plane bending.

B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
7
Fig. 8. The comparison of the experimental UV–vis spectra (black) with calculated transitions (blue). (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
tautomeric form. On the other hand, compounds adopt NH tau-
tomeric form show a new absorption band at >400 nm [42,43].
It is clear from the Table 6 and Fig. 8 that there is no absorption
band observed at >400 nm region in any solvent for the title com-
pound. This behavior can be explained with the pure enol-imine
tautomeric form of the molecule even in solution. According to
the experimental absorption spectra, for all of the solvents, the
molecule exhibits absorption bands at near 350 and 240 nm. The
results obtained by the TD-DFT calculations show the absorption
bands at near 370 and 280 nm. In view of calculated absorption
spectra of OH form, the maximum absorption wavelength cor-
responds to the electronic transition from the highest occupied
molecular orbital HOMO to lowest unoccupied molecular orbital
LUMO with 84% contribution. The other wavelengths with major
contributions can be seen in Table 6.
Natural Bond Orbital (NBO) analysis is a helpful tool for
understanding of delocalization of electron density from occupied
Lewis-type (donor) NBOs to properly unoccupied non-Lewis type
(acceptor) NBOs within the molecule [44]. Table 7 displays the
second-order perturbation theory analysis of Fock matrix in NBO
basis for the OH form of compound and it is clear that the ␲ → ␲*
interactions were common in the molecule. Then, we can say that
the electronic transitions are mainly derived from the contribu-
tion of bands ␲ → ␲*. The most important result obtained from the
experimental and calculated electronic absorption spectra is the
absence of band associated with the NH tautomeric form.
Table 6
Experimental and calculated electronic transitions in gas phase and various solvents.
Experimental
DFT/B3LYP/6-31+G(d,p)
ꢀ (nm) [A]a
ꢀ (nm) [f]b
Major contributionsc
Gas
–
356 [1.169]
312 [0.014]
H → L (81%)
–
H-1 → L (80%)
H → L + 3 (9%)
–
CHCl3
350 [0.791]
246 [0.105]
371 [1.336]
284 [0.015]
H → L (84%)
H → L + 1 (77%)
H-3 → L (17%)
H → L + 2 (77%)
H-1 → L (6%)
3.4. Second-order non-linear optical (NLO) properties
216 [0.170]
273 [0.054]
261 [0.016]
The quantum chemistry based prediction of non-linear opti-
cal properties of a molecule has an important role for the design
of materials in modern communication technology, signal pro-
cessing and optical interconnections [45]. Organic molecules are
commonly studied because of their larger NLO susceptibilities aris-
ing ␲-electron cloud movement from donor to acceptor, fast NLO
response times, high laser damage thresholds and low dielectric
constants. Although the organic molecules have these advantages,
they have several NLO disadvantages, too: they have generally low
thermal stability and they may undergo a facile relaxation to ran-
dom orientation [46]. In addition, in the UV–vis region, the low
energy transitions result in a trade-off between the non-linear effi-
ciency and optical transparency [47,48]. But the usage of organic
molecules as ligands can overcome these disadvantages. Schiff base
compounds are the commonly investigated class of them because
they are widely used as ligands [49].
H-3 → L (68%)
H → L + 1 (18%)
H-2 → L + 1 (6%)
H-4 → L (46%)
H → L + 5 (17%)
H → L + 4 (14%)
249 [0.058]
DMSO
356 [1.016]
246 [0.111]
373 [1.347]
284 [0.014]
H → L (84%)
H → L + 1 (78%)
H-3 → L (17%)
H → L + 2 (75%)
H-2 → L (6%)
274 [0.059]
262 [0.016]
H-1 → L (5%)
H-3 → L (32%)
H → L + 4 (28%)
H → L + 3 (24%)
H → L + 1 (7%)
H-4 → L (46%)
H → L + 5 (13%)
H → L + 4 (11%)
248 [0.059]
The total static dipole moment ꢁtot, the average linear polar-
izability ˛¯ and the first hyperpolarizability ˇ can be calculated by
using the Eqs. (2), (3) and (4), respectively [45]:
a
A: absorbance.
f: oscillator strength.
H: HOMO; L: LUMO.
b
ꢁtot = (ꢁ2x + ꢁy2 + ꢁ2z )1/2
(2)
c

8
B. Kos¸ar et al. / Spectrochimica Acta Part A 93 (2012) 1–9
Table 7
Selected second-order perturbation energies E(2) associated with i → j delocalization in gas phase for OH form.
b
Donor (i)
Acceptor (j)
E(2) a (kcal mol−1
)
εj − εi (a.u.)
F(i, j)c (a.u.)
BD(2) C1 C6
BD(2) C1 C6
BD(2) C2 C3
BD(2) C2 C3
BD(2) C2 C3
BD(2) C4 C5
BD(2) C4 C5
BD(2) C7 N1
BD(2) C12 C13
BD(2) C12 C13
BD(2) C8 C9
BD(2) C8 C9
BD(2) C8 C9
BD(2) C10 C11
BD(2) C10 C11
LP(1) N1
BD*(2) C2 C3
BD*(2) C4 C5
BD*(2) C1 C6
BD*(2) C4 C5
BD*(2) C7 N1
BD*(2) C2 C3
BD*(2) C1 C6
BD*(2) C8 C9
BD*(2) C8 C9
BD*(2) C10 C11
BD*(2) C7 N1
BD*(2) C12 C13
BD*(2) C10 C11
BD*(2) C12 C13
BD*(2) C8 C9
BD*(1) O1 H1
BD*(2) C1 C6
11.79
26.46
25.28
14.09
27.89
31.09
12.43
10.77
19.43
18.98
10.56
19.21
20.50
20.59
21.01
45.37
37.90
0.28
0.27
0.27
0.26
0.26
0.27
0.27
0.36
0.28
0.29
0.26
0.28
0.28
0.27
0.27
0.52
0.33
0.053
0.079
0.074
0.056
0.078
0.083
0.053
0.060
0.067
0.067
0.048
0.066
0.068
0.067
0.068
0.149
0.105
LP(2) O1
a
Stabilization energy.
b
Energy difference between i (donor) and j (acceptor) NBO orbitals.c Fock matrix element of i and j NBO orbitals.
Table 8
Donmez et al. [51]. In −o3u0r present work, their values are 3.751 D,
Calculated dipole moments (D), polarizability (Å3) and the first hyperpolarizabil-
ity (×10−30 esu) components for (E)-5-(diethylamino)-2-[(4-propylphenylimino)
methyl]phenol.
3
3
˚
˚
esu for OH form and 3.358 D, 48.842 A ,
47.169 A , 40.059 × 10
19.367 × 10−30 esu for NH form. If we recall Table 7, ␲-electron
cloud movement must be responsible for the non-linear optical
properties of the tautomers. According to the high values of the first
hyperpolarizability of the tautomeric forms, the title compound can
really be a good applicant in the development of NLO materials.
Components
OH
NH
1.0548
ꢁx
ꢁy
ꢁz
−2.8685
−2.2887
0.7790
−3.0713
0.8579
˛
˛
80.8582
1.6397
86.1568
1.1587
xx
4. Conclusions
xy
˛
˛
˛
˛
35.5604
−1.2749
0.2576
25.0896
44.3237
−1.7955
−3.7174
−0.9624
−0.8781
0.8747
36.0159
−1.8246
−1.3018
24.3544
23.6349
−4.3662
−4.4285
−0.9377
−0.7466
0.8671
yy
xz
In this study, we have synthesized a Schiff base compound,
(E)-5-(diethylamino)-2-[(4-propylphenylimino)methyl]phenol
and characterized by spectroscopic and structural techniques as
well as the theoretical methods. X-ray, FT-IR and UV–vis methods
confirm the OH form of the title compound. The PES scan process
in gas phase shows that the potential barrier is 4.957 kcal mol−1
to transition from the OH tautomeric form to the NH tautomeric
form. This intramolecular proton transfer affects the molecular
geometry by changing the aromaticity of C1/C6 ring and lengths
of the indicative bonds. In addition, according to the calculated
results, the increased polarity of solvent brings about proton
transfer in intramolecular hydrogen bonding. And finally, the high
values of the first hyperpolarizabilities of tautomers show that the
title compound is a good applicant for NLO.
yz
zz
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
ˇxyz
ˇyyz
ˇxzz
ˇyzz
ˇzzz
0.2671
−0.0187
−0.7428
−0.5185
0.2111
−0.6844
−0.5447
0.2681
1
3
˛¯ = (˛xx + ˛yy + ˛zz
)
(3)
(4)
We hope the results of this study will help researchers to design
and synthesis new materials.
1/2
2
2
ˇ = [(ˇxxx + ˇxyy + ˇxzz
)
+ (ˇyyy + ˇxxy + ˇyzz
)
+ (ˇzzz + ˇxxz + ˇyyz)2]
In order to investigate the NLO properties of the title com-
pound, the components of dipole moment, polarizability and the
first hyperpolarizability have been calculated using polar = ENONLY
input to Gaussian03 at the level of B3LYP/6-31+G(d,p) in gas
phase. The components and calculated values of total static dipole
moment ꢁ, the average linear polarizability ˛¯ and the first hyper-
polarizability ˇ are listed in Table 8. The calculated values of
components show difference depend on the size of basis sets
used. But it is difficult to decide which basis set generates the
reliable values because there are no reported experimental val-
ues for the title compound in literature. For this reason, we used
the basis set 6-31+G(d,p) as in all this work. The calculated ˇ
components have been converted into electrostatic units (esu)
(1 a.u. = 8.6393 × 10−33 esu).
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