Experimental and computational studies on zwitterionic (E)-2-(1-(2-(4-methylphenylsulfonamido)ethyliminio)ethyl) phenolate

Article abstract of DOI:10.1007/s11224-010-9641-7

The Schiff base compound (E)-2-(1-(2-(4-methylphenylsulfonamido)ethyliminio)ethyl) phenolate has been synthesised and characterized by IR, UV-Vis, and X-ray single-crystal determination. Ab initio calculations have been carried out for the title compound using the density functional theory (DFT) and Hartree-Fock (HF) methods at 6-31G(d) basis set. The calculated results show that the DFT/B3LYP and HF can well reproduce the structure of the title compound. Using the TD-DFT and TD-HF methods, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD-DFT method and the experimental ones is determined. Molecular orbital coefficient analyses reveal that the electronic transitions are mainly assigned to n → π* and π → π* electronic transitions. To investigate the tautomeric stability, optimization calculations at B3LYP/6-31G(d) level were performed for the NH and OH forms of the title compound. Calculated results reveal that the OH form is more stable than NH form. In addition, molecular electrostatic potential and NBO analysis of the title compound were performed at B3LYP/6-31G(d) level of theory.

Full text of DOI:10.1007/s11224-010-9641-7

Struct Chem (2010) 21:1027–1036
DOI 10.1007/s11224-010-9641-7
ORIGINAL RESEARCH
Experimental and computational studies on zwitterionic (E)-2-
(1-(2-(4-methylphenylsulfonamido)ethyliminio)ethyl) phenolate
•
Gokhan Alpaslan Hasan Tanak
•
¨
Ays¸en Alaman Agar Ahmet Erdonmez
•
•
˘
¨
S¸amil Is¸ık
Received: 16 April 2010 / Accepted: 6 July 2010 / Published online: 18 July 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The Schiff base compound (E)-2-(1-(2-(4-
methylphenylsulfonamido)ethyliminio)ethyl) phenolate has
been synthesised and characterized by IR, UV–Vis, and
X-ray single-crystal determination. Ab initio calculations
have been carried out for the title compound using the
density functional theory (DFT) and Hartree–Fock (HF)
methods at 6-31G(d) basis set. The calculated results show
that the DFT/B3LYP and HF can well reproduce the
structure of the title compound. Using the TD-DFT and
TD-HF methods, electronic absorption spectra of the title
compound have been predicted and a good agreement with
the TD-DFT method and the experimental ones is deter-
mined. Molecular orbital coefficient analyses reveal that
the electronic transitions are mainly assigned to n ? p*
and p ? p* electronic transitions. To investigate the tau-
tomeric stability, optimization calculations at B3LYP/6-
31G(d) level were performed for the NH and OH forms of
the title compound. Calculated results reveal that the OH
form is more stable than NH form. In addition, molecular
electrostatic potential and NBO analysis of the title com-
pound were performed at B3LYP/6-31G(d) level of theory.
Introduction
Schiff bases and their complexes are of great interest in
many fields of chemistry and biochemistry because of their
versatile metal-binding ability [1, 2]. Moreover, Schiff
bases also have biological activities such as antimicrobial
[3, 4], antifungal [5], antitumor [6, 7] activities, and her-
bicidal properties [8]. On the industrial scale, they have a
wide range of applications, such as in dyes and in pigments
[9]. Schiff base compounds display interesting photochro-
mic and thermochromic features in the solid state and can
be classified in terms of these properties [10]. Photo- and
thermo-chromism arise via H-atom transfer from the
hydroxy O atom to the N atom [11, 12]. Such proton
exchanging materials can be utilized for the design of
various molecular electronic devices [13]. In general,
Schiff bases display two possible tautomeric forms, the
phenol-imine (OH) and the keto-amine (NH) forms.
Depending on the tautomers, two types of intramolecular
hydrogen bonds are observed in Schiff bases: O–HꢀꢀꢀN in
phenol-imine [14, 15] and N–HꢀꢀꢀO in keto-amine [16–18]
tautomers (Scheme 1).
Keywords Schiff base ꢀ Zwitterionic ꢀ MEP ꢀ NBO ꢀ
DFT ꢀ HF
Although Schiff bases exhibit two possible tautomeric
forms, OH and NH, another structural form, the zwitterion,
is also observed. Zwitterionic forms of Schiff bases are
regarded as a variant of their NH forms, but they can easily
be distinguished from keto (NH) tautomers by their N?–H
bond distances [19]. Zwitterionic forms of Schiff bases
have an intramolecular ionic hydrogen bond (N?–HꢀꢀꢀO-),
and their N?–H bonds are longer than the standard inter-
¨
G. Alpaslan ꢀ H. Tanak (&) ꢀ A. Erdonmez ꢀ S¸. Is¸ık
Department of Physics, Faculty of Arts and Sciences,
Ondokuz Mayıs University, Kurupelit, 55139 Samsun, Turkey
e-mail: htanak@omu.edu.tr
˚
atomic separations observed in neutral N–H bonds (0.87 A)
[19]. The other ionic bonds in the zwitterions, C=N? and
C–O-, are not as distinctive as indicators as is the N?–H
bond, because the NH form of Schiff bases in the solid state
can be regarded as a resonance hybrid of two canonical
˘
A. A. Agar
Department of Chemistry, Faculty of Arts and Sciences,
Ondokuz Mayıs University, Kurupelit, 55139 Samsun, Turkey
123

1028
Struct Chem (2010) 21:1027–1036
Scheme 1 Tautomeric forms of
o-hydroxy Schiff bases
N
R
N
R
N
R
O
H
O
H
O
H
phenol-imine
OH Form
keto-amine
zwitterionic
NH Forms
suitable for X-ray analysis were obtained from ethylalcohol
by slow evaporation (Yield: % 54; m.p., 421–423 K).
Chemical diagram of the title compound is shown in
Scheme 2.
Crystal structure determination
A yellow crystal of the compound with dimensions of
0.50 9 0.41 9 0.32 mm3 was mounted on goniometer of a
STOE IPDS II diffractometer. Measurements were per-
formed at room temperature (296 K) using graphite
Scheme 2 (E)-2-(1-(2-(4-methylphenylsulfonamido)ethyliminio)
ethyl)phenolate(C17H20N2O3S)
˚
monochromated MoKa radiation (k = 0.71073 A).
The systematic absences and intensity symmetries indi-
cated the monoclinic P21/n space group. A total of
9145 reflection (2705 unique) within the h range of
[2.06° \ h \ 26.0°] were collected in the w scan mode.
Cell parameters were determined using X-AREA software
[30]. The intensities collected were corrected for Lorentz
and polarization factors, absorption correction (l =
0.21 mm-1) by integration method via X-RED32 software
[30]. The structure was solved by direct methods using
SHELXS-97 [31]. The maximum peaks and deepest hole
structures, namely the keto tautomer and the zwitterionic
form [20, 21]. Different methods were used to show the
presence of the enol, keto, and zwitterionic forms, among
1
them are UV–Vis, IR, MS, H-, 13C-, 14N-NMR spectros-
copy, and X-ray crystallography techniques [22–29].
In this article, we report the synthesis, characterization,
and crystal structure of the Schiff base compound (E)-2-
(1-(2-(4-methylphenylsulfonamido)ethyliminio)ethyl) phe-
nolate (Scheme 2) as well as the theoretical studies on it
using the HF/6-31G(d) and DFT/B3LYP/6-31G(d) meth-
ods. The properties of the structural geometry, molecular
electrostatic potential (MEP), and natural bond orbital
(NBO) analysis for the title compound at the B3LYP/6-
31G(d) level were studied. These studies are valuable for
providing insight into molecular properties of Schiff base
compounds.
-3
˚
observed in the final Dq map were 0.24 and -0.27 eA
,
respectively. The scattering factors were taken from
SHELXL-97 [31]. The molecular graphics were done using
ORTEP-3 for Windows [32]. The data collection condi-
tions and parameters of refinement process are listed in
Table 1.
Computational methods
The molecular geometry is directly taken from the X-ray
diffraction experimental result without any constraints.
In the next step, the DFT calculations with a hybrid
functional B3LYP (Becke’s Three parameter hybrid func-
tional using the LYP correlation functional) at 6-31G(d)
basis set and the Hatree–Fock calculations at 6-31G(d)
basis set by the Berny method [33, 34] were performed
with the Gaussian 03W software package [35] and Gauss-
view visualization program [36]. The vibrational frequency
calculations at the same levels of theory revealed no
imaginary frequencies, indicating that an optimal geometry
at these levels of approximation was found for the title
Experimental and computational methods
Synthesis
The compound (E)-2-(1-(2-(4-methylphenylsulfonamido)-
ethyliminio)ethyl) phenolate was prepared by reflux a
mixture of a solution containing 2-hydroxyacetophenone
(0.05 g 0.367 mmol) in 20-mL ethanol and a solution con-
taining N-(2-aminoethyl)-p-toluenesulfonamide (0.078 g
0.367 mmol) in 20-mL ethanol. The reaction mixture was
stirred for 1 h under reflux. The crystals of (E)-2-(1-(2-
(4-methylphenylsulfonamido)ethyliminio)ethyl)phenolate
123

Struct Chem (2010) 21:1027–1036
1029
the B3LYP/6-31G(d) method. The molecular electrostatic
potential, V(r), at a given point r(x, y, z) in the vicinity of a
molecule, is defined in terms of the interaction energy
between the electrical charge generated by the molecule’s
electrons and nuclei and a positive test charge (a proton)
located at r. For the system studied the V(r) values were
calculated as described previously using the equation [46],
Table 1 Crystallographic data for the title compound
Crystal data
Chemical formula
Crystal shape/color
Formula weight
Crystal system
C17H20N2O3S
Prism/yellow
332.41
Monoclinic
P21/n
Space group
Z
X
ZA
qðr0Þ
˚
a = 11.4472(6) A
Unit cell parameters
VðrÞ ¼
ꢁ
dr0
ð1Þ
r0 ꢁ r
˚
b = 11.1176(4) A
RA ꢁ r
A
˚
c = 13.4873(7) A
where ZA is the charge of nucleus A, located at RA, q(r0) is
the electronic density function of the molecule, and r0 is the
dummy integration variable. In addition, NBO analysis was
performed at the B3LYP/6-31G(d) level by means of the
NBO 3.1 program within the Gaussian 03W package [47].
3
˚
1639.36(13) A
Volume
Z
4
Dx (Mg cm-3
)
1.347
l (mm-1
)
0.214
F000
704
Crystal size (mm3)
Data collection
Diffractometer/meas. meth
Absorption correction
Tmin
0.50 9 0.41 9 0.32
Results and discussion
STOE IPDS II/w-scan
Integration
0.903
Description of the crystal structure
Tmax
0.954
The title compound, an Ortep-3 view of which is shown in
Fig. 1, crystallizes in the monoclinic space group P21/
n with Z = 4 in the unit cell. The asymmetric unit in the
crystal structure contains only one molecule. The molecule
adopts a folded conformation and dihedral angle between
the aromatic ring systems is 6.57(5)°. It is also known that
Schiff bases may exhibit photochromism depending on the
planarity or non-planarity, respectively [48]. The C7=N1
No. of measured, independent and 9145, 3194, 2705
observed reflections
Criterion for observed reflections
Rint
I [ 2r(I)
0.023
26
hmax
Refinement
Refinement on
F2
R[F2 [ 2r (F2)], wR, S
No. of reflection
No. of parameters
Weighting scheme
0.034, 0.097, 1.05
˚
˚
[1.303(2) A] and C2–O1 [1.301(2) A] bonds of the title
compound are the most important indicators of the tauto-
meric type. While the C2–O1 bond is of a double bond for
the keto-amine tautomer, this bond displays single bond
character in phenol-imine tautomer. In addition, the C7–N1
bond is also a double bond in phenol-imine tautomer and of
single bond length in keto-amine tautomer [49, 50].
However, these bond distances have intermediate values
3194
219
w = 1/[r2(F20) ? (0.0843P)2]
P = (F20 ? 2F2c)/3
0.001
(D/r)max
-3
)
˚
Dqmax, Dqmin (e A
0.24, -0.27
˚
between single and double C–O (1.362 and 1.222 A,
˚
respectively) and C–N (1.339 and 1.279 A, respectively)
compound. The electronic absorption spectra were calcu-
lated using the time-dependent density functional theory
(TD-DFT) and Hartree–Fock (TD-HF) methods [37–40].
Also, it is calculated in ethanol solution using the Polar-
izable Continuum Model (PCM) [41–44]. To investigate
the tautomeric stability, some properties such as total
energy, HOMO and LUMO energies, and the chemical
hardness [45] for the OH and NH forms of the title com-
pound were obtained at B3LYP/6-31G(d) level in gas
phase. These properties were also examined for the title
compound in solvent media with three kinds of solvent
(chloroform, ethanol, and water) using the PCM method.
To investigate the reactive sites of the title compound
the molecular electrostatic potential was evaluated using
bond distance [51]. The shortened C2–O1 bond and the
slightly longer C7–N1 bond provide structural evidence for
the zwitterionic tautomeric form of the title compound.
Additionally, the bonds of C1–C7, C7–N1, and N1–C9
with their respective bond distances of 1.443(2), 1.303(2),
˚
and 1.457(2) A indicate the zwitterionic character of the
title compound [52]. The bond lengths of the title com-
pound are in good agreement with those of the related
structures 2-[(E)-2-(4-methylbenzenesulfonamido)ethyli-
miniomethyl]-4-nitrophenolate [53] and (E)-2-hydroxy-6-
[(4-propylphenyl)iminiomethyl]phenolate [54].
The title compound is stabilized by N–HꢀꢀꢀO and
C–HꢀꢀꢀO type hydrogen bonds. There is
a strong
123

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Struct Chem (2010) 21:1027–1036
intramolecular N1?–H1ꢀꢀꢀO1- hydrogen bond (Fig. 1) with
N1ꢀꢀꢀO1 distance shorter than the sum of the van der Waals
˚
radii of O and N (3.07 A) [55]. Atom N2 in the molecule at
(x, y, z) act as hydrogen-bond donor to atom O1 in the
molecule at (1 - x, -y, -z), so forming a cyclic centro-
symmetric R22(18) [56] dimer centered. The C–HꢀꢀꢀO
hydrogen bonds link the molecules into a C(5) chain along
the b axis (Fig. 2). In addition, there is a C–Hꢀꢀꢀp interac-
tion of C17–H17C and the phenyl ring. Perpendicular
distance between atom H17C and the plane of the phenyl
˚
ring is 3.185 A. The details of the hydrogen bonds are
summarized in Table 2.
Optimized geometries
The optimized parameters (bond lengths, bond angles, and
dihedral angles) of the title compound have been obtained
at HF and B3LYP methods with the 6-31G(d) basis set.
The calculated results are listed in Table 3. When the
X-ray structure of the compound is compared with its
optimized counterparts (see Fig. 3), conformational dis-
crepancies are observed between them. The most remark-
able discrepancies exist in the orientation of the phenyl and
tolyl rings in the compound. According to X-ray study,
dihedral angle between these two rings is 6.57(5)°, whereas
it has been calculated as 66.77° for B3LYP and 64.04° for
HF. The orientation of the both the rings is defined by the
torsion angles C2–C1–C7–N1 [-5.3(2)°], C7–N1–C9–C10
[85.04(19)°], C9–C10–N2–S1 [-135.82(12)°] and N2–S1–
C11–C16 [-100.86(13)°] which have been calculated as
9.77°, 100.61°, 160.74°, and -95.12° for B3LYP, and
11.20°, 94.03°, 160.64°, and -98.12° for HF, respectively.
As seen from Table 3, most of the calculated bond
lengths and the bond angles are slightly different from the
experimental ones. We noted that the experimental results
belong to the solid phase and theoretical calculations
belong to the gaseous phase. In the solid state the experi-
mental results are related to molecular packing, but in gas
phase the isolated molecules are considered in the theo-
retical calculations. The biggest differences of bond lengths
between the experimental and the predicted values are
Fig. 1 Ortep
3 diagram of the title compound. Displacement
ellipsoids are drawn at the 50% probability level and H atoms are
shown as small spheres of arbitrary radii
Fig. 2 Packing diagram of the title compound
˚
Table 2 Hydrogen-bond geometry (A, °)
˚
found at N2–S1 bond with the difference being 0.081 A for
˚
B3LYP method, and C4–C5 bond with a value 0.059 A for
D–HꢀꢀꢀA
D–H
HꢀꢀꢀA
DꢀꢀꢀA
D–HꢀꢀꢀA
HF method. While the biggest differences for the bond
angles are found as 3.96° at C7–N1–C9 for B3LYP and
3.56° at N1–C7–C8 for HF. According to above compari-
sons, the biggest differences of bond lengths and bond
angles mainly occur in the groups involved in the hydrogen
bonds [i.e., N2–S1, C4–C5, C7–N1–C9 and N1–C7–C8],
which can also easily be understood taking into account the
intra- and intermolecular hydrogen bond interactions
present in the crystal.
N1–H1ꢀꢀꢀO1
0.91(2)
1.68(2)
2.5056(17) 149(2)
N2–H18ꢀꢀꢀO1i
C12–H12ꢀꢀꢀO3ii
0.838(19) 1.949(19) 2.7816(17) 172.4(18)
0.93
C17–H17CꢀꢀꢀCg1iii 0.96
Note: Symmetry transformations used to generate equivalent atoms.
(Cg1 is the centroid of the C1–C6 ring)
2.51
3.309(2)
3.937 (3)
143.8
136
3.185
D donor, A acceptor
(i) 1 - x, -y, -z; (ii) 3/2 - x, -1/2 ? y, ‘ - z; (iii) 1/2 ? x, 1/2
- y, 1/2 ? z
123

Struct Chem (2010) 21:1027–1036
1031
Table 3 Selected molecular structure parameters
Table 3 continued
Parameters
Experimental
HF/
6-31G(d)
B3LYP/
6-31G(d)
Parameters
Experimental
HF/
6-31G(d)
B3LYP/
6-31G(d)
˚
Bond lengths (A)
C15–C14–C13
C15–C14–C17
C13–C14–C17
C14–C15–C16
C15–C16–C11
C7–N1–C9
118.13(17)
120.88(19)
120.99(19)
121.43(17)
119.38(16)
126.26(14)
119.36(11)
119.13(8)
106.72(8)
107.86(7)
108.17(8)
107.16(8)
107.28(7)
118.58
121.22
120.18
121.00
119.47
128.93
120.52
121.88
105.78
106.55
107.58
107.34
106.84
3.56
118.39
121.05
120.53
121.19
119.21
130.22
118.96
122.64
105.56
106.11
107.45
107.42
106.69
3.96
C1–C6
1.410(2)
1.425(2)
1.443(2)
1.301(2)
1.416(2)
1.365(3)
1.374(3)
1.366(3)
1.303(2)
1.490(2)
1.457(2)
1.519(2)
1.4577(19)
1.382(2)
1.385(2)
1.7613(16)
1.373(3)
1.387(3)
1.378(3)
1.509(3)
1.378(3)
1.6011(13)
1.4245(12)
1.4313(12)
1.439
1.462
1.405
1.235
1.452
1.344
1.433
1.345
1.322
1.509
1.449
1.527
1.455
1.383
1.390
1.771
1.379
1.394
1.387
1.510
1.386
1.638
1.426
1.430
0.059
0.0261
1.427
1.467
1.426
1.278
1.438
1.371
1.421
1.372
1.330
1.508
1.454
1.538
1.463
1.395
1.398
1.797
1.391
1.403
1.400
1.510
1.395
1.683
1.461
1.465
0.081
0.0283
C1–C2
C1–C7
C2–O1
C2–C3
C3–C4
C10–N2–S1
C4–C5
O2–S1–O3
C5–C6
O2–S1–N2
C7–N1
O3–S1–N2
C7–C8
O2–S1–C11
C9–N1
O3–S1–C11
C9–C10
N2–S1–C11
C10–N2
Max dif.
C11–C16
C11–C12
C11–S1
RMSE
1.227
1.181
Torsion angles (°)
C10–C9–N1–C7
C9–C10–N2–S1
C12–C11–S1–N2
C10–N2–S1–C11
N1–C9–C10–N2
C2–C1–C7–N1
C6–C1–C7–N1
85.04(19)
-135.82(12)
77.15(14)
65.17(12)
69.28(17)
-5.3(2)
94.03
160.64
81.37
100.61
160.74
83.80
C12–C13
C13–C14
C14–C15
C14–C17
C15–C16
N2–S1
70.46
71.10
60.10
59.26
11.20
9.77
177.99(15)
-170.90
-174.58
O2–S1
O3–S1
Max dif.
RMSE
In order to compare the theoretical results with the
experimental values, root mean square error (RMSE) is
used and the values of which for bond lengths and bond
Bond angles (°)
C6–C1–C2
C6–C1–C7
C2–C1–C7
O1–C2–C3
O1–C2–C1
C3–C2–C1
C4–C3–C2
C3–C4–C5
C6–C5–C4
C5–C6–C1
N1–C7–C1
N1–C7–C8
C1–C7–C8
N1–C9–C10
N2–C10–C9
C16–C11–C12
C16–C11–S1
C12–C11–S1
C13–C12–C11
C12–C13–C14
118.88(15)
121.13(15)
119.91(15)
120.87(16)
122.10(14)
117.02(16)
121.67(19)
121.24(19)
119.27(19)
121.88(19)
118.85(14)
120.10(15)
121.00(15)
112.69(13)
111.06(12)
120.25(16)
120.38(12)
119.33(13)
119.20(17)
121.60(17)
118.38
121.81
119.77
119.74
123.81
116.42
121.86
121.60
119.18
122.48
120.17
116.54
123.27
111.41
109.71
120.44
120.12
119.43
119.44
121.04
119.10
121.10
119.64
120.29
123.13
116.57
121.81
121.28
119.36
121.83
117.88
120.20
121.90
110.05
110.08
120.77
119.86
119.34
119.19
121.22
˚
angles are calculated as 0.0283 A and 1.181° for B3LYP
˚
method, and 0.0261 A and 1.227° for HF method. A logical
method for globally comparing the structures obtained with
the theoretical calculations is by superimposing the
molecular skeleton with that obtained from X-ray diffrac-
˚
˚
tion, giving a RMSE of 1.792 A for B3LYP and 1.837 A
for method HF (Fig. 3). According to these results, it may
be concluded that while the HF calculations well reproduce
the bond lengths, the B3LYP method is better in predicting
the bond angles and 3-D geometry of the title compound.
IR spectroscopy
The FT-IR spectra of the title compound were recorded in
the 4000–400 cm-1 region using KBr pellets on a
Schmadzu FT-IR 8900 spectrophotometer. The spectra
contain some characteristic bands of the stretching vibra-
tions of the NH, CH, CN, CO, and SO2 groups. The IR
spectrum of the title compound shows two bands at 3371
123

1032
Struct Chem (2010) 21:1027–1036
Fig. 3 Atom-by-atom
superimposition of the
structures calculated
(a = HF; b = DFT) over the
X-ray structure for the title
compound
and 3191 cm-1 due to N–H stretching vibrations.
This difference between the N–H stretching vibration fre-
quencies is because of the intramolecular N1?–H1ꢀꢀꢀO1-
hydrogen bonding which leads a shift to lower wavenum-
ber. The weak bands in the range 3070–3000 cm-1 cor-
respond to the symmetric and asymmetric stretching
vibrations of aromatic and methyl CH bonds. The charac-
teristic region of 1500–1700 cm-1 can be used to identify
the proton transfer of Schiff bases. The title compound
shows a strong band at 1610 cm-1 which is assigned to
C=N stretching vibration. Another characteristic region of
which indicates that the title compound is in zwitterionic
form and not in keto in ethanol solvent.
Electronic absorption spectra of the title compound were
calculated by TD-DFT and TD-HF methods based on the
B3LYP/6-31G(d) and HF/6-31G(d) levels optimized
structure in gas phase, respectively. For TD-HF calcula-
tions, the absorption wavelengths are obtained at 297, 215,
and 176 nm. It is obvious that these bands are not corre-
sponding to the experimental results, which shows that
using the TD-HF method here to predict the electronic
absorption spectra is not reasonable. For TD-DFT calcu-
lations, the theoretical absorption bands are predicted at
388, 275, and 224 nm and can easily be seen that they
correspond to the experimental absorption ones. In addition
to the calculations in gas phase, TD-DFT calculations of
the title compound in ethanol solvent were performed by
using PCM model. The PCM calculations reveal that the
calculated absorption bands have slight blue-shifts with
values of 379, 269, and 221 nm when compared with the
the Schiff bases derivative spectrum is 1100–1400 cm-1
,
which is attributed to C–O stretching vibrations. The C–O
stretching vibration was observed at 1388 cm-1 which
confirms the presence of phenolate group in the compound.
The symmetric and asymmetric SO2 stretching vibrations
can occur in the regions 1125–1150 and 1295–1330 cm-1
respectively [57]. In our study, these SO2 stretching
vibrations were observed at 1064 and 1285 cm-1
,
.
The presence of N–H, C=N, and C–O stretching vibrations
strongly suggest that the title compound has the zwitter-
ionic form in the solid state.
Electronic absorption spectra
The UV–Vis electronic absorption spectra of the title
compound were recorded within 200–600-nm range on a
Unicam UV–Vis spectrophotometer in EtOH solvent. The
observed spectra showed three bands at 360 (loge =
5.008), 288 (loge = 4.657), and 242 nm (loge = 4.628).
The maximum absorption wavelength is assigned to
intramolecular electronic transfer band of the C=N group
which contributes to n ? p* transition and the other bands
may be due to p ? p* transitions of the phenyl and tolyl
rings. These values are similar to those found in related
compound [29]. According to some studies on the UV–Vis
spectrum of o-hydroxylated Schiff bases, the band at
[400 nm indicates presence of the keto-amine form
[58–61]. In our study, any band belonging to keto-amine
form was not observed with a value greater than 400 nm,
Fig. 4 Molecular orbital surfaces and energy levels given in paren-
theses for the HOMO - 2, HOMO, LUMO, and LUMO ? 1 of the
title compound computed at B3LYP/6-31G(d) level
123

Struct Chem (2010) 21:1027–1036
1033
gas-phase calculations of TD-DFT method. According to
the investigation on the frontier molecular orbital (FMO)
energy levels of the title compound, we can find that the
corresponding electronic transfers happened between the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO), HOMO - 2 and
LUMO, HOMO - 2 and LUMO ? 1 orbitals, respec-
tively. Figure 4 shows the distributions and energy levels
of the FMOs computed at the B3LYP/6-31G(d) level for
the title compound. As seen from Fig. 4, in the HOMO and
LUMO, electrons are mainly delocalized on the phenolate
ring and the atoms of imine group; in the LUMO ? 1,
electrons are delocalized on the tolyl ring and sulfonamide
fragment and in the HOMO - 2, electrons are delocalized
on the whole structure.
Fig. 5 Molecular electrostatic potential map calculated at B3LYP/6-
31G(d) level
Molecular orbital coefficients analyses based on opti-
mized geometry indicate that, for the title compound, the
frontier molecular orbitals are mainly composed of
p-atomic orbitals, so aforementioned electronic transitions
are mainly derived from the contribution of bands n ? p*
and p ? p*.
In order to evaluate the solvent effect to the aforemen-
tioned properties of the title compound, we carried out
calculations in three kinds of solvent (water, ethanol, and
chloroform) with the B3LYP/6-31G(d) level using the
PCM model and the results are given in Table 4. From
Table 4, we can conclude that the total molecular energies
obtained by PCM method decrease with the increasing
polarity of the solvent, while the dipole moments and
hardness will increase with the increase of the polarity of
the solvent. According to these results, the stability of the
title compound increases in going from the gas phase to the
solution phase.
Energetics and stability
The NH and OH tautomerism of the title compound is
given in Scheme 1. To investigate the tautomeric stability,
optimization calculations at B3LYP/6-31G(d) level were
performed for the NH and OH forms of the title compound.
Some physicochemical properties such as total, HOMO,
and LUMO energies, dipole moment and chemical hard-
ness (g) were also calculated with the same level of theory
and the results were given in Table 4. The chemical
hardness is quite useful to rationalize the relative stability
and reactivity of chemical species. Hard species having
large HOMO–LUMO gap will be more stable and less
reactive than soft species having small HOMO–LUMO gap
[62]. As seen from Table 4, the total energy of the OH
form is lower than the NH form, while chemical hardness
of the OH form is greater than the NH one, which indicates
that the OH form of the title compound is more stable than
its NH form in gas phase.
Molecular electrostatic potential
MEP is related to the electronic density and is a very useful
descriptor in determining sites for electrophilic and nucle-
ophilic reactions as well as hydrogen bonding interactions
[63, 64]. The electrostatic potential V(r) is also well suited
for analyzing processes based on the ‘‘recognition’’ of one
molecule by another, as in drug–receptor and enzyme–
substrate interactions, because it is through their potentials
that the two species first ‘‘see’’ each other [65, 66].
Table 4 Calculated energies, dipole moments, frontier orbital energies, and harnesses
OH form gas phase
(e = 1)
NH form gas phase
(e = 1)
Chloroform
(e = 4.9)
Ethanol
(e = 24.55)
Water
(e = 78.39)
Etotal (Hartree)
EHOMO (eV)
ELUMO (eV)
g (eV)
-1393.153562
-5.959
-1.439
2.26
-1393.150209
-5.429
-1.819
1.805
-1393.163211
-5.396
-1.712
1.842
-1393.167035
-5.387
-1393.167751
-5.386
-1.681
-1.675
1.853
1.855
l (D)
8.102
8.385
9.696
10.139
10.225
123

1034
Struct Chem (2010) 21:1027–1036
Table 5 Second-order perturbation theory analysis of the Fock matrix in NBO basis, calculated at B3LYP/6-31G(d) level
Donor orbital (i)
Acceptor orbital (j)
E(2)c (kcal/mol)
ej - ei (a.u.)a
Fij (a.u.)b
LP(1) O1
LP(2) O1
LP(1) O1
LP(2) O1
LP(3) O1
LP(1) O3
LP(3) O3
a
BD(1) N1–H1
BD(1) N1–H1
BD(1) N2–H18
BD(1) N2–H18
BD(1) N2–H18
BD(1) C12–H12
BD(1) C12–H12
6.28
17.76
5.14
2.85
1.50
0.61
0.49
1.40
0.97
1.62
1.19
1.17
1.56
1.09
0.084
0.119
0.082
0.053
0.041
0.027
0.022
Energy difference between donor and acceptor i and j NBO orbitals
Fij is the Fock matrix element between i and j NBO orbitals
E(2) means energy of hyper conjugative interactions
b
c
To predict reactive sites for electrophilic or nucleophilic
formally unoccupied (antibond or Rydgberg) non-Lewis
NBO orbitals correspond to a stabilizing donor–acceptor
interaction.
attack for the investigated molecule, the MEP at the
B3LYP/6-31G(d) optimized geometry was calculated.
The negative (red) regions of the MEP are related to
electrophilic reactivity and positive (blue) regions to
nucleophilic reactivity, as shown in Fig. 5. As can be seen
from the figure, this molecule has several possible sites for
electrophilic attack. Negative electrostatic potential regions
(red color) are mainly localized over the O1 atom and the
O2 and O3 atoms of the sulfonamide group. The negative
V(r) values are -0.069 a.u. for O3 atom which is the most
negative region, -0.067 a.u. for O1 atom and -0.064 a.u.
for O2 atom. However, a maximum positive region (blue
color) is localized on the C9–H9A bond with a value of
?0.049 a.u., indicating a possible site for nucleophilic
attack. According to these calculated results, the MEP map
shows that the negative potential sites are on oxygen atoms
as well as the positive potential sites are around the
hydrogen atoms. These sites give information concerning
the region from where the compound can have metallic
bondings and intermolecular interactions. So, Fig. 5 con-
firms the existence of the intermolecular C12–H12ꢀꢀꢀO3
and N2–H18ꢀꢀꢀO1 interactions.
In order to investigate the intra and intermolecular
interactions, the stabilization energies of the title com-
pound were performed by using second-order perturbation
theory. For each donor NBO(i) and acceptor NBO(j), the
stabilization energy E(2) associated with electron delocal-
ization between donor and acceptor is estimated as [68, 69]
ꢀ
ꢁ
2
Fij
Eð2Þ ¼ ꢁqi
ð2Þ
ej ꢁ ei
where qi is the donor orbital occupancy, ej, ej are diag-
onal elements (orbital energies), and Fij is the off-
diagonal NBO Fock matrix element. The results of
second-order perturbation theory analysis of the Fock
Matrix at B3LYP/6-31G(d) level of theory are presented
in Table 5.
NBO analysis revealed that the n(O1) ? r(N1–H1)
interactions give the strongest stabilization to the system of
the title compound by 24.04 kcal mol-1, and strengthen
the intramolecular N1–H1ꢀꢀꢀO1 hydrogen bond. The lone
pairs of O1 also donate its electrons to r-type antibonding
orbital for N2–H18. The total stabilization energy of
N2–H18ꢀꢀꢀO1 intermolecular hydrogen bonding is
9.49 kcal mol-1. There is another NBO interaction of the
n(O3) ? r(C12–H12) imply the existence of C12–
H12ꢀꢀꢀO3 hydrogen bond which has the total stabilization
energy 1.1 kcal mol-1. We can say that the energy of the
C12–H12ꢀꢀꢀO3 hydrogen bond we obtained is reliable when
it is compared with some values in literature. For example,
Desiraju [70] has estimated the energy of the C–HꢀꢀꢀO
NBO analysis
NBO analysis provides an efficient method for studying
intra- and intermolecular bonding and interaction among
bonds, and also provides a convenient basis for investi-
gating charge transfer or conjugative interaction in
molecular systems [67]. The larger the E(2) value, the
more intensive is the interaction between electron donors
and electron acceptors, i.e., the more donating tendency
from electron donors to electron acceptors and the
greater the extent of conjugation of the whole system.
Delocalization of electron density between occupied
Lewis-type (bond or lone pair) NBO orbitals and
hydrogen bond as 1.0–2.0 kcal mol-1
.
Thus, it is apparent that N–HꢀꢀꢀO and C–HꢀꢀꢀO inter-
molecular interactions significantly influence crystal pack-
ing in this molecule.
123

Struct Chem (2010) 21:1027–1036
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