A new 2,2′-oxydianiline derivative symmetrical azomethine compound containing thiophene units: Synthesis, spectroscopic characterization (UV–Vis, FTIR, 1H and 13C NMR) and DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2018.05.021

In this study, a new symmetrical azomethine compound, N,N′-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine OPBTS (5), having two thiophene rings and N, O donor groups, was successfully prepared by a simple condensation reaction of 2-hydroxy-5-(thiophen-2-yl)benzaldehyde (3) and 2,2′-oxydianiline (4). Characterization of OPBTS was performed by the analysis of UV–Vis., FTIR, ¹H and ¹³C NMR spectroscopic results and elemental analysis. The optimized molecular geometry, sum of electronic and thermal free energies (SETFEs), dipole moment, IR frequencies, ¹H and ¹³C NMR chemical shift values, UV–Vis. spectroscopic parameters, HOMO-LUMO energies, molecular electrostatic potential (MEP) map and atomic charges of OPBTS were calculated by using Density Functional Theory (DFT/B3LYP/6-311 + G(d, p)) method in the gas phase and various solvents. The theoretical results were compared to the experimentally obtained data and all results were found to be compatible. The experimental and theoretical results confirmed the proposed molecular structure for the new synthesized bis-azomethine derivative (OPBTS).

Full text of DOI:10.1016/j.molstruc.2018.05.021

Accepted Manuscript
A new 2,2'-oxydianiline derivative symmetrical azomethine compound containing
1
thiophene units: Synthesis, spectroscopic characterization (UV-Vis, FTIR, H and
13
C NMR) and DFT calculations
Emel Ermiş, Kaan Durmuş, Ömer Uğur Aygüzer, Halil Berber, Mustafa Güllü
PII:
S0022-2860(18)30581-7
10.1016/j.molstruc.2018.05.021
MOLSTR 25193
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 13 March 2018
Revised Date: 2 May 2018
Accepted Date: 7 May 2018
Please cite this article as: E. Ermiş, K. Durmuş, Ö.Uğ. Aygüzer, H. Berber, M. Güllü, A new 2,2'-
oxydianiline derivative symmetrical azomethine compound containing thiophene units: Synthesis,
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spectroscopic characterization (UV-Vis, FTIR, H and C NMR) and DFT calculations, Journal of
Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.05.021.
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A new 2,2'-oxydianiline derivative symmetrical azomethine compound containing
thiophene units: Synthesis, spectroscopic characterization (UV-Vis, FTIR, 1H and 13C
NMR) and DFT calculations
Emel Ermişa, *, Kaan Durmuşa, Ömer Uğur Aygüzera, Halil Berbera, Mustafa Güllüb
aAnadolu University, Faculty of Science, Department of Chemistry, 26470 Eskişehir, Turkey
bAnkara University, Faculty of Science, Department of Chemistry, 06100 Ankara, Turkey
*Corresponding author: Emel Ermiş; Tel.: +90 222 335 0580; Fax: +90 222 320 4910
E-mail address: eermis@anadolu.edu.tr
ABSTRACT
In this study, a new symmetrical azomethine compound, N,N'-oxydiphenylenebis(5-
(thiophen-2-yl)salicylidenimine OPBTS (5), having two thiophene rings and N, O donor
groups, was successfully prepared by a simple condensation reaction of 2-hydroxy-5-
(thiophen-2-yl)benzaldehyde (3) and 2,2'-oxydianiline (4). Characterization of OPBTS was
1
performed by the analysis of UV–Vis., FTIR, H and 13C NMR spectroscopic results and
elemental analysis. The optimized molecular geometry, sum of electronic and thermal free
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13
energies (SETFEs), dipole moment, IR frequencies, H and C NMR chemical shift values,
UV-Vis. spectroscopic parameters, HOMO-LUMO energies, molecular electrostatic potential
(MEP) map and atomic charges of OPBTS were calculated by using Density Functional
Theory (DFT/B3LYP/6-311+G(d, p)) method in the gas phase and various solvents. The
theoretical results were compared to the experimentally obtained data and all results were
found to be compatible. The experimental and theoretical results confirmed the proposed
molecular structure for the new synthesized bis-azomethine derivative (OPBTS).
Keywords: 2-Hydroxy-5-(thiophen-2-yl)benzaldehyde; 2,2'-Oxydianiline; Vibrational
spectroscopy; NMR chemical shifts; Density Functional Theory (DFT).
1. Introduction
The azomethine compounds, which are also called Schiff bases or imines, constitute an
important class of compounds exhibiting biological activity, used as intermediates in organic
synthesis, ligands for coordination complexes and materials for organic electronics [1-8].
These compounds are easily accessible by a one step condensation reaction between carbonyl

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compounds and amines [9-11]. In addition, they are obtained under milder reaction conditions
without any requirement of the purification because water is the only byproduct [8, 11, 12].
The azomethine compounds containing various 5-membered aryl heterocycles, especially
thiophene derivatives, are encouraging materials for optoelectronic applications, and some
derivatives of these compounds can be synthesized in good yields with a simple, economic
and environmentally friendly method [7, 12-14]. Thiophene derivatives are considered to be
promising building blocks for most organic functional materials such as electrochromic
devices (ECD), organic light-emitting diodes (OLED), organic photovoltaic (OPV) cells, and
organic field effect transistors (OFET), nonlinear optics (NLO) and supercapacitors [11, 15-
20]. Moreover, azomethine derivatives bearing thiophene ring have been extensively
investigated, because these compounds exhibit a variety of medicinal and biological activities
such as anti-inflammatory, antibacterial, analgesic, anticancer and antimicrobial [21-25]. For
this reason, the design and synthesis of new molecules including the thiophene ring have
received a great deal of attention.
Quantum chemical calculations are one of the modern tools used to estimate the various
physical and chemical properties of molecules in biological and chemical systems. In
addition, these calculation methods widely contributes to the spectroscopic studies performed
experimentally [3, 21, 26-29]. The Density Functional Theory (DFT) has been intensively
employed for the calculation of various properties of some thiophene derivatives such as
molecular structure, UV-Vis, FT-IR and NMR spectra, HOMO-LUMO energies, charge
distributions and nonlinear optical (NLO) behaviors. In many studies with such compounds,
reliable calculation results consistent with experimental data have been obtained [3, 21, 22,
26, 30-32].
In this study, the synthesis of a novel 2,2'-oxydianiline derivative symmetrical
azomethine compound, N,N'-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine OPBTS
(5), containing N, O donor groups and two thiophene rings (Scheme 1) was reported. The
1
chemical structure of OPBTS was experimentally characterized by UV-Vis, FTIR, H NMR
and 13C NMR spectroscopic methods and elemental analysis. The DFT method at the B3LYP
level with the 6-311+G(d, p) basis set was used to calculate the optimized molecular
geometry, IR frequencies, 1H and 13C NMR chemical shifts, UV-Vis spectra, HOMO-LUMO
energies, molecular electrostatic potential (MEP) map and atomic charges of OPBTS
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molecule. The experimental data of OPBTS obtained from UV-Vis, FTIR, H and C NMR
spectra were compared to the calculated results obtained from the DFT method.

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2. Experimental
2.1. Materials and methods
5-Bromosalicylaldehyde, bis(triphenylphosphine)palladium (II) dichloride [Cl2Pd(PPh3)2], 2-
tributylstannylthiophene and 2,2'-oxydianiline were commercially obtained from Sigma-
Aldrich and Across. All reagents and organic solvents were used without further purification.
The melting points (m.p.) of the synthesized compounds was determined by using a Sanyo
Gallenkamp melting point apparatus. Electronic absorption spectra of OPBTS in various
solvents such as toluene, chloroform (CHCl3), tetrahydrofuran (THF), dichloromethane
(DCM), acetone, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were attained
by using SHIMADZU UV-3150 UV-VIS-NIR spectrophotometer. Fourier Transformed
Infrared (FTIR) spectroscopic analysis was performed with KBr pellet by a Perkin Elmer
1
Spectrum 100 FT-IR spectrometer. H (400 MHz) and 13C (100 MHz) Nuclear Magnetic
Resonance (NMR) spectra were recorded on an Agilent DD2 NMR (400 MHz) Spectrometer.
13
1H and C chemical shifts were measured relative to tetramethylsilane in DMSO-d6, spectral
splitting; s, singlet; d, doublet; dd, double doublet; t, triplet; q, quartet; m, multiplied; and br,
broad. The elemental analysis of the synthesized compounds was determined by an
ELEMENTAR VARIO ELIII elemental analyzer (CHNS).
2.2. Synthesis
2.2.1. Synthesis of 2-hydroxy-5-(thiophen-2-yl)benzaldehyde (3):
The compound (3) was synthesized according to the procedure described in the literature [20].
5-Bromosalicylaldehyde (1) (2.847 g, 14.16 mmol) and bis(triphenylphosphine)palladium (II)
dichloride [Cl2Pd(PPh3)2] (0.497 g, 0.708 mmol) were placed in a 100 mL two-necked round-
bottomed flask and they were dissolved in anhydrous dimethylformamide (DMF, 25 mL)
under nitrogen atmosphere. Then, 2-tributylstannylthiophene (2) (6.75 mL, 21.24 mmol) was
added via a dropping funnel. The reaction mixture was stirred at 80 °C for 16 h. After the
reaction completed, the mixture was cooled and added a saturated NH4Cl (100 mL). The
organic phase was then collected by diethyl ether extraction (Et2O, 5x25 mL), washed with
water, dried over anhydrous MgSO4, and then filtered. The crude product was washed with
hexane (3x20 mL) and purified by column chromatography using silica-gel solid phase and a
mixture of dichloromethane: hexane (1:6) as eluent. The yellow product (3) was obtained in
53% (1.495 g) yield, (m.p. 106-109 °C). FTIR (KBr, cm-1) 3601-3440 (νO-H), 3104-2921 (νAr
C-H), 2852 ( νAld C-H), 1667 (νC=O), 1649-1479 (νAr C=C), 1280-1171 (νC-O), 824-717 (νC-S-C),
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698. H NMR (400 MHz, DMSO-d6) δ (ppm) 10.88 (s, 1H), 10.26 (s, 1H), 7.80 (dd, 1H, J =

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7.2, J= 2.4 Hz), 7.77 (d, 1H, J= 2.4), 7.45 (d, 1H, J= 5.0 Hz), 7.38 (d, 1H, J= 3.6 Hz), 7.06
(dd, 1H, J = 5.0, J= 3.6 Hz), 7.02 (d, 1H, J= 8.4 Hz). 13C NMR (100 MHz, DMSO-d6) δ (ppm)
191.39, 160.65, 142.75, 133.79, 128.92, 125.90, 125.50, 125.40, 123.43, 122.87, 118.61.
Elemental anal. calcd. for C11H8O2S: C, 64.69; H, 3.95; O, 15.67; S, 15.70%. Found: C,
64.46; H, 3.99; S, 15.32%.
2.2.2. Synthesis of N,N'-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine, OPBTS (5):
A solution of 2-hydroxy-5-(thiophen-2-yl)benzaldehyde (3) (1.34 g, 2.0 mmol) in ethanol (30
mL) was added to a solution of 2,2’-oxydianiline (2-aminophenyl ether) (4) (0.66 g, 1.0
mmol) in ethanol (20 mL) with continuous stirring, and the mixture was refluxed for 24 h.
The completion of the reaction was monitored through TLC. The reaction mixture was then
cooled to room temperature and ethanol was removed. The obtained residue was washed with
ethanol and dried at room temperature. The product was recrystallized from the mixture of
ethanol:kloroform (1:2). The yield of orange product OPBTS was 46% (1.72 g), (m.p. 172-
175 °C). FTIR (KBr, cm-1) 3436.04 (νO-H phenolic), 3268.00-3120.01 (νAr C-H), 1621.17 (νC=N),
1
1575.90-1489.00 (νAr C=C), 816.62 and 681.50
(
νC-S-C). H NMR (400 MHz, DMSO-d6) δ
(ppm) 13.27 (s, 1H, ArO-H), 9.02 (s, 1H, -CH=N), 7.82 (d, 1H, J= 2.4 Hz, Ar-H), 7.61 (dd,
1H, J= 8.4, J=2.4 Hz, Ar-H), 7.55 (dd, 1H, J= 7.4 Hz, J=1.8 Hz, Ar-H), 7.40 (dd, 1H, J= 5.2
Hz, J=1.2 Hz, Th-H), 7.30 (dd, 1H, J= 3.6 Hz, J=1.2 Hz, Th-H), 7.26-7.23 (m, 2H, Ar-H),
7.04 (dd, 1H, J= 5.2 Hz, J=3.6 Hz, Th-H), 6.94 (dd, 1H, J= 8.0 Hz, J=1.6 Hz, Ar-H), 6.87 (d,
1H, J= 8.8 Hz, Ar-H). 13C NMR (100 MHz, DMSO-d6) δ (ppm) 163.73, 160.46, 150.20,
143.07, 138.89, 130.94, 129.48, 128.91, 128.85, 125.53, 125.08, 123.03, 120.58, 119.82,
119.48, 117.92. Elemental anal. calcd. for C34H24N2O3S2: C, 71.31; H, 4.22; N, 4.89; O, 8.38;
S, 11.20%. Found C, 70.86; H, 4.21; N, 4.90; S, 10.87%.
3. Theoretical calculations
In the present study, ChemDraw Professional 16 Individual ASL SN Win [33],
Gaussian 09 program [34] and GaussView 5.0.8 [35] were used to perform DFT level
calculations on a personal computer. The theoretical calculations for OPBTS were carried out
using DFT (Density Functional Theory)/B3LYP (Becke's three parameter exact exchange-
functional (B3) combined with gradient-corrected correlational functional of Lee, Yang, Parr
(LYP)) method with the 6-311+G(d, p) basis set [36, 37]. The geometry of the most stable
conformation was optimized for OPBTS with the Gaussian09 program DFT/B3LYP method

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at 6-311+G(d, p) level. The optimized geometry was employed to calculate the frequencies in
1
13
the gas phase and solvent media at functional B3LYP method. H and C nuclear magnetic
resonance (NMR) chemical shift values of the studied compound were computed using the
standard GIAO/TMS B3LYP/6-311+G(d, p) (Gauge-Independent Atomic Orbital) approach
in the gas phase and in DMSO with the Gaussian 09 package. The UV-Vis spectra
calculations of OPBTS were performed by using in the TD-DFT calculation at the B3LYP
level with 6-311+G(d, p) basis set in the gas phase and various solvents (toluene, CHCl3,
THF, DCM, acetone, DMF and DMSO). In addition, the sum of electronic and thermal free
energy (SETFE), dipole moment, the HOMO and LUMO energy values, Mulliken atomic
charges and molecular electrostatic potential (MEP) map for OPBTS were calculated by
DFT/B3LYP method at the same basis set.
4.
Results and discussion
4.1. Synthesis
In this study, 2-hydroxy-5-(thiophen-2-yl)benzaldehyde (3) as the basic precursor was
synthesized according to the procedure in the literature [20]. Then, the synthesis of N,N'-
oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine, OPBTS (5) was accomplished in
ethanol by condensation reaction of 2-hydroxy-5-thiophen-2-ylbenzaldehyde (3) and 2,2’-
oxydianiline (4) (Scheme 1).
Scheme 1. Synthesis of N,N'-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine, OPBTS (5).

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The elemental analysis results of OPBTS (anal. calcd. for C34H24N2O3S2: C, 71.31; H, 4.22;
N, 4.89; O, 8.38; S, 11.20%. Found C, 70.86; H, 4.21; N, 4.90; S, 10.87%) confirmed the
proposed molecular formula as C34H24N2O3S2. In addition, the chemical structure of OPBTS
1
was identified by UV-Vis, FTIR and H-13C NMR spectroscopic methods. The geometric,
electronic and spectroscopic (UV-Vis, IR and NMR) behaviors for OPBTS were also
investigated by the DFT calculations.
4.2. Geometry analysis
In this study, the proposed conformational structures (5a-5k) for OPBTS were shown in
Fig. 1. In order to confirm the favoured structure, the calculations were performed by
Chem3D program at dihedral drive-single plot command. The relative energies of 5a-5k were
calculated using MM2 command and the results were given Table 1. According to the
calculations, the most favourable structure was found to be as 5a (35.4694 kcal/mol). Three-
dimensional optimized structures of 5a-5k were also shown in Fig. S1 (Supplementary
material). (For conformational structure analysis, the scan command in Gaussian 09 program
was not preferred in this study).
Fig. 1. The structures of 5a-5k conformations for N,N'-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine,
OPBTS (5).
Table 1. The calculated relative energies of 5a-5k conformations for OPBTS.
5a
5b
5c
5d
5e
5f
5g
5h
5i
5k
35.4694 35.7487 48.4475 48.7439 48.7776 49.1158 52.0229 54.2631 66.1060 71.3708
E(kcal/mol)
*

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The optimized molecular structure with B3LYP/6-311+G(d, p) level of The calculated
relative energies of 5a-5k conformations for OPBTS given in Fig. 2. In this study, all
calculations were performed by using the optimized molecular structure 5a of OPBTS.
Fig. 2. The optimized molecular structure 5a of OPBTS with the atom numbering.
4.3. Energies and dipole moments
The sum of electronic and thermal free energies (SETFEs) and dipole moments of
OPBTS were calculated by B3LYP level with 6-311+G(d, p) basis set in the gas phase and
various solvent media (toluene, CHCl3, THF, DCM, acetone, DMF and DMSO), and the
calculation results were summarized in Table 2. The theoretical results showed that the
SETFEs and the dipole moments increase with the increasing polarity of the solvent in
general. It can be seen from the Table 2, the dipole moments for the studied compound in
various solvents change from 4.1865 D to 4.7241 D. The solvent effects cause a rise in the
dipole moments as they lead to increase in the delocalized charge of the molecules [38].

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Table 2. The sum of electronic and thermal free energies (SETFEs) and dipole moments (ꢀ) in the gas phase and solvent media for OPBTS.
Gas phase
Toluene
(ε=2.38)
CHCl3
(ε=4.81)
THF
(ε=7.52)
DCM
(ε=9.08)
Acetone
(ε=21.01)
DMF
(ε=38.25)
DMSO
(ε=47.00)
(bε=1.0)
aSETFE (kcal/mol)
-1531815.641
3.6569
-1531822.567
4.1865
-1531821.098
4.4429
-1531822.408
4.5510
-1531822.798
4.5841
-1531823.906
4.6804
-1531823.748
4.6664
-1531824.396
4.7241
ꢀ (D)
aSETFE: Sum of Electronic and Thermal Free Energy.
bε: (Dielectric Constant) https://www.organicdivision.org/orig/organic_solvents.html

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4.4. Intramolecular hydrogen bonding
The intramolecular hydrogen bond lengths for the optimized geometry of OPBTS were
calculated using the DFT calculation method at the B3LYP/6-311G+(d, p) level in the gas
phase and various solvents, and the results are given in Table 3. It was determined that the
calculated hydrogen bond lengths in the gas phase were the same as the calculated hydrogen
bond lengths in the solvent media. As it can be seen from the Table 3, the intramolecular
hydrogen-bond lengths of 42H…13N and 43H…14N were calculated to be 1.73152 Å and
1.69707 Å, respectively. These lengths are accepted to be within the medium strength of
hydrogen bond association [39, 40]. The intramolecular ‒31O−42H…13N and ‒
30O−43H…14N hydrogen bonds generate six-membered temporary rings, which stabilized the
phenol-imine form of the symmetrical OPBTS molecule. This type of intramolecular
hydrogen bonds is a common feature of o-hydroxy Schiff base derivatives [38, 41, 42].
According to the calculation results, other possible hydrogen bond lengths for 42H….15O,
43H….15O, 42H….14N and 43H….13N were found to be 3.10516, 2.90967, 3.65564 and
4.86573 Å, respectively (Table 3). These results indicated the absence of hydrogen bonds
between these atoms.
Table 3. The theoretical intramolecular hydrogen bond lengths at the B3LYP/6-311G+(d,p) level and the
formation of six-membered rings with the intramolecular 31O−42H…13N and 30O−43H…14N hydrogen bonds
for OPBTS.
42H…13N
43H…14N
42H…15O
43H…15O
42H…14N
43H…13N
1.73152
1.69707
3.10516
2.90967
3.65564
4.86573
H-bond length (Å)

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4.5. Vibrational spectroscopy (IR)
IR spectroscopy is an efficient method for identifying functional groups in organic
molecules. In this study, the experimental and theoretical FTIR spectra of OPBTS were
shown in Fig. S2 (Supplementary material), and the related vibrational frequencies were
compared in Table 4. The FTIR spectrum of OPBTS was experimentally recorded in the
range of 4000-400 cm-1 using KBr pellet. The theoretical IR frequency values for the
optimized geometry were determined by the DFT/B3LYP method with the 6-311+G(d, p)
basis set in the gas phase. The agreement between the experimental and theoretical
frequencies is good in general. Some differences between these frequencies result from the
use of gas phase molecules in the DFT calculations.
Table 4. The experimental and theoretical characteristic IR frequencies (cm-1) of OPBTS.
Experimental wavenumber
(cm-1, KBr)
Theoretical wavenumber
Vibrational assignment
[cm-1, B3LYP/6-311+g(d, p)]
ν (30O-H43) and ν (31O-H42) (ArO-H)
ν (C-H) of benzene and thiophene
3436.04 (s, br)
2926.69 and 3027.01
3268.68-3167.41
3268.00-3120.01 (w, br)
ν (16C-H52) and ν (17C-H53) of
azomethine
3014.00 (w, br)
3055.81 and 3050.78
ν (16C=N14) and ν (17C=N13)
ν (C=C) of benzene and thiophene
ν (24C-30O) and (25C-31O)
ν (6C-N13) and ν (12C-N14)
ν (5C-15O-C7)
1621.17 (s)
1645.69 and 1643.89
1621.93-1439.64
1292.24-1289.55
1257.74 and 1235.90
1251.44
1575.90-1489.00 (m)
1292.80 (w)
1244.50 (w)
1225.20 (w)
ν (16C-H52) and ν (17C-H53) of
azomethine
1403.57 and 1401.58/ 1009.68
and 1017.15
1402.10 (s) and 1160.90 (s)
1111.62 (s)-1096.50 (s)
816.62 (m) and 681.50 (m)
707.24 (m)
ν (C-H) of benzene
999.35-967.67
ν (32C-37S-C36) and ν (33C-38S-C39) of
thiophene rings
821.22 and 684.52-675.78
712.89-709.53-708.41
ν (C-H) of thiophene
Abbreviation: w, weak; m, medium; s, strong; vs, very strong; br, broad.
The symmetrical compound OPBTS contains the functional groups such as phenolic
hydroxyl, azomethine, phenyl ether and aromatic benzene and thiophene rings. The O‒H
stretching vibrations, which are very sensitive to inter- and intra-molecular hydrogen bonding,
are observed at in the region 3650-3200 cm-1 in FTIR spectrum. In phenols without
substituents, the O‒H stretching vibration frequency was reported to be 3657 cm-1 in the gas
phase [28, 43]. In the present study, the phenolic O‒H vibration of the studied compound was

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observed at 3436.04 cm-1 experimentally, and the theoretical IR frequency for this vibration
was found to be as 2926.69 and 3027.01 cm-1. In the experimental IR spectrum, the broad and
very strong band (a width of about 600 cm-1) centered at 3436.04 cm-1 can be assigned to the
stretching of O–H bond. This band indicates the presence of intramolecular hydrogen bond. In
addition, the observed stretching frequency at 2849.90 cm-1 defines the existence of O‒H...N
intramolecular hydrogen bond, this is also compatible with the reported values in the
literature. The phenolic C‒O band observed at 1292.80 cm-1 for OPBTS also shows that the
compound is in the form of the phenol-imine (O‒H...N intramolecular hydrogen bond) in solid
state [42, 44]. As a result, the observed frequencies for OPBTS confirm that the tautomeric
equilibrium favors the form of phenol-imine which is stabilized by the six-membered
temporary rings (with ‒31O−42H…13N and ‒30O−43H…14N hydrogen bonds).
The C-H stretching vibrations of the aromatic structures arise in the region of 2900-
3150 cm-1 [31, 32, 43]. In the experimental FTIR spectrum, it has been observed that the O‒H
vibration band, which occur in a very wide and strong form due to the strong intramolecular
hydrogen bonding (O‒H...N), covers these aromatic C‒H vibration bands. The weak shoulder
bands at between 3268.00 and 3120.01 cm-1 were assigned experimentally for the stretching
vibration of aromatic C‒H (for thiophene and benzene), and related vibrations were calculated
at the range of 3268.68-3167.41 cm-1 for the B3LYP/6-311+G(d, p) level of theory. The weak
shoulder band at 3014.00 cm-1 attributed to 16C‒H52 and 17C‒H53 stretching vibrations of
azomethine groups for the studied compound. These stretching frequencies were calculated at
the 3055.81 and 3050.78 cm-1 using the same calculation method.
The –C=N‒ stretching vibration of azomethine compounds regularly appears in the
region of 1500-1700 cm-1 [26, 45]. In this research, the strong absorption band at 1621.17 cm-
1
in the FTIR spectrum confirms the existence of −C=N− group. The vibrational frequencies
of −C=N− groups (16C=N14 and 17C=N13) were theoretically calculated at 1645.69 cm-1 and
1643.89 cm-1, respectively.
The characteristic IR bands of aromatic groups of OPBTS were emerged
experimentally in the range of 1575.90-1489.00 cm-1, corresponding to –C=C− stretching
vibrations of benzene and thiophene rings. The theoretical ‒C=C‒ vibrations were determined
in the region of 1621.93-1439.64 cm-1 for OPBTS. The C‒S‒C stretching vibrations of
thiophene rings were observed at 816.62 cm-1 and 681.50 cm-1 experimentally, and these
bands were calculated at 821.22 and 684.52 cm-1 for the B3LYP method with the 6-
311+G(d,p) basis set.

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The correlation graph of the experimental and theoretical vibrational frequencies for
OPBTS was shown in Fig. 3 and the correlation coefficient was obtained as 0.9854. As it can
be seen from the correlation graph, the experimental frequencies are in good agreement with
the calculated values. The corresponding vibrational frequency values support the chemical
structure of OPBTS.
4000
y = 0.9699x + 12.801
R² = 0.9854
3000
2000
1000
1000
2000
3000
4000
Experimental wavenumber (cm-1)
Fig. 3. The correlation graph of the experimental and theoretical vibrational IR frequencies of OPBTS.
4.6. 1H and 13C Nuclear Magnetic Resonance Spectroscopy (NMR)
NMR spectroscopy is a significant tool for determining the structure of an organic
compound. DFT approach is quite useful for the prediction of NMR spectra and the
investigation of relationship between molecular structure and their chemical shifts. Therefore,
the use of experimental and theoretical methods allows evaluation and prediction of the
1
13
structure of molecules [27-30, 46]. In this present investigation, the experimental H and C
NMR spectra of OPBTS molecule were recorded in DMSO-d6 and they are presented in Fig.
1
13
4. The optimized geometry of OPBTS was used to obtain the H and C NMR spectra with
B3LYP method 6-311+G(d, p) basis set using the Gaussian 09 program with the standard
1
gauge-including atomic orbital (GIAO) approach. The experimental and theoretical H and
13C NMR chemical shift values of the studied compound were compared in Table 5.

ACCEPTED MANUSCRIPT
13

ACCEPTED MANUSCRIPT
14
Fig. 4. 1H (a) and 13C (b) NMR spectra of N,N'-oxydiphenylenebis(5-(thiophen-2-yl)salicylidenimine, OPBTS (5) in DMSO-d6 obtained as experimentally.

15
ACCEPTED MANUSCRIPT
Table 5. 1H and 13C NMR chemical shifts δ [ppm] determined as experimental and theoretical for OPBTS.
1H NMR
δcalc. (in
DMSO-d6) DMSO)
13C NMR
δcalc..(in
DMSO-d6) DMSO)
Atoms
δexp.(in
δcalc.(in gas Atoms
phase)
δexp.(in
δcalc.(in gas
phase)
155.67
112.09
126.29
122.00
117.60
138.91
159.64
119.44
129.27
125.71
128.86
117.27
165.10
163.36
117.40
124.40
139.52
13.27
6.94
7.26
7.23
7.55
9.02
7.82
7.61
6.87
7.30
7.04
7.40
12.40
5.54
6.38
6.38
6.40
8.11
6.53
6.69
6.11
6.09
6.12
6.52
12.56
5.31
6.10
6.13
6.13
7.91
6.27
6.42
6.03
5.80
5.86
6.22
143.07
117.92
128.85
123.03
119.82
130.94
160.46
119.48
129.48
125.53
128.91
117.92
163.73
150.20
120.58
125.08
138.89
155.31
112.78
127.72
122.72
118.24
137.90
161.52
119.37
130.12
126.38
129.90
116.69
163.45
161.59
119.58
125.89
140.95
43-H
48-H
49-H
50-H
51-H
52-H
54-H
55-H
56-H
60-H
61-H
62-H
7-C
8-C
9-C
10-C
11-C
12-C
16-C
18-C
20-C
21-C
22-C
23-C
24-C
32-C
34-C
35-C
36-C
The experimental 1H NMR chemical shifts were observed in the range of δ 13.27-6.87ppm in
DMSO-d6, while the theoretical chemical shift values were determined between δ 12.40 and
5.54 ppm (in DMSO)/ δ 12.56 and 5.31 ppm (in gas phase) with B3LYP method 6-311+G(d,
1
p) basis set for the symmetrical OPBTS. The H NMR chemical shift value of the phenolic-
OH proton (H43) was observed at δ 13.27 ppm experimentally and at δ 12.40 ppm (in
DMSO)/ δ 12.56 (in gas phase) theoretically. The proton resonance signal (H52) of the
azomethine group (‒HC=N−) was assigned as a singlet at δ 9.02 ppm experimentally and
computed as δ 8.11 ppm (in DMSO)/ δ 7.91 ppm (in gas phase). These experimental 1H NMR
data for the compound indicate that the tautomeric equilibrium favors the form of phenol-
imine in DMSO-d6 (δ= 13.27 ppm, singlet for OH; δ= 9.02 ppm, singlet for CH=N) [42, 44].
The resonance signals for aromatic protons of OPBTS were attained as a set of doublets,
1
double-doublet and multiplied in the range of δ 7.82-6.87 ppm in the experimental H NMR
spectrum. The chemical shifts of these protons were theoretically attained between δ 6.69 and
5.54 ppm (in DMSO)/ δ 6.42 and 5.31 ppm (in the gas phase).
13
The experimental C NMR resonance signals of OPBTS were observed in the region
of δ 163.73-117.92 ppm in DMSO-d6, while they are calculated at the range of δ 163.45-
13
112.78 ppm (in DMSO)/ δ 165.10-112.09 ppm (in gas phase). The C NMR chemical shift
values of the C7, C12, C16, C24, C32 and C36 atoms, which are close to the oxygen, nitrogen
and sulphur heteroatoms were found to be higher than those of the other carbons in OPBTS
molecule. The 13C NMR resonance signals of these carbon atoms were recorded

16
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experimentally at δ 143.07 ppm (7C), 130.94 ppm (12C), 160.46 ppm (16C), 163.73 ppm
(24C), 150.20 ppm (32C), 138.89 ppm (36C) in DMSO-d6, and calculated at δ 155.31 ppm
(7C), 137.90 ppm (12C), 161.52 ppm (16C), 163.45 ppm (24C), 161.59 ppm (32C), 140.95
ppm (36C) (in DMSO)/ δ 155.67 ppm (7C), 138.91 ppm (12C), 159.64 ppm (16C), 165.10
ppm (24C), 163.36 ppm (32C), 139.52 ppm (36C) (in gas phase). In addition, the
13
experimental and theoretical C NMR chemical shift values of other carbons were observed
in the expected regions (Table 5). The proposed structure of OPBTS was confirmed with the
experimental and theoretical 1H and 13C NMR chemical shift values.
1
The correlations between the experimental and the theoretical H and 13C NMR
chemical shift values were illustrated in Fig. 5 (a,b), and the correlation coefficient values for
them were found to be 0.9880 and 0.9436, respectively. In this study, a good agreement
1
between the experimental and theoretical H and 13C NMR chemical shifts (Table 5) was
achieved.
14
(a)
y = 1.0235x - 1.1899
R² = 0.988
12
10
8
6
6
8
10
12
14
Experimental 1H NMR chemical shift (ppm)

17
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180
160
140
120
100
(b)
y = 1.1451x - 17.602
R² = 0.9436
100
120
140
160
Experimental 13C NMR chemical shift (ppm)
Fig. 5. The correlation graphs for the experimental and theoretical 1H NMR (a) and 13C NMR (b) chemical shifts
of OPBTS.
4.7. UV-Vis. spectroscopy
The UV-Vis electronic absorption spectra of OPBTS were experimentally recorded in
the region of 200-650 nm at the room temperature and the concentration of 1.5x10-5 M in the
organic solvents (toluene, CHCl3, THF, DCM, acetone, DMF and DMSO). The TD-DFT
calculation approach based on the B3LYP/6-311+G(d. p) level for the optimized geometry
has been used to investigate the electronic absorption behaviors of the studied compound in
the gas phase and solvent media. The experimental and theoretical UV-Vis spectra were
indicated in Fig. 6 and the corresponding electronic transition values were given in Table 6. In
the UV-Vis spectra of OPBTS in toluene and acetone, “the overlaps” in the range of 200-320
nm were appeared due to the interactions between OPBTS and solvent. Therefore, the UV-
Vis spectra of OPBTS in toluene and acetone were given in Fig S3 (Supplementary material).

18
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0.700
0.600
0.500
0.400
0.300
0.200
0.100
(b)
Gas phase
CHCl
3
THF
DCM
DMF
DMSO
650.00
600.00
500.00
400.00
Wavelength (nm)
200.00
300.00
Fig. 6. The experimental (a) and theoretical (b) UV-Vis spectra of OPBTS.

19
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Table 6. The experimental and theoretical UV-Vis spectroscopic parameters for OPBTS.
Experimental
λmax (nm) (A=Absorption)
Theoretical
Oscillatör strength (f)
Solvent
λmax (nm)
322.85
0.2886
0.5707
0.6003
0.5749
0.5796
0.5616
0.5747
0.5816
Gas phase
Toluene
CHCl3
THF
-
302.00 (A=0.504)
374.00 (A=0.185)
275.00 (A=0.711)
370.00 (A=0.184)
288.50 (A=0.725)
374.00(A=0.186)
275.00 (A=0.581)
368.00 (A=0.150)
315.50
312.52
312.52
312.52
312.52
312.52
312.52
DCM
369.50 (A=0.173)
Acetone
DMF
287.50 (A=0.732)
375.50 (A=0.178)
300.50 (A=0.890)
375.50 (A=0.263)
DMSO
As it can be seen from Table 6, the UV-Vis absorption bands for OPBTS in various
organic solvents were experimentally observed between 275.00 and 375.50 nm, while the
related wavelengths were theoretically determined in 312.52 nm (322.85 nm in the gas phase
and 315.50 nm in toluene). Also, the maximum absorption band of OPBTS in the gas phase
was calculated as 322.85 nm. According to the experimental absorption spectrum, OPBTS
molecule exhibits two absorption bands in various solvents (Fig. 6a). The absorption bands
below 300 nm in different organic solvents can be assigned to π-π* transitions of the
azomethine (‒HC=N‒) group and aromatic benzene and thiophene rings. The absorption
bands observed in the range of 300-400 nm result from the n-π* transitions in azomethine and
phenoxy groups of OPBTS. Usually, the absorption bands appearing below 400 nm in the
experimental UV-Vis spectrum of the 2-hydroxy Schiff base derivatives indicate the phenol-
imine form, while the bands observed above 400 nm show the keto-amine form of the related
molecule [28, 35, 41]. Since the absorption band above 400 nm was not observed in the
experimental and theoretical UV-Vis spectra for this study, it can be said that OPBTS was in
the form of phenol-imine in the selected organic solvents.
4.8. Investigation of the HOMO and LUMO energies
The Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular
Orbital (LUMO) and their band gap are very important quantum parameters in the
determination of the chemical stability of the molecule. These molecular orbitals play an
important role in optical and electrical properties and electronic absorption spectra. It can be
said that a molecule with a small HOMO-LUMO orbital gap is highly polarizable and usually

20
ACCEPTED MANUSCRIPT
possess a high chemical reactivity [27, 46, 47]. In this study, the HOMO and LUMO energies
of OPBTS were calculated at B3LYP/6-311+G(d, p) level in the gas phase and different
solvents, and the obtained results were listed in Table 7. 3D plots of the HOMO and LUMO
for the studied molecule were illustrated in Fig. 7. As seen from Table 7, it was observed that
the HOMO and LUMO energy values increase with an increase in the polarity of solvent. The
energies of the HOMO and LUMO for OPBTS were calculated as -5.613 eV and -2.258 eV
in the gas phase (-5.926 eV and -2.281 eV in DMSO), respectively. The HOMO of the studied
compound is localized on the aromatic benzene and thiophene rings (the left side of the
phenyl ether bridge), while LUMO is concentrated on the aromatic benzene rings (the right
side of the phenyl ether bridge). In addition, both of the HOMO and LUMO frontier orbitals
are localized on azomethine groups. As shown in Fig. 7, the energy gap (ꢁE) between the
HOMO and LUMO in the gas phase was determined as 3.36 eV, while this band gap in
DMSO was found to be as 3.65 eV.
Table 7. The calculated HOMO and LUMO energies and the energy gaps (ꢁE) of OPBTS in the gas phase and
Gas phase
Toluene
CHCl3
THF
DCM Acetone
DMF
DMSO
-5.613
-5.779
-5.853
-5.882
-5.890
-2.274
3.62
-5.915
-2.279
3.64
-5.912
-5.926
EHOMO (eV)
ELUMO (eV)
∆Egap (eV)
-2.258
3.36
-2.261
3.52
-2.269
3.58
-2.273
3.61
-2.278
3.63
-2.281
3.65
different solvents.
∆Egap= ELUMO-EHOMO
*

21
ACCEPTED MANUSCRIPT
Fig. 7. HOMO and LUMO 3D plots of OPBTS obtained with B3LYP/6-311+G(d, p) method in the gas phase
(a) and in DMSO (b).
4.9. Mulliken atomic charges
The calculation of Mulliken atomic charges is used to examine the electronic charge
distribution of atoms in a molecular system. The atomic charges, which are related to the
electronic properties and reactivity of molecules, have an important role in the applications of
quantum chemical calculations [28, 47]. The Mulliken atomic charges of OPBTS were
calculated using B3LYP/6-311G+(d, p) level in the gas phase. To examine the solvent effect
on the charge distribution, the atomic charges of the studied compound were determined in
the various solvents with different polarities (toluene, CHCl3, THF, DCM, acetone, DMF and
DMSO). The obtained results were summarized in Table 8. The Mulliken atomic charges for
OPBTS tend to increase with an increase in the polarity of solvent. The negative atomic
charges were delocalized on the electronegative N13, N14, O15, O30 and O31 atoms. O15
atom has the most negative charge in the gas phase (-0.574 au.) and solvent media (-0.592 au.
in DMSO). The carbon atoms, which are excluded C18, C19, C23, C32, C33, C36 and C39
atoms, are positively charged. It was determined that the 24C and 25C atoms which are
connected to the electronegative oxygen atoms (30O and 31O) have bigger positive atomic
charge than the other carbon atoms in the gas phase (0.334 and 0.322 a.u. respectively) and
solvent media. On the other hand, the C36 and C39 atoms possess negative charges due to the

22
ACCEPTED MANUSCRIPT
delocalization of electrons on the sulphur atoms in the thiophene rings. The Mulliken atomic
charges for the 37S and 38S atoms of thiophene rings were calculated as 0.329 and 0.333 au.
in the gas phase.

23
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Table 8. The Mulliken atomic charges of OPBTS in the gas-phase and various solvent media.
Gas phase Toluene
(ε=1.0) (ε=2.38)
0.066 0.076
CHCl3
(ε=4.81)
THF
(ε=7.52)
DCM
(ε=9.08)
Acetone
DMF
DMSO
Atom No
(ε=21.01) (ε=36.70) (ε=46.70)
1 C
2 C
0.082
0.084
0.084
0.086
0.019
0.036
0.028
0.193
0.164
0.229
0.059
0.025
0.015
0.070
0.132
-0.579
-0.576
-0.591
0.349
0.334
-0.150
-0.130
0.043
0.038
0.115
-0.018
0.329
0.318
-0.005
0.079
0.033
0.086
-0.230
-0.209
-0.524
-0.525
0.209
0.079
-0.225
0.313
0.311
-0.224
0.082
0.212
0.086
0.019
0.036
0.028
0.193
0.164
0.229
0.059
0.025
0.015
0.070
0.132
-0.579
-0.576
-0.591
0.349
0.334
-0.150
-0.130
0.043
0.038
0.115
-0.018
0.329
0.319
-0.005
0.079
0.033
0.085
-0.229
-0.209
-0.524
-0.525
0.209
0.079
-0.226
0.313
0.311
-0.224
0.082
0.212
0.087
0.020
0.036
0.028
0.192
0.164
0.229
0.059
0.025
0.015
0.070
0.131
-0.580
-0.576
-0.592
0.350
0.336
-0.151
-0.131
0.044
0.037
0.115
-0.018
0.329
0.318
-0.006
0.079
0.033
0.087
-0.231
-0.210
-0.525
-0.525
0.210
0.079
-0.225
0.312
0.310
-0.224
0.082
0.212
0.012
0.029
0.028
0.203
0.165
0.226
0.050
0.020
0.014
0.061
0.146
-0.566
-0.566
-0.574
0.321
0.293
-0.137
-0.114
0.024
0.047
0.101
-0.013
0.334
0.322
0.001
0.077
0.042
0.058
-0.192
-0.178
-0.514
-0.516
0.197
0.071
-0.240
0.329
0.333
-0.236
0.076
0.199
0.015
0.032
0.029
0.198
0.165
0.229
0.055
0.022
0.014
0.066
0.139
-0.573
-0.571
-0.583
0.338
0.316
-0.145
-0.123
0.035
0.042
0.108
-0.015
0.332
0.321
-0.002
0.078
0.037
0.072
-0.213
-0.195
-0.519
-0.521
0.204
0.075
-0.232
0.320
0.321
-0.229
0.078
0.206
0.017
0.034
0.029
0.195
0.165
0.229
0.057
0.023
0.014
0.068
0.135
-0.576
-0.574
-0.588
0.344
0.326
-0.148
-0.127
0.040
0.040
0.112
-0.017
0.331
0.320
-0.004
0.079
0.035
0.079
-0.222
-0.203
-0.522
-0.523
0.207
0.077
-0.229
0.316
0.316
-0.227
0.080
0.209
0.018
0.035
0.029
0.194
0.165
0.229
0.058
0.024
0.015
0.069
0.134
-0.578
-0.575
-0.589
0.346
0.330
-0.149
-0.129
0.041
0.039
0.113
-0.017
0.330
0.319
-0.004
0.079
0.034
0.082
-0.225
-0.206
-0.523
-0.524
0.208
0.078
-0.227
0.314
0.314
-0.225
0.081
0.210
0.019
0.035
0.029
0.194
0.165
0.229
0.058
0.024
0.015
0.069
0.133
-0.578
-0.575
-0.590
0.347
0.331
-0.149
-0.129
0.042
0.038
0.113
-0.017
0.330
0.319
-0.005
0.079
0.034
0.083
-0.227
-0.207
-0.523
-0.524
0.208
0.078
-0.227
0.314
0.313
-0.225
0.081
0.211
3 C
4 C
5 C
6 C
7 C
8 C
9 C
10 C
11 C
12 C
13 N
14 N
15 O
16 C
17 C
18 C
19 C
20 C
21 C
22 C
23 C
24 C
25 C
26 C
27 C
28 C
29 C
30 O
31 O
32 C
33 C
34 C
35 C
36 C
37 S
38 S
39 C
40 C
41 C

24
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4.1. Molecular Electrostatic Potential (MEP) surface analysis
The MEP mapping is a very useful identifier in the investigation of the molecular
structure with its chemical property relationships. The MEP surfaces are utilized to identify
the reactive regions of the molecule during the electrophilic and nucleophilic attacks with
intermolecular and intramolecular hydrogen bonding interactions. In MEP map, the different
values of electrostatic potential at the surface are represented by different colors. While the
most negative electrostatic potential region is illustrated by red color, the most positive
electrostatic potential region is represented by blue color, and the green color indicates the
zero potential region [47-50].
The MEP surface of OPBTS was plotted using DFT/B3LYP level with 6-311+G(d,p)
basis set to estimate the reactive sites of electrophilic and nucleophilic attack on the
investigated molecule. The MEP map of studied compound was shown in Fig. 8. According to
the present calculations, the MEP map indicated that the negative potential sites were on the
oxygen atoms (15O and 30O, 31O) of oxyphenyl and two hydroxyl groups and the positive
potential sites were around the hydrogen atoms. Therefore, it can be said that the electron-rich
oxygen atoms are suitable to the electrophilic attacks, while the electron-deficient hydrogen
atoms are appropriate to nucleophilic attacks. As it can be seen from the figure, the regions on
the aromatic rings and nitrogen atoms (N13 and N14) are neutral as represented by green
color. These results indicate the existence of the intramolecular ‒13N…42H–31O– and ‒
14N…43H–30O– interactions between the nitrogen atoms and the hydroxyl groups.
Fig. 8. 3D plots of MEPs from two different perspectives of OPBTS on the B3LYP/6–311+G(d. p) basis set.

25
ACCEPTED MANUSCRIPT
5. Conclusion
In this present study, a novel symmetrical azomethine compound containing thiophene
1
13
units, OPBTS was synthesized and experimentally characterized by using H and C NMR,
FTIR, UV-Vis spectroscopic methods and elemental analysis. The optimized geometry, IR
1
13
vibrational frequencies, H and C NMR chemical shifts, the electronic absorption spectra,
Mulliken atomic charges, HOMO-LUMO energies and the MEP map of OPBTS have been
calculated at DFT/B3LYP level of theory with 6-311+G(d. p) basis set using the Gaussian09
1
package. The obtained theoretical H and 13C NMR, FT-IR, UV-Vis spectroscopic results
were reported and compared with the experimental data. The comparison between the
theoretical and the experimental parameters showed that the calculated results from
DFT/B3LYP method are in good agreement with the experimental values. The absorption
bands of OPBTS were observed below 400 nm in the experimental and theoretical UV-Vis
spectra. This result indicates that the bis azomethine compound, OPBTS was in phenol-imine
form in the gas phase and solvent media. The Mulliken atomic charges for the studied
compound tend to increase with an increase in the polarity of solvent. It was determined that
the negative atomic charges delocalized on the electronegative N13, N14, O15, O30 and O31
atoms, and O15 atom possesses the most negative charge in the gas phase (-0.574 au.) and
solvent media (-0.592 au. in DMSO). The 24C and 25C atoms have bigger positive atomic
charge than the other carbon atoms in the gas phase (0.334 and 0.322 a.u. respectively) and
solvent media. The MEP analysis indicated that the negative potential sites were on the
oxygen atoms (15O and 30O, 31O) of oxyphenyl and two hydroxyl groups and the positive
potential sites were around the hydrogen atoms. According to the calculation results, the
HOMO and LUMO energy values increase with an increase in the polarity of solvent, and the
HOMO and LUMO are localized on azomethine groups. The energy gap (ꢁE) between the
HOMO and LUMO was determined as 3.36 eV in the gas phase, while this band gap was
found to be as 3.65 eV in DMSO. As a result, this study can be a guide for the design,
synthesis and various applications of new materials such as the studied compound.
Acknowledgments
This study was supported by Anadolu University Scientific Research Projects (Anadolu
University BAP) through project 1705F213. The authors also gratefully thank to Anadolu
University BAP for supporting Gaussian 09 and Gauss View 5.0 programs with the projects
(Project No 1102F027 and 1304F064) and its financial support for Agilent DD2 NMR

26
ACCEPTED MANUSCRIPT
Spectrometer (Project No. 1306F110). The authors thank to Dr. İlhami ÇELİK for the NMR
analysis.
Supporting information
Supplementary data related to this article are available (Figure S1, S2 and S3).
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