Synthesis, characterization and intramolecular proton transfer of 3,3′-dihydroxy-4,4′-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)] -bis-phenol

Article abstract of DOI:10.1016/j.molstruc.2014.05.003

A newly synthesized Schiff base, 3,3′-dihydroxy-4,4′-[5-methyl- 1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol, was characterized experimentally. Its geometries optimization, tautomerization, assignment of the IR bands and NMR chemical shifts were calculated by using density functional theory (DFT) method. In addition, the atoms in molecules (AIM) analysis was employed for investigation of its tautomerization. Four possible tautomers of the investigated Schiff base were optimized in both of the gas and solution phases. The Schiff base has no planar structure, but each of the benzene rings is in a separate plane. In the most stable tautomer, the phenolic protons of the two OH groups are engaged in the intramolecular-hydrogen bond with the azomethine nitrogens. Good consistency between the theoretical and experimental results confirms validity of the optimized geometry. Also, kinetics and mechanism of the intramolecular-proton transfer of the studied Schiff base was demonstrated theoretically.

Full text of DOI:10.1016/j.molstruc.2014.05.003

Accepted Manuscript
Synthesis, characterization and intramolecular proton transfer of 3,3’-Dihy-
droxy-4,4’-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol
Hossein Eshtiagh-Hosseini, S. Ali Beyramabadi, Ali Morsali, Masoud Mirzaei,
Hamed Chegini, Morteza Elahi, Mohammad Ali Naseri
PII:
S0022-2860(14)00462-1
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.05.003
MOLSTR 20602
Reference:
Journal of Molecular Structure
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 November 2013
30 April 2014
2 May 2014
Please cite this article as: H. Eshtiagh-Hosseini, S. Ali Beyramabadi, A. Morsali, M. Mirzaei, H. Chegini, M. Elahi,
M.A. Naseri, Synthesis, characterization and intramolecular proton transfer of 3,3’-Dihydroxy-4,4’-[5-methyl-1,3-
phenylenebis(nitrilomethylidyne)]-bis-phenol, Journal of Molecular Structure (2014), doi: http://dx.doi.org/
10.1016/j.molstruc.2014.05.003
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Synthesis, characterization and intramolecular proton transfer of 3,3'-
Dihydroxy-4,4'-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-
phenol
Hossein Eshtiagh-Hosseinia, S. Ali Beyramabadib,*, Ali Morsalib, Masoud Mirzaeia, Hamed
Cheginib, Morteza Elahib, Mohammad Ali Naseric
a Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran
b Department of Chemistry, Mashhad branch, Islamic Azad University, Mashhad, Iran
c Department of Chemistry, Faculty of Science, Birjand University, Birjand, Iran
* Corresponding author (Beyramabadi):
Tel.: +98 511 8414182; fax: +98 511 8414182
E-mail addresses: beiramabadi@yahoo.com, beiramabadi6285@mshdiau.ac.ir.
1

Abstract
A
newly
synthesized
Schiff
base,
3,3'-Dihydroxy-4,4'-[5-methyl-1,3-
characterized experimentally. Its
phenylenebis(nitrilomethylidyne)]-bis-phenol, was
geometries optimization, tautomerization, assignment of the IR bands and NMR chemical
shifts were calculated by using density functional theory (DFT) method. In addition, the
Atoms in molecules (AIM) analysis was employed for investigation of its tautomerization.
Four possible tautomers of the investigated Schiff base were optimized in both of the gas and
solution phases. The Schiff base has no planar structure, but each of the benzene rings is in a
separate plane. In the most stable tutomer, the phenolic protons of the two –OH groups are
engaged in the intramolecular-hydrogen bond with the azomethine nitrogens. Good
consistency between the theoretical and experimental results confirms validity of the
optimized geomrtry. Also, kinetics and mechanism of the intramolecular-proton transfer of
the studied Schiff base was demonstrated theoretically.
Keywords: Schiff Base; DFT; Assignment; Intramolecular Proton Transfer, AIM analysis,
Tautomerization.
2

1. Introduction
The Schiff bases and their metal complexes have a large number of applications especially in
the fields of biochemistry, catalyst, synthesis and as an intermediate in a number of
enzymatic reactions [1–4]. The tetradentate ONNO donor Schiff bases form a main group of
compounds, salen or salophen, which serve in a wide variety of applications [5].
Hydrogen bond systems are one of the key interactions that determine the structure,
functionality, and dynamic processes in a large kind of systems. Therefore, the
understanding and comprehensive studies of the nature of hydrogen bonds is important for
biological and chemical systems [6].
In view of our interest in synthesis of new Schiff bases [7-14], herein we report synthesis,
experimental and theoretical characterization of a new Schiff base, 3,3'-Dihydroxy-4,4'-[5-
methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol = L. In addition, tautomerism of
the L was investigated using the DFT approaches.
In the structure of the titled compound, there are two OH…N intramolecular-hydrogen
bonds, competition of which has important role in stability of the tautomeric structures. A
major aim of this paper is to study hydrogen bonding in aminohydroxyaryl Schiff bases and
to theoretically describe the proton-transfer process and particular tautomeric forms, too.
2. Experimental
2.1. Material and methods
All of used chemicals were obtained from Merck Company and were used without any
further purification. Melting points were determined using an electrothermal 9100 melting
point apparatus. The IR spectra were obtained using a Buck 500 infrared spectrophotometer.
3

Elemental analysis (C, H, N) was performed on a Heraeus elemental analyzer CHN-O-
1
Rapid. The H and 13C NMR spectra were obtained on a Bruker Drx-500 Avance
spectrometer (500.13 MHz) in DMSO. Mass spectra were recorded on a Shimadzu-GC-
Mass-Qp 1100 Ex.
2.2. Synthesis of the titled Schiff base (C21H18N2O4)
A solution of 2.36 g (6.66 mmol) 3-methylbenzene-1,2-diamine and 1.88 g (13.61 mmol) of
dihydroxybenzaldehyde in toluene (150 mL) was refluxed under vigorous stirring with a
Dean-Stark for 3 h. The resulting suspension was kept at room temperature prior to being
filtered, washed with ethanol (2×10 mL) and dried with diethyl ether (2×10 mL) to afford a
deep red solid. The solid was recrystallized for further purification. (Yield: 65%, m.p.> 250
°C, the molecular ion peaks, m/z (M+) =363).
3. Computational details
The DFT clculations have been performed using B3LYP functional [15] as implemented in
the Gaussian 03 program package [16]. The 6-311+G(d,p) basis set was used. Here, the
Polarizable-Continuum Model (PCM) [17] has been employed for investigation of solute-
solvent interactions. All geometries were fully optimized in both of the gas phase and
methanolic solution. The optimized geometries were used to calculate the vibrational
frequencies, NMR chemical shifts and AIM analysis.
1
The H and 13C NMR chemical shifts were predicted with respect to tetramethylsilane
(TMS), where the GIAO method was used [18]. Since the DFT-calculated vibrational-
frequencies are usually higher than the experimental ones, they were scaled using a factor of
0.9614 [19].
4

The AIM topological analysis was performed in accordance with Bader’s approach [20]
using AIMall package [21]. The DENSITY= CURRENT option was used to generate the
wavefunction files.
3. Results and discussion
3.1. Tautomerism
There are four possible tautomers for the titled compound, geometries of which are fully
optimized in both of the gas phase and methanolic solution. Their PCM optimized structures
are shown in Fig. 1, which are named as La, Lb, Lc and Ld.
5

O3
H4
H3
C1
La
H8
C10 C9
H9
O4
C2
C6
C3
O1
H1
C4 H5
H10
C11
C8
O2
H2
C5
C12 C13
H18
H6
H11
N1
C7
C14
N2
C20
H7
H12
C19
C18
C15
C16
H17
H13
H14
C17
H16
C21
H15
.
6

Figure 1. The B3LYP optimized geometries of the La, Lb, Lc and Ld tautomers of the titled
Schiff base.
In continuation, the tautomerization of the titled Schiff base was investigated using the
PCM model. Otimized geometries of the transition states are shown in Fig. 2.
The La tautomer is more stable than the Lb, Lc and Ld tautomers by15.27, 30.58 and
29.66 kJ.mol-1 in the gas phase and 3.38, 5.90 and 10.95 kJ.mol-1, in the PCM model,
⎯⎯→
respectively. The La
Lb tautomerization involves the TSLa-Lb as the transition state.
←⎯⎯
In this tautomerism, the intramolecular-proton transfer (IPT) from O2 to N2 atom is
occurring by activation energy (Ea) of 12.12 kJ.mol-1 in the gas phase, which reduces to 5.66
kJ.mol-1 by considering the solvent effects in PCM model.
7

⎯⎯→
⎯⎯→
Lb one. The former
The La
Lc tautomerism is in competition with the La
←⎯⎯
←⎯⎯
⎯⎯→
involves the TSLa-Lc transition state. The Ea of the La
Lc pathway is higher than the
←⎯⎯
-1
⎯⎯→
La
Lb one by 8.04 and 1.69 kJ.mol , in the gas phase and PCM model, respectively.
←⎯⎯
⎯⎯→
⎯⎯→
Lc pathways can be
Difference of the computed Eas for the La
Lb and La
←⎯⎯
attributed to the inductive effect of the methyl substitution.
In the Ld species, both of the H1 and H2 protons are bonded to the azomethine nitrogens.
⎯⎯→ ⎯⎯→
←⎯⎯
The Lb
Ld and Lc
Ld tautomerisms are passed through the TSLb-Ld and
←⎯⎯
←⎯⎯
TSLc-Ld, respectively, the Eas of which are 14.02 and 13.30 kJ.mol-1 in the gas phase, and
8.42 and 6.71 kJ.mol-1, in the PCM model, respectively.
Hence, from kinetical point of view, tautomerization of La to Ld may be progressed in
⎯⎯→
⎯⎯→
⎯⎯→
⎯⎯→
Lc Ld.
the both pathways: the La
Lb
Ld or the La
←⎯⎯
←⎯⎯
←⎯⎯
←⎯⎯
8

Figure 2. The B3LYP optimized geometries of the TSLa-Lb, TSLb-Ld, TSLa-Lc and TSLc-Ld.
9

3.2. Geometry optimization
The elemental analysis results for the titled Schiff base (Anal. Calcd. for C20H20N2O4: C,
70.39; H, 6.16; N, 7.14. Found: C, 67.68; H, 4.89; N, 8.28 %) confirm the proposed formula.
The titled Schiff base has four tautomers, the most stable of which is the OHOH form (La
tautomer). Optimized geometry of the La with labeling of its atoms is shown in Fig. 1.
Selected-structural parameters of this tautomer are listed in Table 1. The calculated-
structural parameters are in agreement with the data reported for the similar compounds [7-
14,22-28].
The calculated-bond lengths are in the expected range [29]. All of three benzene rings are
essentially planar, but each of them lies in a separate plane. The two side-benzene rings
make a dihedral angle of approximately 20° with each other. In the diamine-bridge region,
the middle-benzene ring has a dihedral angle of about 60° with the side benzene rings.
Table 1. Selected structural parameters of the La tautomer.
Bond
Bond length (Å)
Angle (°)
Dihedral angle (°)
O1-H1
H1-N1
N1-O1
O2-H2
H2-N2
C1-O1
C1-C2
O3-H4
1.00
1.71
2.62
1.00
1.70
1.35
1.39
0.97
H1-O1-C1
O1-H1-N1
O1-C1-C2
C1-C2-C3
C2-C3-O3
O1-C1-C6
C1-C6-C7
C6-C7-N1
107.1
148.5
118.6
119.8
117.0
121.1
121.6
122.2
H1-O1-C1-C2
C1-C2-C3-C4
C2-C3-O3-H4
C2-C3-C4-H5
O1-C1-C6-C7
C5-C6-C7-N1
C6-C7-N1-C15
C7-N1-C15-C16
0.2
-0.1
179.9
-179.9
0.2
179.7
179.1
-64.83
10

C2-H3
C4-C5
1.08
1.38
1.45
1.09
1.29
1.42
1.41
1.51
1.40
1.39
1.40
1.41
1.29
1.45
1.42
1.35
1.00
4.80
7.76
1.41
1.09
1.08
C7-N1-C15
N1-C15-C16
C15-C16-C21
C16-C17-C18
C3-O3-H4
120.9
122.1
122.5
122.5
110.2
121.4
121.4
122.0
121.6
121.1
106.9
148.8
148.5
120.3
119.3
C20-C15-C16-C21
C15-C16-C17-C18
C18-C19-C20-H18
C1-O1-O2-C8
176.2
-0.7
C6-C7
178.1
-40.1
-32.2
162.8
-37.8
126.3
-13.9
-12.8
23.4
C7-H7
C7-N1
C4-C1-C8-C11
C16-C19-C27-C24
C7-N1-N2-C14
C2-C1-C9-C8
N1-C15
C15-C16
C16-C21
C16-C17
C17-C18
C18-C19
C19-N2
C14-N2
C14-C13
C13-C8
C8-O2
C19-N2-C14
N2-C14-H12
N2-C14- C13
C14- C13-C8
C13-C8-O2
O1-N1-N2-O2
C3-C1-C8-C9
C8-O2-H2
C1-C6-C13-C8
C2-C5-C12-C9
C2-C5-C20-C17
O2-H2…N2
O1-H1…N1
C14- C13-C12
C15- C20-H18
24.1
125.9
C1-C6-C15-C16
C8-C13-C19-C18
-66.4
-147.8
C18-C19- C20-H18
178.1
O2-H2
N1-N2
O1-O2
N2-C19
C17-H16
C20-H18
11

The resorcinol OH groups are in the same plane with the aromatic rings (Table 1). The H1
and H2 atoms are engaged in intramolecular-hydrogen bonds with the azomethine nitrogens,
N1 and N2 atoms, respectively. The calculated N…H and N-O distances are 1.71 and 2.621
Å, respectively. These H-bond interactions decrease electron density of the O1-H1 and O2-
H2 bonds. So that the O1-H1 and O2-H2 bonds are longer than the O3-H4 and O4-H9 ones
by 3.2 pm.
The C7=N1 and C14=N2 bonds are in the same plane with the corresponding resorcinol.
Both of the C15-N1 and C19-N2 bond lengths are appropriate size for the C-N single bond,
while both of the C7-N1 and C14-N2 bonds are corresponding to the C=N double bond. As
expected, the methyl substitution is in the same plane with the benzene ring. The calculated
structural parameters are in agreement with the previously reported data for the similar
Schiff bases and their complexes [7-14,22-28].
3.3. NMR spectra
1
The DFT and experimental H- and 13C-NMR chemical shifts (δ) of the La species are
gathered in Table 2, where the atoms are numbered as in Fig. 1. Good agreement between
the calculated and experimental chemical shifts confirms suitability of the optimized
geometry for the La tautomer as the most stable form of the L.
The H4 and H9 hydrogens are only exceptions, where the experimental chemical shifts
are significantly higher than the calculated ones. These atoms may be engaged in the
intermolecular-hydrogen bonds. In addition, the calculated chemical shifts are corresponding
to the isolated molecule in the gas phase, while the experimental data are from DMSO
solutions. Obviously, the solvent molecules interact with the -OH protons.
A broad signal at 13.60-13.80 ppm is assiged to the H1 and H2 phenolic protons. Their
12

engagement in the intramolecular H-bond (O-H…N), shifts their signals upfield [7-14,30].
13

Table 2. Experimental and DFT computed 1H- and 13C-NMR chemical shifts of 3,3'-Dihydroxy-4,4'-[5-methyl-1,3-
phenylenebis(nitrilomethylidyne)]-bis-phenol in DMSO solution, δ [ppm].
1H NMR
13C NMR
Atom
Atom
Atom position
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Atom position
Exp.
Theo.
position
position
H2
H1
13.75
13.24
4.05
3.93
8.85
8.85
7.33
7.17
7.06
7.06
6.91
H3
H8
6.81
6.69
5.98
5.83
2.37
2.22
1.93
C1
C8
163.8
163.8
163.4
163.4
163.4
163.4
162.5
162.5
147.9
147.4
134.9
173.33
172.09
170.72
168.00
167.56
165.05
158.04
154.70
138.23
138.23
137.05
C16
C20
C13
C6
131.8
129.8
119.1
112.4
110.6
108.2
108.2
102.7
102.7
17.4
130.92
125.02
119.27
117.87
115.58
107.66
107.66
107.47
107.38
19.00
13.60-13.80
6.30-6.50
H4
10.33
10.33
8.80
H5
C7
H9
H10
H14
H13
H15
C3
H12
H7
2.30
2.30
2.30
C10
C14
C15
C19
C5
C15
C4
8.80
H16
H18
H6
C11
C2
7.30-7.50
C9
H11
H17
C12
C17
C21
14

3.4. Vibrational spectroscopy
Nowadays, theoretical assignment of the spectra provides a quantitative framework for
understanding and identification of the chemical compounds [7-14,31-33].
Here, the vibrational modes of the La tautomer were analyzed by comparing the DFT and
experimental IR spectra. Assignment of the selected-vibrational frequencies is gathered in
Table 3. The obtained results could be useful in identification of similar compounds.
A broad bond in the 3430-2600 cm-1 spectral region is attributed to overlapping of the O-
H and C-H stretching vibrations [7-14,34,35]. Deconvolution of this region is given in Table
3. The most intensive band is related to the stretching vibrations of the O1-H1 and O2-H2
bonds. Due to the engagement of the H1 and H2 in intramolecular-hydrogen bonds, the O1-
H1 and O2-H2 stretching vibrations have lower energies than the corresponding vibrations
for the O3-H4 and O4-H9 bonds.
The very intensive band in the 1660-1500 cm-1 region is related to the azomethine C=N
bonds. Also, the C-O stretching vibrations result in strong bands at 1223 cm-1 (Table 3) [7-
14,34,35].
Table 3. Selected experimental and calculated IR vibrational frequencies (cm-1) of the 3,3'-
Dihydroxy-4,4'-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol.
Experimental
787(m)
Calculated
813
Vibrational assignment
δ (O1-H1, O2-H2)
op
840 (s)
880
δ (C20-H18)
op
976 (m)
1114 (s)
1188(s)
950
Breathing of benzene rings
1099
1211
1276
1331
1432
δ (aromatic hydrogens)
ip
υ(C6-C7, C13-C14, C15-N1, C25-N2)
υ(C-O)
1223(vs)
1342 (m)
δ (C7-H7, C14-H12)
ip
δ (methyl)
sci
15

1496 (s)
1512 (s)
1597(vs)
1481
1524
υ(C-O)+υasym(C=C) benzene rings
υ
sym(C=C) bridging benzene ring
sym(C=C) benzene rings
1543-1584
1594
υ
υ(C7=N1, C14=N2)
1630(vs)
1601
υ(C=N)+ υsym(C=C) benzene ring of resorcinol
2912
υ
sym(C-H) methyl
2929
υ(O2-H2, C14-H12)
υ(O1-H1, C7-H7)
υ(O2-H2, C14-H12)
2941
2950
3430-2600 (br)
2960,2987
υ
asym(C-H) methyl
3041-3067
3076
υ(C-H) aromatic
υ(C2-H3, C9-H8)
υ(O3-H4, O4-H9)
3661
Abbreviations: op, out-of-plane; ip, in-plane; m, medium; s, strong; vs, very strong; br, broad.
3.5. Electrostatic potential map
The electrostatic potentials, VS(r), of the La, Lb, Lc and Ld are shown in Fig. 3, with the
negative potential shown in red and the positive shown in blue. As shown in Fig. 3, there is a
region of positive VS(r) in the most external part of H2 (the region located at the
continuation of O2…H2) in all the structures, but the negative VS(r) is located at the
outermost part of N2. Negative potential around the N2 atom of the La is very prominent
that shows the occurrence of IPT between O2 and N2 [36]. The interaction between the
negative VS(r) region of N2 and the positive VS(r) region of H2 is one of the reasons for
intramolecular hydrogen bond formation of La.
16

Figure 3. Electrostatic potential map of the La, Lb, Lc, Ld and TsLa-Lb.
3.6. Topological analysis
The Bader theory is a very applicable tool for analyzing hydrogen bonding. Analysis of
the properties of BCPs has often been used for the assessment of the nature of hydrogen
bonds [37-39]. The parameters derived from the Bader theory, as an example of the
Laplacian of the electron density
, the electron energy density HC [the sum of the kinetic
electron energy density (GC) and the potential electron energy density (VC)], and -GC/VC,
indicate the type of interaction. For a negative value of a Laplacian, there is no doubt that
interaction or bond formation could be covalency. If
is noncovalent. If is positive but HC is negative, and the -GC/VC is smaller than 1, then
the interaction may be classified as partly covalent in nature [36,40].
and HC are positive, the interaction
The topological parameters, such as
,
, GC, VC, and HC at the BCP of N…HO
BCP
17

bond are listed in Table 4. Also, the molecular graphs are shown in Fig. 4.
Table 4: Topological properties at the BCP of N…HO bond and NH…O in La, Lb, Lc, Ld,
TsLa-Lb, TsLa-Lc, TsLa-Ld, TsLb-Ld, TsLc-Ld.
2
O–H
BCP
Atom
ρ
∇ ρ
GC
VC
HC
-GC/VC
La
49
O2-H2
0.321112
-2.18513
0.069836
-0.68596
-0.61612
0.101808
Lb
Lc
Ld
TsLaLb
TsLaLc
TsLaLd
TsLbld
TsLcLd
N–H
La
Lb
Lc
Ld
TsLaLb
TsLaLc
TsLaLd
TsLbld
TsLcLd
35
49
39
36
40
49
42
22
BCP
35
37
37
37
23
34
27
24
16
O2-H2
O2-H2
O2-H2
O2-H2
O2-H2
O1-H1
O2-H2
O8 - H2
Atom
N2-H2
N2-H2
N2-H2
N2-H2
N2-H2
N2-H2
N1-H1
N1-H2
N2-H2
0.043893
0.32231
0.044601
0.159305
0.044504
0.322144
0.154596
0.156884
ρ
0.136244
-2.19936
0.137359
-0.13626
0.137235
-2.1968
0.03682
-0.03958
-0.68895
-0.04047
-0.21937
-0.04036
-0.68848
-0.20895
-0.21421
VC
-0.05132
-0.51358
-0.05032
-0.51285
-0.22573
-0.51361
-0.05042
-0.23391
-0.23027
-0.00276
-0.6194
0.930268
0.100958
0.924286
0.422355
0.925006
0.101153
0.56027
0.569113
-GC/VC
0.770315
0.091342
0.776424
0.091604
0.308546
0.091499
0.77549
0.069555
0.037404
0.092652
0.037336
0.069642
0.117069
0.121908
GC
0.039531
0.046912
0.039072
0.046979
0.069648
0.046995
0.039101
0.164592
0.160762
-0.00306
-0.12672
-0.00303
-0.61884
-0.09188
-0.09230
HC
-0.01179
-0.46667
-0.01125
-0.46587
-0.15608
-0.46662
-0.01132
-0.06931
-0.06951
-0.10075
-0.11844
∇ ρ
2
0.054195
0.316307
0.053352
0.315597
0.17829
0.316043
0.053451
0.182953
0.180715
0.110972
-1.67904
0.111288
-1.67556
-0.34574
-1.67849
0.111123
-0.38112
-0.365
0.70367
0.698134
The molecular graph shows the existence of a BCP between the H2 and N2 atoms, which
are linked by two bond paths. The topological structure indicates that the intramolecular
hydrogen bond existed in La. Table 4 shows that at the BCP of the hydrogen bond, the
is
positive, while HC is negative, and the ratio of GC and VC, -GC/VC, is between 0.5 and 1;
all of the topological parameters showed that the intramolecular hydrogen bond, is partly
covalent in nature [41].
The
at the BCP increased, which means that the hydrogen bond was stronger than
BCP
that in La. Compared with the La tautomer, in the Lb and Lc ones, the absolute values of
, GC, VC, and HC increased, while –GC/VC decreased, which means that the covalent
portion increased in the Lb and Lc tautomers.
18

3.7. AIM analysis on the RCP
The RCP is a point of minimum electron density within the ring surface and a maximum
on the ring line [42]. Table 5 gives the electron density
at the RCP and it’s
of the
RCP
ring (H–O–C=C–C–N), which created by the hydrogen bond formation. The distance
between the RCP and the BCP of the hydrogen bond is also listed in Table 5.
The extended dRCP BCP also prophesied that the hydrogen bond becomes stronger in the Lb
tautomer. This as well means that the properties of RCP:
and
could probably be
RCP
treated as measures of the intramolecular hydrogen bond strength.
Table 5. The topological properties at the RCP, the distance between RCP and BCP in La,
Lb, Lc, Ld, TsLa-Lb, TsLa-Lc, TsLa-Ld.
dRCP
RCP
BCP
La
0.018444
0.11645
0.108571
0.109327
0.115713
0.158086
0.109211
0.115808
0.829353
0.822264
0.823856
0.827736
0.939767
0.823176
0.82775
0.927735
0.926849
Lb
0.016959
0.017043
0.018339
0.023235
0.017033
0.018359
Lc
Ld
TSLa-Lb
TSLa-Lc
TSLa-Ld
TSLc-Ld 0.023209
TSLb-Ld 0.023221
0.158041
0.158083
19

20

Figure 4. The QTAIM molecular graph of the La, Lb, Lc, Ld, TsLa-Lb, TsLb-Ld, TsLa-
Lc, TsLc-Ld and TsLa-Ld (small green spheres, small red squares, and lines represent
BCP, RCP, and bond paths, respectively)
4. Conclusion
Here, the 3,3'-Dihydroxy-4,4'-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol
has been synthesized and characterized experimentally. The newly synthesized species was
investigated computationally by using the DFT methods. Geometries of the four possible
tautomers of the L were fully optimized in the gas phase and methanolic solution using the
PCM model. In both of the gas phase and PCM model, the La is the most stable tautomer of
the L.
In all tautomers, the L has no planar structure. The H1 and H2 phenolic protons are
engaged in the intramolecular hydrogen bond (_O-H…N), which affects considerably their
NMR chemical shifts and energy of their O-H stretching vibration in the IR spectra, too.
This intramolecular hydrogen bonds increase stability of the investigated Schiff base.
Its tautomerization could be occurred in two different pathways. In competition IPTs
between two sites of the La, proton transferring between O2 and N2 could be occurred in
lower barrier energy than the O1 to N1 pathway, which is confirmed by the Electrostatic
21

potential maps of the structures and Topological parameters by AIMall package.
The computed parameters are in good agreement with the values reported for the similar
compounds. On the other hand, the DFT calculated IR frequencies and NMR chemical shifts
also confirm the experimental data, showing validity of the optimized geometries for the
investigated species.
Acknowledgement
We gratefully acknowledge financial support of this investigation by Iran National
Science Foundation, INSF (Project Number 87020068).
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27

Graphical abstract
28

Highlights
ꢀ A new ONNO tetradentate Schiff base has been synthesized and characterized
experimentally and theoretically, too.
ꢀ Geometries of its tautomers were optimized.
ꢀ Theoretical assignments of the IR and NMR spectra have been computed.
ꢀ Its tautomerism was demonstrated by kinetical and AIM analysis.
29