Synthesis, spectroscopic investigations and computational study of monomeric and dimeric structures of 2-methyl-4-quinolinol

Article abstract of DOI:10.1007/s11164-015-2084-4

The present study aimed to determine an efficient and solvent-free method to synthesize 2-methyl-4-quinolinol (2MQ, also known as 4-hydroxy-2-methylquinoline) and includes spectroscopic investigations and computational studies. Molecular geometry and vibrational wavenumbers of 2MQ were investigated using the density functional (DFT/B3LYP) method with 6-311++G(d,p) and 6-311++G(2d,p) basis sets. According to calculations, the keto form of 2MQ is more stable than the annual form, and the dimeric conformation is predicted to be more stable than the monomeric conformations. A detailed analysis of the nature of the hydrogen bonding, using topological parameters such as electronic charge density, Laplacian, kinetic and potential energy density evaluated at the bond critical point, is also presented. The ¹H nuclear magnetic resonance chemical shifts of the molecule were calculated by the GIAO method. The molecule orbital contributions were studied by using total (TDOS) and partial (PDOS) density of states. The UV-visible spectrum of the compound was recorded and the electronic properties, such as HOMO and LUMO energies, were investigated by the time-dependent DFT (TD-DFT) approach. The linear polarizability (α) and the first-order hyperpolarizability (β) values of the investigated molecule were computed using DFT quantum mechanical calculations. The results show that the 2MQ molecule may have a nonlinear optical comportment with non-zero values. The stability and charge delocalization of the molecule was studied by natural bond orbital analysis. In addition, a molecular electrostatic potential map of the title compound was studied for predicting the reactive sites. Local reactivity descriptors, such as Fukui functions, local softness and electrophilicity indices analyses, were studied to determine the reactive sites within the molecule.

Full text of DOI:10.1007/s11164-015-2084-4

Res Chem Intermed
DOI 10.1007/s11164-015-2084-4
Synthesis, spectroscopic investigations
and computational study of monomeric and dimeric
structures of 2-methyl-4-quinolinol
1
,2
1
•
Seied Ali Pourmousavi
1
•
Ayoub Kanaani
Kobra Khorsi Damghani
1
•
Fatemeh Ghorbani
,2
•
1
3
Davood Ajloo
•
Mohamad Vakili
Received: 19 December 2014 / Accepted: 28 April 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract The present study aimed to determine an efficient and solvent-free
method to synthesize 2-methyl-4-quinolinol (2MQ, also known as 4-hydroxy-2-
methylquinoline) and includes spectroscopic investigations and computational
studies. Molecular geometry and vibrational wavenumbers of 2MQ were investi-
gated using the density functional (DFT/B3LYP) method with 6-311??G(d,p) and
6
-311??G(2d,p) basis sets. According to calculations, the keto form of 2MQ is
more stable than the annual form, and the dimeric conformation is predicted to be
more stable than the monomeric conformations. A detailed analysis of the nature of
the hydrogen bonding, using topological parameters such as electronic charge
density, Laplacian, kinetic and potential energy density evaluated at the bond cri-
1
tical point, is also presented. The H nuclear magnetic resonance chemical shifts of
the molecule were calculated by the GIAO method. The molecule orbital contri-
butions were studied by using total (TDOS) and partial (PDOS) density of states.
The UV–visible spectrum of the compound was recorded and the electronic prop-
erties, such as HOMO and LUMO energies, were investigated by the time-depen-
dent DFT (TD-DFT) approach. The linear polarizability (a) and the first-order
hyperpolarizability (b) values of the investigated molecule were computed using
DFT quantum mechanical calculations. The results show that the 2MQ molecule
may have a nonlinear optical comportment with non-zero values. The stability and
charge delocalization of the molecule was studied by natural bond orbital analysis.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2084-4)
contains supplementary material, which is available to authorized users.
&
Seied Ali Pourmousavi
pourmousavi@du.ac.ir
1
2
3
School of Chemistry, Damghan University, 36716-41167 Damghan, Iran
Institute of Biological Sciences, Damghan University, 36716-41167 Damghan, Iran
Department of Chemistry, Ferdowsi University of Mashhad, 91775-1436 Mashhad, Iran
123

S. A. Pourmousavi et al.
In addition, a molecular electrostatic potential map of the title compound was
studied for predicting the reactive sites. Local reactivity descriptors, such as Fukui
functions, local softness and electrophilicity indices analyses, were studied to de-
termine the reactive sites within the molecule.
1
Keywords 2-Methyl-4-quinolinol Á DFT Á Hydrogen bonded dimer Á FT-IR Á H
NMR and UV spectra Á NBO analysis
Introduction
Quinoline nucleus occurs in several natural compounds displaying a broad range of
biological and pharmaceutical activity [1–3]. The synthesis of quinolone derivatives
using a variety of methods has been documented [4–9].
We herein report the preparation of 2MQ by condensation of aniline with ethyl
acetoacetate in the presence of sulfuric acid adsorbed silica gel (H SO –silica)
2
4
under solvent-free conditions. The resulting enaminone system was subject to acid-
catalyzed cyclization to yield 2MQ.
The present communication deals with structural, electronic and spectroscopic
investigations of monomeric and dimeric forms of 2MQ, with the long-term
objective of gaining better insight into such compounds. Particular attention has
been focused on the effect of intermolecular N–HÁÁÁO hydrogen bonding, on the
bond distances, calculated frequencies and the nature of hydrogen bonding, by
calculating topological parameters at the bond critical points using the atoms-in-
molecule theory.
The aim of this work was to investigate the molecular structure and conduct a
vibrational study of the molecule, due to its biological and pharmaceutical
importance as analyzed by the density functional theory using 6-311??G(d,p) and
6
-311??G(2d,p) basis sets. In order to obtain a description of the molecular
vibrations of 2MQ, complete vibrational analysis has been carried out, providing
detailed information about intramolecular vibrations in the entire mid-infrared
region and especially in the fingerprint region. The ab initio quantum mechanical
method is widely used for simulating IR and Raman spectra [10, 11].
The changes in electron density (ED) in the r* and p* antibonding orbitals and
(
2)
stabilization energies E have been calculated by NBO analysis to acquire clear
evidence of stabilization originating in the hyperconjugation of hydrogen-bonded
interaction.
1
In this work, infrared (IR), H nuclear magnetic resonance (NMR) and ultraviolet
UV)–Vis spectra of 2MQ are reported both experimentally and theoretically.
(
Thermal stability of 2MQ was studied by thermo gravimetric analysis (TGA).
HOMO and LUMO analyses have been used to elucidate information regarding
charge transfer within the molecule. Molecular quantities, such as electronic
chemical potential (l), global hardness (g), global softness (S), electronegativity (v),
electrophilicity indices (x) and Fukui functions, are calculated and discussed.
Moreover, molecular electrostatic potential (MEP) surface is plotted over the
optimized geometry to elucidate the reactivity of the title compound. Reactive
1
23

Synthesis, spectroscopic investigations and computational…
nucleophilic and electrophilic sites in the MEP surface are compared with the fitting
point charges to electrostatic potential. The standard thermodynamic functions, such
ꢀ
ꢁ
À
Á
À
Á
as molecular energy (E), heat capacity C0 , entropy S and enthalpy H , are
0
m
0
m
p;m
reported on the basis of vibrational analyses and statistical thermodynamics.
Experimental
H SO –silica was prepared by our previously reported method [12]. All the reagents
2
4
and chemicals were obtained from Merck and used without further purification. The
development of reactions was monitored by thin-layer chromatography (TLC) on
Merck pre-coated silica gel 60 F254 aluminum sheets, and visualized by UV light.
IR spectra were recorded on a Shimadzu FT-IR 8300 Spectrophotometer using the
KBr pellet technique. The FT-Raman spectrum of 2MQ in the solid phase was
recorded employing a 180° back-scattering geometry and a Bomem MB-154
Fourier transform Raman spectrometer. It was equipped with a ZnSe beam splitter
and a TE cooled InGaAs detector. Rayleigh filtration was afforded by two sets of
two holographic technology filters. The spectra were accumulated for 1500 scans
-
1
1
with a resolution of 2 cm . The laser power at the sample was 300 mW. H NMR
spectra was recorded at ambient temperature on a BRUKER AVANCE DRX-
4
00 MHz spectrophotometer using CDCl as the solvent and TMS as an internal
3
standard. The ultraviolet absorption spectrum was examined in the range of
00–800 nm using a Perkin-Elmer lambda 25 recording spectrophotometer. TGA
2
was recorded using a Perkin-Elmer model in order to determine its thermal stability.
The analysis was carried out in an atmosphere of nitrogen at a heating rate of 25 °C
per 10 min for the temperature range of 150–750 °C. Cyclic voltammetry
measurements were performed by means of AUTOLAB PGSTAT20 potentiostat–
galvanostat (EcoChemie, Netherlands). The electrochemical properties of 2MQ
-
3
(
c = 2 9 10 M) were investigated by cyclic voltammetry with acetonitrile/0.1 M
as the solvent in the presence of tetrabutylammonium perchlorate (TBAP) as the
supporting electrolyte, using Pt working and counter electrodes and Ag/AgCl as a
reference electrode at ambient temperature (27 °C). Prior to the measurements, the
solution was purged with argon to remove residual oxygen.
Preparation of 2MQ using H SO –silica
2
4
To a mixture of a ethyl acetoacetate (1 mmol) and aniline (1.2 mmol), H SO –silica
2
4
(
0.15 g) was added. The mixture was stirred for 8 min at room temperature. After
completion of the reaction, the reaction mixture was stirred in an oil bath at 140 °C,
and TLC was used to monitor the progress of the reaction. At the end of the
reaction, the resulting mixture was mixed with ethyl acetate and filtered to remove
H SO –silica. The filtrate was washed with NaHCO (3 9 7 mL) and then washed
2
4
3
with water (5 mL), and the solvent was removed under reduced pressure to give the
product, which was recrystallized from ethanol. After recrystallization from ethanol,
123

S. A. Pourmousavi et al.
1
the 2MQ was obtained as a white solid. Yield: 67 %, H-NMR (400 MHz, CDC1 ):
3
1
H ), 7.62–7.65 (t, J = 7.2 Hz, 1H, H ), 7.38–7.41 (t, J = 7.2 Hz, 1H, H ), 6.31 (S,
1.75 (S, 1H, H ), 8.37–8.39 (d, J = 8 Hz, 1H, H ), 7.74–7.76 (d, J = 8 Hz, 1H,
a e
b
c
d
1
H, H ), 2.52 (S, 3H. H ).
f
g
Calculations
Geometry optimizations for the monomeric and dimeric forms of 2MQ were
calculated at the B3LYP [13–15] level using 6-31G(d), 6-311G(d,p),
6
-311??G(d,p), 6-311??G(2d,p), and LanL2DZ basis sets. All the calculations
were performed using the Gaussian 03 W program package [16], with default
convergence criteria and without any constraint on the geometry [17].
In this investigation, we observed that the calculated frequencies were slightly
greater than the fundamental frequencies. These discrepancies are corrected either
by computing anharmonic corrections explicitly [18], or by introducing a scale
factor. In the present study, a selective scaling factor of 0.9961 was used for
-
1
-1
wavenumbers \1700 cm
and 0.9556 was used for [1700 cm
at
-
1
6
-311??G(d,p); and 0.9963 was used for wavenumbers \1700 cm and 0.9959
-
1
was used for [1700 cm at 6-311??G(2d,p) [19]. The vibrational wavenumber
assignments were carried out by combining the results of the GaussView 4.1.2
program [20] and the VEDA4 program [21]. The assignments of experimental
frequencies are based on the observed band frequency and intensity changes in the
infrared and Raman spectra, confirmed by establishing a one-to-one correlation
between observed and theoretically calculated frequencies. For clear observation of
these bands, band deconvolution techniques in the IR spectrum could be used.
Lorentzian function was utilized for deconvolution of IR spectra using the Genplot
package.
The nuclear magnetic resonance (NMR) chemical shift calculations were
performed using the Gauge-Included Atomic Orbital (GIAO) method [22] in
chloroform.
The time-dependent density functional theory (TD-DFT) was carried out to find
electronic transitions. To calculate functional group contributions to the molecular
orbitals, the total density of states (TDOS or DOS) the partial density of states
(
PDOS) spectra were prepared using the program GaussSum 2.2 [23]. NBO
calculations [24] were performed using the NBO 3.1 program as implemented in the
Gaussian 03 W package at the DFT/B3LYP level in order to understand
intramolecular delocalization or hyperconjugation. The HOMO and LUMO were
calculated by the B3LYP/6-311??G(2d,p) method. AIM 2000 software [25] was
applied to obtain electron density at the hydrogen bond critical points, according to
Bader’s atoms in molecules (AIM) theory [26] at the B3LYP/6-311??G(d,p) and
B3LYP/6-311??G(2d,p) levels.
The transition state (TS) structure of the photochromism reaction was obtained
by using the QST3 method.
1
23

Synthesis, spectroscopic investigations and computational…
Results and discussion
Noting recent reports on the use of sulfuric acid immobilized on silica (H SO –
2
4
silica) [27–30] for various organic transformations, herein we wish to report an
efficient procedure for the preparation of 2MQ in the presence of a catalytic amount
of H SO –silica under solvent-free conditions.
2
4
The synthesis of 2MQ is outlined in Scheme 1. At first, condensation of aniline
with ethyl acetoacetate gave the enaminone in the presence of H SO –silica at room
2
4
temperature. In this reaction, enaminone was achieved in high yield. The resulting
enaminone system was subject to acid-catalyzed cyclization under heating condition
at 140 °C to yield 67 % 2MQ. In each case, the reaction was monitored by TLC and
was discontinued when further consumption of the starting material was no longer
observed.
The catalyst played a crucial role in the success of the reaction, in terms of time
and yield. In the absence of the catalyst, the reaction of aniline with ethyl
acetoacetate could be carried out, but the product was obtained in very low yield
(
10 %) after a prolonged reaction time.
The possibility of recycling the catalyst is one of the advantages of this
procedure. The recovered catalyst was activated by heating at 100 °C under vacuum
for 10 h, and was reused for the preparation of 2MQ. The recovered catalyst, after
activation, was reused for four successive times without any appreciable loss in its
activity.
NH2
N
H SO -Silica
RT, Solvent Free Conditions
2
4
H
+
O
OEt
O
O
H SO -Silica
OEt
2
4
0
1
40 C
H
N
N
or
OH
O
Scheme 1 The synthesis of 2MQ
123

S. A. Pourmousavi et al.
The present H SO –silica catalyzed procedure provides an efficient method for
2
4
the preparation of 2MQ under thermal conditions. The significant advantages of this
procedure are: (1) lower reaction temperature compared to the Cornad Limpach
method (up to 250 °C [9]); (2) solvent-free conditions, (3) H SO –silica is an
2
4
inexpensive, easily available and environmentally benign solid support acid that can
be separated by simple filtration; and (4) the reusability of catalyst.
Geometrical parameters
The compound can occur in two tautomeric forms, the enol and keto forms (Fig. 1).
The two tautomeric monomer forms of 2MQ were subjected to geometry
optimization at various levels of theory using the Gaussian03 package [16]. The
energy difference between the keto and enol forms obtained from DFT method was
1
6.52 (calculated at the B3LYP/6-311??G(d,p) level) (see Table 1). According to
all the calculations, the keto form of 2MQ was predicted to be more stable than the
enol tautomer, and hence was used for theoretical investigations. Because of the
hydrogen bonding formation, dimerization of 2MQ considerably reduces the energy,
5
2.7 kJ/mol (at the B3LYP/6-311??G(d,p). These values indicate that the dimer
form is quite stable in comparison to its monomeric form. The energy difference
between dimer and keto forms suggests that there is a hydrogen bond in the dimeric
form.
Fig. 1 a Tautomer equilibrium of 2MQ, b OH form and c NH form of the optimized structures obtained
using B3LYP/6-311G(d,p) level
1
23

Synthesis, spectroscopic investigations and computational…
Table 1 The absolute total electronic energies (Hartree) and the energy differences of 2MQ at different
basis sets
Calculation levels
E (enol)
E (keto)
DE
B3LYP/6-31G(d)
-516.471676
-516.604319
-516.613034
-516.627215
-516.390452
-516.479090
-516.610419
-516.619333
-516.632948
-516.404302
19.45
16.00
16.52
15.04
36.33
B3LYP/6-311G(d,p)
B3LYP/6-311??G(d,p)
B3LYP/6-311??G(2d,p)
B3LYP/LanL2DZ
E absolute total electronic energies (Hartree); DE energy differences between enol and keto at kJ/mol
The calculated optimized geometry parameters (bond lengths and bond angles) of
MQ are compared in Table 2 with the experimental X-ray data of 2-(4-
2
fluorophenyl)quinolin-4(1H)-one [31], which is structurally similar to 2MQ. Taking
into account that the molecular geometry in the vapor phase may be different from
the solid base, owing to extended hydrogen bonding and stacking interactions, there
is reasonable agreement between the calculated and experimental geometric
parameters. When comparing the theoretical and the experimental bond lengths, the
theoretical parameters are slightly deviated from the experimental data. For the title
compound, it was found that the carbon–carbon bond lengths in the benzene ring
˚
were intermediates between typical C–C single (1.54 A) and C=C double (1.34 A)
˚
˚
bonds. The greater bond length of C –C (1.405 A) is due to delocalization of
3
4
electron density due to the adjacent quinoline ring. According to an international
crystallography table [32], the C=O bond length in the aromatic carboxylic group
˚
conforms to an average value of 1.226 A. The experimental C=O bond length is
˚ ˚
1.261 A [33]. The calculated value of C=O (1.250 A) in the 2MQ dimer reported in
Table 2 is in good agreement with experimental data.
In crystal structure, the molecules are linked in centro-symmetric dimers by N–
HÁÁÁO hydrogen bonds [34]. The optimized molecular structure of the hydrogen-
bonded dimer is shown in Fig. 2. The computed bond lengths and bond angles of the
dimer conformer differ slightly from the X-ray data. The strength of the H-bonds
may also be found from the elongation of dX–H(X = O, N or F) and the red shifts of
˚
tXH. The OÁÁÁH distance in the hydrogen-bonded dimer is found to be 1.864 A. The
N–H and C=O distances involved in the hydrogen bonds are lengthened by 0.018
˚
and 0.019 A respectively upon dimerization. The N–HÁÁÁO angle is calculated to be
1
74.11°, while the C=OÁÁÁH angle is 127.58°.
Vibrational analysis
The title compound consists of 21 atoms, which have 57 normal modes. Those
normal modes of 2MQ have been assigned according to the detailed motions of the
individual atoms. Experimental Fourier transform infrared spectroscopy (FT-IR)
and FT-Raman spectra, with corresponding theoretically simulated FT-IR and FT-
Raman spectra for 2MQ, are shown in Figs. 3 and 4, respectively. The observed IR,
123

S. A. Pourmousavi et al.
Table 2 The optimized geometric parameters of 2MQ and comparison with experimental results, bond
˚
lengths in angstrom (A) and bond angles in degrees (°)
a
b
a
b
Parameters
Monomer
Dimer
Exp.
Parameters
Monomer
Dimer
Exp.
Bond lengths ( A˚ )
C
C
C
C
C
C
C
1
2
3
4
5
6
3
–C
–C
–C
–C
–C
–C
2
3
4
5
6
1
1.381
1.403
1.405
1.401
1.381
1.401
1.3843
1.008
1.387
1.369
1.405
1.402
1.409
1.364
1.399
1.379
0.898
N
15–C13
1.369
1.483
1.230
1.451
1.358
1.081
1.500
1.369
1.360
1.464
1.261
1.421
1.369
0.930
_
1.408
1.412
1.408
1.386
1.408
1.387
1.027
C
C
C
C
C
C
4
–C
9
1.476
1.250
1.446
1.369
1.084
1.503
9
–O20
–C12
9
12–C13
12–H14
13–C16
–N15
N
15–H21
Bond angles (°)
C
1
C
2
C
2
C
3
C
3
C
4
C
4
C
5
C
5
C
6
C
6
C
1
–C
2
–C
3
–C
3
–C
4
–C
4
–C
5
–C
5
–C
6
–C
6
–C
1
–C
1
–C
2
–C
3
119.6
119.51
120.43
120.55
118.83
120.10
120.78
117.53
120.30
119.79
119.93
120.66
120.56
119.4
C
C
C
C
C
C
C
C
C
C
5
4
4
4
3
3
9
9
–C
–C
–C
–C
4
3
9
9
–C
9
120.8
119.1
114.9
122.0
118.3
122.8
122.5
117.6
123.9
119.9
120.4
115.8
121.07
119.01
115.63
121.26
118.16
122.99
122.17
118.37
123.74
119.45
120.09
116.17
121.9
–C
4
120.4
120.5
118.8
120.4
120.9
117.3
120.4
119.7
119.7
120.6
120.5
120.4
119.6
118.4
119.7
121.0
119.5
120.1
119.8
119.5
121.0
120.3
–N15
–C12
–O20
120.0
115.5
120.9
118.4
121.9
122.9
118.6
123.4
118.6
119.9
116.7
–N15
–C
5
–C
9
–C
6
–N15–H21
–N15–C13
–C12–C13
–C12–H14
–H10
–H11
–C
–H
–C
–H
1
12–C13–C16
13–C12–H14
7
2
N
N
15–C13–C12
15–C13–C16
8
Intermolecular H bond lengths
N–H
N–O
C=O
a
_
_
_
1.027
2.887
1.250
0.898
OÁÁÁH
N–H–O
–
–
–
1.864
174.11
127.58
1.915
2.768
1.261
158.0
139.9
C = OÁÁÁH
For atom numbering from Fig. 1
b
Ref. [29]
Raman bands and calculated wavenumbers (scaled) and assignments are given in
Table 3. A deconvoluted IR spectrum of 2MQ was depicted in Fig. S1. For
complete vibrational analysis, the vibrational modes are discussed under five
headings: (1) N–H group vibration, (2) C=O group vibration, (3) C–N vibration, (4)
methyl group vibration, and (5) phenyl ring vibration.
N–H group vibration
Intermolecular hydrogen bonding is simply revealed in the deviation of theoretical
wavenumbers of N–H stretching modes. For heterocyclic compounds containing an
-
N–H group, an N–H stretching band is observed in the 3500–3000 cm region, the
1
1
23

Synthesis, spectroscopic investigations and computational…
Fig. 2 Optimized dimer structure of 2MQ at B3LYP/6-311??G(2d,p) level
1
00
8
6
4
2
0
0
0
0
0
4
00
600
800
1000 1200 1400 1600 1800 2500 3000 3500
-1
Wavenumber (cm )
Fig. 3 FT-IR spectrum of 2MQ
position of which depends upon the degree of hydrogen bonding [35, 36]. In the
present work, the theoretical calculation indicates that the scaled frequency value at
-
610 cm is assigned to N–H stretching vibrations. The PED that corresponds to
1
3
this vibration is a pure mode with contribution of 100 %. The recorded spectra fail
123

S. A. Pourmousavi et al.
1
00
8
6
4
2
0
0
0
0
0
2
00
400
600
800 1000 1200 1400 1600 1800 2500 3000 3500
-1
Wavenumber (cm )
Fig. 4 FT-Raman spectrum of 2MQ
to show such a band in the above-mentioned region, but it is observed at 3426 and
3
-
350 cm in FT-IR and FT-Raman spectrum as a medium band, and is assigned to
1
N–H stretching vibrations. Two members of this class of compounds, which have
been the subject of experimental studies, are 4-pyridone [37] and 2-pyridon [38]. By
comparing this experimental property, the trend of hydrogen bond strength is
obtained:
2MQ\4Àpyridone\2Àpyridone:
The N–H in-plane bending vibrations usually occur in the region of
-
650–1580 cm [39]. The N–H in-plane bending vibrations are calculated at
1
1
-
652, 1622, 1547 cm , and match well with the experimental FT-IR and FT-
1
1
Raman peaks. The wavenumber obtained at 435 cm
-
1
by FT-IR spectra and
by FT-Raman spectra are assigned to NH2 out-of-plane bending
-
1
438 cm
vibrations. This assignment is in agreement with the literature [40].
C=O group vibration
-
1
The appearance of strong bands in the FT-IR around 1800–1650 cm in aromatic
compounds indicates the presence of a carbonyl group and is due to the C=O-
stretching motion [41]. Formation of hydrogen bonding lowers the bond strength
and thus t(C=O) absorption occurs at lower wavenumbers. The C=O stretching
-1
-
1
vibration of the 2MQ is observed at 1642, and 1627 cm in FT-IR and 1643 cm
in FT-Raman spectrum, and these wavenumbers are in good coherence with the
empirical value. The dominant in-plane C=O-bending vibration is calculated at
-
14 cm , in complete conformity with Rastogi et al. [42]. The bands recorded at
1
6
-
05 cm in FT-IR spectrum and 609 cm in FT-Raman spectrum are assigned to
1
-1
6
1
23

Synthesis, spectroscopic investigations and computational…
123

S. A. Pourmousavi et al.
1
23

Synthesis, spectroscopic investigations and computational…
123

S. A. Pourmousavi et al.
dCO of 2MQ. The C=O out-of-plane bending vibration is computed in the region
-
40 ± 80 cm [43]. The predicted value at 149 cm is assigned to this vibration
1
-1
5
in 2MQ. According to the literature [44], this assigned value is in the expected
range.
C–N vibration
The C–N stretching frequency is a rather difficult task, since there are problems in
identifying these frequencies from other vibrations [45]. Silverstein et al. [46]
-
1
assigned C–N stretching vibrations in the region of 1386–1266 cm for aromatic
-
1
amines. Bands at 1375, 1298, 1246, 1288, and 1247 cm in FT-IR and FT-Raman
weres assigned to C=N stretching vibrations, respectively.
Methyl group vibrations
-
The band at 2991 cm is assigned to CH asymmetric stretching [47]. The C–H
1
3
stretching in methyl groups occurs at lower frequencies than use of the aromatic ring
-
1
(
3100–3000 cm ). In the present work, CH asymmetric stretching is found at
3
-
908 cm in FT-IR, and 2988, and 2920 cm in the Raman spectrum. CH3
1
-1
2
-
1
symmetric stretching is found at 2762 cm in FT-IR. In many molecules, the
symmetric deformation appears with an intensity varying from medium to strong,
-
and is expected in the range 1380 ± 25 cm [48]. According to our deconvolution
1
results, the IR spectrum of 2MQ indicates the presence of one band. We observed
-
1
-1
the d CH frequencies at 1420 cm (see Fig. S1) and at 1426 cm (FT-Raman).
s
3
-
1
In FT-IR and FT-Raman, the bands observed at 1471, 1446 and 1473 cm
respectively, are assigned to the scissoring modes for the CH group.
,
3
The rocking vibrations of CH group are generally observed in the region
3
-
070–1010 cm [49]. The bands at 1075 and 1027 cm in the IR spectrum and at
1
-1
1
1
-
1
027 cm (FT-Raman) are assigned as qCH mode for the 2MQ. The twisting
3
modes were not observed in the FT-IR, because they appear at very low frequency.
-
1
The bands at 189 and 112 cm (B3LYP) are assigned as the twisting mode sCH .
3
-
1
The calculated value of m(C–CH ) was 666 cm . The experimentally observed
value in the FT-IR spectrum at 654 cm confirms the assignment on comparison
with the calculated value. In our present study, the strong bands at 325 and
3
-
1
-
1
2
30 cm in FT-Raman spectrum are assigned to d(C–CH3).
Phenyl ring vibration
The phenyl ring spectral region chiefly involves C–H, C–C stretching, and C–C–C
as well as H–C–C– bending vibrations. The bands due to the ring C–H stretching
vibrations are observed as a collection of weak-to-moderate bands in the region of
-
1
3
100–3000 cm . The calculated wavenumbers for the CH-stretching modes were
-
1
found in the range of 3196–3153 cm and were matched with the experimental FT-
IR and FT-Raman spectra. The PED for these modes suggested that these were pure
modes. The aromatic C–H in-plane bending modes of benzene and its derivatives
-
1
are observed in the region of 1300–1000 cm [50, 51]. In our title molecule, weak
1
23

Synthesis, spectroscopic investigations and computational…
to medium bands observed in FT-IR as well as in FT-Raman spectrum at 1554–1138
-
1
and 1553–1144 cm are assigned to C–H in-plane bending vibrations for aromatic
ring and show good agreement with computed wavenumbers by the B3LYP/6-
3
bending were found throughout the region of 1554–1138 cm . The C–H wagging
11??G(2d,p) method. In the case of 2MQ, vibrations involving the C–H in-plane
-
1
-
or out of bending vibrations) mode started appearing from 999 cm and had
1
(
-
1
contribution up to 683 cm and was assigned well in the spectra. In general, the
torsional modes appear in the low-wavenumber regions. In the present case, the
-
1
calculated normal modes below 200 cm wavenumbers are mainly the torsional
modes.
Generally, the C–C stretching vibrations in aromatic compounds give bands in
-
1
the region of 1430–1650 cm [52]. In the present study, the frequencies observed
-
1
in the FT-IR spectrum at 1608, 1597, 1554, and 1471 cm are assigned to C–C
stretching vibrations. The same vibrations appear in the FT-Raman spectrum at
-
610, 1596, 1553, and 1473 cm . All the rings stretching modes are expected to
1
1
appear in the expected range. As expected, all the bands are observed with very
weak and medium intensities. The C–C–C bending vibrations always occurred
-
1
below 600 cm
[53]. In our present study, strong bands observed in FT-IR
-
1
-1
spectrum at 715, 654 and 605 cm as weak bands, and 631, 406, and 270 cm as
a medium to strong bands in FT-Raman spectrum, are assigned to C–C–C in-plane
and out-of-plane bending vibrations, respectively.
NBO analysis
Natural bond orbital (NBO) calculations were performed using the NBO 3.0
program as implemented in the Gaussian03 package at the DFT/B3LYP level. A
useful aspect of the NBO is that it provides an accurate method for studying
intramolecular and intermolecular bonding and interaction among bonds, and also
gives an efficient basis for investigating charge transfer or conjugative interaction in
various molecular systems [54]. In order to characterize the intramolecular and
intermolecular interactions quantitatively, a second-order perturbation theory is
applied that gives the energy lowering associated with such interactions. For each
donor NBO (i) and acceptor NBO (j), the strength of interaction (or stabilization
(2)
energy) E associated with electron delocalization between donor and acceptor is
estimated by the second order energy lowering as in [55, 56].
(2)
For each donor (i) and acceptor (j) the stabilization energy E associated with
the delocalization i ? j is determined as
2
ðFijÞ
ð2Þ
E
¼ DEij ¼ qi
ð1Þ
2
ðE À E Þ
j
i
where q is the donor orbital occupancy, E and E are diagonal elements, and F is
i
i
j
ij
the off-diagonal NBO Fock matrix element.
The second-order perturbation theory analysis of the Fock matrix, in the NBO
basis for 2MQ, calculated at B3LYP/6-311??G(2d,p) is presented in Table 4,
which shows p-conjugation/resonance due to top-electron delocalization in ring,
123

S. A. Pourmousavi et al.
1
23

Synthesis, spectroscopic investigations and computational…
123

S. A. Pourmousavi et al.
primary and secondary hyperconjugative interactions. The p-conjugation/resonance
due to p-electron delocalization is involved due to the p ? p* interactions, whereas
the primary hyperconjugative interactions due to the various types of orbital
overlaps such as r ? p*, p ? r*, n ? r*, and secondary hyperconjugative
interactions due to the orbital overlap r ? r*. The interactions p(C1–
C ) ? p*(C –C , C –C ), p(C –C ) ? p*(C –C , C –C ) and p(C5–
2
3
4
5
6
3
4
1
2
5
6
C ) ? p*(C –C , C –C )are responsible for conjugation of respective p-bonds in
6
1
2
3
4
phenyl ring. The electron density at the conjugated p bonds (1.97989–1.61480) of
phenyl ring and p* bonds (0.42295–0.27305) of phenyl ring indicate strong p-
electron delocalization within the ring, leading to a maximum stabilization of
energy at 22.85 kcal/mol. The second-order perturbation theory analysis of Fock-
matrix in NBO basis shows that strong intermolecular hyper-conjugative interac-
tions are formed by orbital overlap between n(O) and n(N) with p*(C –C ), p*(C –
3
4
12
C ), p*(C –C ) and p*(C –C ) bond orbitals, which result in ICT causing
1
3
4
9
9
12
stabilization of the system.
Second-order perturbation theory analysis of the Fock matrix in NBO basis: (1)
from monomer unit 1 to unit 2, and (2) from monomer unit 2 to unit 1, is given in
Table 4. In the dimer, the primary hyperconjugative interactions from the monomer
unit (1) to the unit (2) due to n (O )/n (O ) ? r*(N –H ) stabilized the
1
20
2
20
36
42
molecule up to 6.98 and 13.30 kcal/mol, and confirms the presence of classical
interaction of N –H ÁÁÁO .
3
6
42
20
Charge distribution
The calculation of atomic charges plays an important role in the application of
quantum mechanical calculations to molecular systems [57]. The Mulliken [58]
population analysis in the 2MQ molecule (monomer and dimer) is calculated using
B3LYP level with 6-311??G(d,p) basis set as listed in Table S1. The plot of
Mulliken atomic charge is shown in Fig. 5.
The charge distribution of the molecule has an important influence on the
vibrational spectra. The charge distribution of 2MQ shows that the carbon atom
0
0
0
0
.7
.5
.3
.1
-
-
-
-
0.1 C 1 C 2 C 3 C 4 C 5 C 6 H 7 H 8 C 9 H H C12 C
Atoms
13 14 15 16 17 18 19 20 21
H
N
C
H H H O H
10
11
0.3
0.5
0.7
Monomer
Dimer
Fig. 5 Mulliken population analysis chart (monomer and dimer) of 2MQ
1
23

Synthesis, spectroscopic investigations and computational…
1
Fig. 6 The experimental H NMR spectra of 2MQ
attached with hydrogen atoms is negative, whereas the remaining carbon atoms are
positively charged. The high positive charge at C is due to the effect of an oxygen
9
atom attached to C . Moreover, Mulliken atomic charges also show that the H
9
21
atom has more positive atomic charges than the other hydrogen atoms. This is due to
the presence of an electronegative nitrogen atom (N ); the hydrogen atom (H )
attracts the positive charge from the nitrogen atom (N ). The presence of high
1
5
21
1
5
negative charge on O atom and net positive charge on H atom may suggest the
formation of intramolecular interaction in solid forms [59].
NMR spectrum
1
The H NMR spectrum of the title compound was recorded in CDCl (Fig. 6) and
3
the chemical shifts are in good agreement with Ref. [5, 60]. An attached hydrogen
or nearby electron-withdrawing atom or group can decrease the shielding and move
the resonance of the attached proton towards a higher frequency. By contrast, an
electron-donating atom or group increases the shielding and moves the resonance
towards a lower frequency [61]. The geometry of the title compound, together with
1
that of tetramethylsilane (TMS), is fully optimized. H NMR chemical shifts are
123

S. A. Pourmousavi et al.
1
Table 5 Experimental isotropic H NMR chemical shifts of 2MQ in comparison with calculated theo-
retical values
Experiment B3LYP/6-
B3LYP/6-
31?G(d,p)
B3LYP/6-
311G(d,p)
pbepbe/6-
31?g(d,p)
Assignment
311??G(d,p)
a
2
6
7
7
7
8
1
a
.526
.317
.400
.640
.602
.389
1.752
2.4321
6.2404
7.5682
7.7769
7.7928
8.6367
9.6321
2.37627
7.2052
7.5765
7.7407
7.8276
8.7048
9.574
1.44073
5.1525
6.7385
6.7205
6.756
2.2958
6.2364
7.5805
7.7284
7.7940
8.6148
9.5384
H(methyl)
H
H
H
H
H
H
14
8
11
7
7.5551
9.2692
10
21
Mean value
calculated with the GIAO approach at B3LYP/6-311??G(d,p), B3LYP/6-
1?G(d,p), B3LYP/6-311G(d,p) and pbepbe/6-31?g(d,p) levels [22]. The chemical
shift of any ‘X’ proton (CS ) is equal to the difference between isotropic magnetic
3
X
shielding (IMS) of TMS and proton (x). It is defined by the equation: CS = -
X
1
IMSTMS 2 IMS . The experimental and calculated H NMR chemical shifts (d in
X
ppm) of 2MQ are given in Table 5. The chemical shifts of aromatic protons of
organic compounds are usually observed in the range of 7.00–8.00 ppm, while the
aliphatic protons resonance in the high field. The signals of the four aromatic
1
protons ( H) of the title compound were theoretically calculated to be
7
.5682–8.6367 ppm
(B3LYP/6-311??G(d,p),
and
were
observed
at
.400–8.389 ppm experimentally. We predicted H10 at 8.389 ppm in the lowest
7
field, and H at 7.400 ppm in the highest field of the aromatic region. The rotation of
8
methyl groups causes their hydrogen atoms to show unique NMR peaks as well. The
maximal deviation of chemical shifts obtained by B3LYP/6-311??G(d,p) (PCM)
level is 2.120 for H . The reason is that because of the electron withdrawing
2
1
inductive effect of O, the activity of H21 is increased, leading to formation of
intermolecular hydrogen bonds, and making them difficult to predict.
Molecular electrostatic potential (MEP) surface
In order to investigate the chemical reactivity of the molecule, molecular electrostatic
potential (MEP) surface is plotted over the optimized electronic structure of 2MQ
using density functional B3LYP level with the 6-311??G(2d,p) basis set. The
molecular electrostatic potential (MEP) surface is a plot of electrostatic potential
mapped onto the constant electron density surface. The MEP surface is superimposed
on top of the total energy density as a shell. A useful feature to study reactivity is that an
approaching electrophile will be attracted to negative regions where the electron
distribution effect is dominant. MEP is related to the electronic density and is a very
useful descriptor in understanding sites for electrophilic and nucleophilic reactions as
well as hydrogen bonding interactions [62].
1
23

Synthesis, spectroscopic investigations and computational…
-7.107e-2
7.107e-2
Charges
e)
Electric Potential
(a.u)
(
C
C
C
C
C
C
H
H
C
H
H
C
C
H
N
C
H
H
H
O
H
1
2
-0.046
-0.316
0.372
-0.256
-0.025
-0.239
0.131
0.155
0.757
0.131
0.131
0.119
0.434
0.149
-0.276
-0.632
0.157
0.146
0.146
-0.542
0.251
-14.761
-14.760
-14.712
-14.767
-14.772
-14.772
-1.097
-1.085
-14.712
-1.112
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
-1.106
-14.784
-14.704
-1.105
-18.328
-14.748
-1.090
-1.085
-1.085
-22.423
-0.985
Fig. 7 Molecular electrostatic potential surface (left) and the point charges, electric potential values
right) on an atom of 2MQ (for interpretation of the references to color in this figure, the reader is referred
to the web version of the article). (Color figure online)
(
The negative (red) regions of MEP were related to electrophilic reactivity and the
positive (blue) regions to nucleophilic reactivity. Figure 7 shows the computation-
ally observed MEP surface map with the fitting point charges to the electrostatic
potential V(r) for 2MQ. The different values of the electrostatic potential at the
surface are represented by different colors. Potential increases in the order are
red \ orange \ yellow \ green \ blue. The predominance of the light-green
region in the MEP surface corresponds to a potential halfway point between the
two extreme regions red and dark-blue color. The color code of the maps is in the
range between -0.07107 a.u. (dark red) and 0.07107 a.u. (dark blue) in the
compound, where blue indicates the strongest attraction and red indicates the
strongest repulsion.
Table 6 Theoretical and experimental absorption wavelength kmax(nm) and oscillator strengths (f) of
2
MQ using TD-DFT/6-311G??(2d, p)
Gas phase Ethanol Acetonitrile
(f) (f)
In solvent major
a
contribution (C10 %)
Assignment Experimental
abs (nm)
k
max
k
max
(f)
k
max
k
(nm)
(nm)
(nm)
237.0 0.003 235.6 0.346 234.8 0.294 H-3 ? L(50),
n ? r*
208
H-2 ? L(19),
H ? L?1(15)
2
2
64.4 0.033 264.9 0.024 264.4 0.023 H ? L(23),
H ? L?1(73)
p ? r*
n ? p*
234
320
96.7 0.101 299.7 0.143 299.1 0.142 H ? L(94)
123

S. A. Pourmousavi et al.
The average maximum negative and positive electrostatic potential values for
these electrophilic and nucleophilic sites calculated at B3LYP/6-311??G(2d,p) are
about -22.423 and -0.985 a.u. The fitting point charges to those potentials are
-
0.542 (O ) and 0.251 (H ). These sites give information about the region where
20 21
the compound can have intermolecular interaction. The MEP map shows that the
negative potential site is on electronegative atoms, and that the positive potential
sites are around the hydrogen atom. The MEP provides a visual representation of the
chemically active sites and the comparative reactivity of the atoms.
UV–Vis spectra and electronic properties
The UV–Vis spectral analysis of 2MQ was calculated by the TD-B3LYP/6-
3
11??G(2d,p) method, and along with measured UV–Vis data, is summarized in
Table 6. The UV–Vis spectra of 2MQ shown in Fig. 8 were measured in acetonitrile
and ethanol solutions. GaussSum 2.2 program [23] has been used to calculate group
contributions to the molecular orbitals and prepare the density of the state (DOS) as
shown in Fig. 9. The DOS spectrum was created by convoluting the molecular
orbital information with GAUSSIAN cures of unit height. DOS plot shows
population analysis per orbital and demonstrates a simple view of the makeup of the
molecular orbitals in a certain energy range. In Fig. 9, partial density of states
(
PDOS) has also been plotted, showing a percentage contribution of a group to each
molecular orbital. The highest occupied molecular orbitals (HOMOs) and lowest-
lying unoccupied molecular orbitals (LUMOs) are named as Frontier molecular
orbitals (FMOs). HOMO and HOMO-1 are the second highest and lowest
unoccupied molecular orbitals LUMO and LUMO ? 1, as shown in Fig. 10. The
energy gap between the HOMO and LUMO orbital, which is a critical parameter in
determining molecular electrical transport properties because it is a measure of
electron conductivity, was calculated as 4.628 eV for the stable form of the title
molecule. The HOMO shows that the charge density localized mainly on the
carbonyl group, as LUMO is localized on the ring system.
3
Acetonitrile
Ethanol
2
1
0
2
00
300
400
500
Wavelength (nm)
Fig. 8 UV–Vis absorption spectra of 2MQ in different solvents
1
23

Synthesis, spectroscopic investigations and computational…
Fig. 9 DOS and PDOS plot of 2MQ
The calculated absorption wavelengths (kmax) and the experimental wavelengths,
electron transition energies and oscillator strengths in the isolated gas phase,
acetonitrile and ethanol phase (solvent) are listed in Table 6. The recorded UV–Vis
spectrum shows three intense bands at 320, 234 and 208 nm. The maximum
absorption wavelength corresponds to the electronic transition from HOMO ?
LUMO (94 %) and HOMO ? LUMO (23 %), HOMO- ? LUMO ? 1 (73 %) and
HOMO-3 ? LUMO (50 %), HOMO-2 ? LUMO (19 %), HOMO ? LUMO ? 1
(
15 %) with contribution. The electronic absorption peak at 320 nm corresponds to
the transition from the ground to the first excited state, and it is mainly described by
one electron excitation from the HOMO ? LUMO (n ? p*). Moreover, lowering
of the HOMO and LUMO energy gap explains the eventual charge transfer
interactions taking place within the molecule.
The IP is defined as the minimum energy necessary to bring an electron from the
material into a vacuum. In other words, ionization potentials (IP) are the energy
difference between neutral molecules and the corresponding cationic systems. The
electron affinity (EA) is defined as the first unoccupied energy level that injects
electrons coming from a vacuum into where the material would occupy. In other
words, EA is the energy difference between neutral molecules and the correspond-
ing anionic systems. IP and EA are usually used to assess the energy barrier for the
injection of holes and electrons [63]. Obviously, lower reorganization energy is
necessary to achieve a high charge-transfer rate.
123

S. A. Pourmousavi et al.
ELUMO+1= –0.741 ev
ELUMO= –1.485 ev
ΔE=5.711 ev
ΔE=4.628 ev
EHOMO= –6.113 ev
EHOMO-1= –6.452 ev
Fig. 10 The atomic orbital compositions of the frontier molecular orbital for 2MQ
Table 7 Electro chemical properties of the title compound
a
a
IP
a
EA
a
ox
E
b
onset (V)
c
Comp
E
HOMO/ELUMO
IP/EA
EHOMO/ELUMO
NH form
OH form
a
-6.113/-1.485
-5.788/-1.436
7.865
8.047
1.290
0.896
7.865/1.290
8.047/0.896
1.439
5.839/1.211
DFT/B3LYP calculated values
b
-3
Oxidation potential in DMSO (2 9 10 mol L ) containing 0.1 mol L Bu
-1
-1
4
NBF
4
with a scan rate
-1
of 100 mV s
c
ox
E
HOMO was calculated by E ? 4.4 V, and ELUMO = EHOMO - DEHL [57]
Enol form has an IP of 8.047 eV (Table 7), which is bigger than that of the keto
form (7.865 eV), indicating that the hole-transporting ability of keto is better than
that of enol. On the other hand, the electron-transporting ability of a keto with an
EA of 1.29 eV should be better than that of enol with an EA of 0.896 eV.
A cyclic voltammogram of supporting electrolyte is also shown in Fig. 11. The
oxidation and reduction peaks are associated with the HOMO and LUMO levels,
respectively. In the section that deals with experimental results, one should discuss EA
1
23

Synthesis, spectroscopic investigations and computational…
8
6
4
2
0
0
0
0
0
-
-
-
-
20
40
60
80
-
1
-0.5
0
0.5
1
1.5
2
Potential [V]
-3
-1
Fig. 11 Cyclic voltammogram of 2 9 10 M title compound. Scan rate 100 mV s , electrolyte 0.1 M
Bu NBF in acetonitrile
4
4
and IP values rather than HOMO and LUMO levels, since the molecular orbitals
energies are not observable but originate from quantum chemical approximations.
Using cyclic voltammetry, the IP was estimated from the onset of the first oxidation
peak, whereas EA is from the onset of the first reduction peak. The oxidation and
reduction peaks are associated with the HOMO and LUMO levels, respectively. The
HOMO energy level, E , was estimated from bellow equation [64]: E
H
=
HOMO
ox
E
? 4.4 V. The LUMO energy level, E , was obtained by adding the energy of the
L
HOMO–LUMO gap, DEHL, to the EH value, while the energy gap DEHL was
determined from the difference between oxidation and reduction potentials. To bear a
significant charge-transfer character is generally indicative of a HOMO/LUMO
absorption transition. The HOMO/LUMO energy levels that are higher than those
corresponding estimations from the experimental data may be related to various
effects from conformation and solvents, which have not been taken into account here.
Thermal analysis
Thermal analysis of 2MQ was carried out using a Perkin-Elmer model, simulta-
neous thermo gravimetric/differential thermal (TG/DT) analyzer. The sample was
scanned in the temperature range of 150–750 °C at a rate of 25 °C for 10 min. The
TG/DT curve is shown in Fig. 12. TGA-measurement showed that weight of the
complex was constant up to 274 °C; thus, the complex did not contain
crystallization water. The major weight loss (90 %) occurs between 274 and
3
76 °C. At temperatures above 400 °C, the sample weight was almost constant. In
the differential thermal gravimetry (DTG) curve, one peak was identified at
temperatures of 365 °C, indicating that the weight loss by thermal decomposition
occurred in one step (Fig. 12b).
Prediction of hyperpolarizabilitie
Based on the finite-field approach, the non-linear optical parameters, such as dipole
moment, polarizability, anisotropy polarizability and first order hyperpolarizability
123

S. A. Pourmousavi et al.
1
1
20
00
8
6
4
2
0
0
0
0
0
1
00
200
300
400
500
600
700
800
o
Tempruture ( C)
(
a)
0.0
-
-
-
-
0.5
1.0
1.5
2.0
0
200
400
Tempruture ( C)
600
800
o
(
b)
Fig. 12 a TGA and b DTG curves of 2MQ
of 2MQ, are calculated using B3LYP level with 6-311??G(d,p) and 6-311??
G(2d,p) basis sets for more reliability. The numerical values of the above-mentioned
parameters are listed in Table S2. Nonlinear optics deal with the interaction of
applied electromagnetic fields in various materials to generate new electromagnetic
fields, altered in wavenumber, phase, or other physical properties [65]. In the
presence of an applied electric field, the energy of a system is a function of the
electric field. First hyperpolarizability is a third rank tensor that can be described by
a 3 9 3 9 3 matrix. The 27 components of the 3D matrix can be reduced to ten
components due to the Kleinman symmetry [66]. The components of b are defined
as the coefficients in the Taylor series expansion of the energy in the external
electric field. When the electric field is weak and homogeneous, this expansion
becomes
1
23

Synthesis, spectroscopic investigations and computational…
X
X
X
1
2
1
6
0
i
i
j
i
j
k
E ¼ E À
l F À
aij F F À
bijk F F F
i
j
ij
ij
ð2Þ
X
1
i
j
k
l
À 2
cijkl F F F F þ . . .
4
ijkl
i
where E is the energy of the unperturbed molecule, F is the field at the origin, l , a , b
0
i
ij ijk
and cijkl are the components of dipole moment, polarizability, the first hyperpolariz-
abilities, and second hyperpolarizabilities, respectively. The total static dipole mo-
ment, mean polarizability (a ), anisotropy polarizability (Da) and the average value
tot
of the first hyperpolarizability (hbi) can be calculated using the following equations.
ꢀ
ꢁ
1
2
2
x
2
y
2
z
l ¼ l þ l þ l
ð3Þ
1
À
Á
a ¼ 3 axx þ ayy þ azz
ð4Þ
tot
h
i
1
2
1
À
Á2
À
Á2
2
2
xz
2
xy
2
yz
Da ¼ pffi ffiffi axx À ayy þ a À a
þ ða À a Þ þ 6 a þ 6 a þ 6 a
ð5Þ
ð6Þ
yy
zz
zz
xx
2
h
i
1
2
2
2
2
hbi ¼ ðb þ byyy þ b Þ þ ðb þ byzz þ byxxÞ þ ðb þ bzxx þ bzyy
Þ
xxx
zzz
yyy
zzz
Hyperpolarizabilities are very sensitive to the basis sets and levels of theoretical
approach employed [67, 68], and the electron correlation can change the value of
hyperpolarizability. It is already established that the molecular hyperpolarizability
and mechanical stabilities get enhanced in organic molecules containing O–H and
N–H groups, which are involved in hydrogen bond interactions [69]. Urea is one of
the prototypical molecules used in the study of the nonlinear optical (NLO)
properties of molecular systems. Therefore, it has frequently been used as a
threshold value for comparative purposes.
The highest value of dipole moment is found along l . In this direction, the value
y
is equal to -6.327 D. For directions X and Z, the values are equal to 1.232 and
0
.001 D, respectively, as shown in Table S2. Total polarizability (a ) calculated as
tot
-
24
-24
-
10.187 9 10
and -10.166 9 10
esu for title molecule. The first hyperpo-
-
31
larizability values (hbi) of the title compound are equal to 56.642 9 10
and
esu for 2MQ. Theoretically, the first hyperpolarizability for title
compound are 15.19 and 14.91 times the magnitude, respectively, of the standard
-
31
5
5.600 9 10
-
30
NLO material urea (l and b of urea are 1.372 D and 0.3728 9 10
esu). Our title
molecule, with a greater dipole moment and hyperpolarizability value than urea,
shows that the molecule has large NLO optical property.
Thermodynamic functions
On the basis of vibrational calculation at the B3LYP/6-311??G(2d,p) level of
theory, the standard statistical thermodynamic functions of heat capacity (C p0 ;m),
123

S. A. Pourmousavi et al.
0
m
0
m
entropy (S ), enthalpy (H ) and molecular energy (E) for the title compound were
obtained from the theoretically calculated frequencies listed in Table S3. It can be
observed that these thermodynamic functions increase with temperature ranging
from 100 to 800 K, due to the fact that the molecular vibrational intensities increase
with temperature [70]. The corresponding quadratic equations obtained by B3LYP/
6
-311??G(2d,p) are as follows, and the related figure is shown in Fig. 13.
For 2MQ by B3LYP/6-311??G(2d,p)
À
Á
o
p;m
¼ 0:4287 þ 0:1596T À 7 Â 10À5
2
2
C
T
R ¼ 0:9992
o
m
À5
2
2
S ¼ 52:522 þ 0:1547T À 3 Â 10
T
ðR ¼ 1Þ
À
Á
o
m
À4
2
2
H ¼ 1:6339 þ 0:0684T À 1 Â 10
T
R ¼ 0:9999
À
Á
E ¼ 104:100 þ 0:0087T À 5 Â 10À5
T
2
R ¼ 0:9997
2
ð7Þ
All the thermodynamic data supply helpful information for the further study of
MQ. They can be used to compute other thermodynamic energies according to the
2
relationships of thermodynamic functions, and estimate directions of chemical
reactions according to the second law of thermodynamics in a thermo chemical field
[71, 72]. Note that all thermodynamic calculations were done in gas phase and they
could not be used in solution.
The values of some thermodynamic parameters [such as zero-point vibrational
energy (ZPVE), thermal energy, specific heat capacity, rotational constants,
rotational temperature and entropy] of the title compound are listed in Table S4.
The variation in ZPVE seems to be significant. The ZPVE is much higher by the
1
1
1
1
1
80
60
40
20
00
8
6
4
2
0
0
0
0
0
0
C
p,m
0
S
m
m
0
H
E
0
100
200
300
400
500
600
700
800
900
Temperature (K)
Fig. 13 Correlation graph of thermodynamic properties and temperatures by B3LYP/6-311??G(2d,p)
for 2MQ molecule
1
23

Synthesis, spectroscopic investigations and computational…
B3LYP/6-311??G(d,p) level than by the B3LYP/6-311??G(2d,p) level. The total
energies are found to decrease with the increase of the basis set dimension.
Reactive descriptors of 2MQ
Global reactivity parameters
The energies of frontier molecular orbitals (EHOMO, ELUMO), energy band gap
(
EHOMO–ELUMO), electronic chemical potential (l), electro negativity (v), global
hardness (g), global softness (S), electrophilicity indices (x) and dipole moment (D)
73] of 2MQ are listed in Table 8. On the basis of EHOMO and ELUMO, these are
[
calculated by using the below equations.
1
l ¼ 2 ðEHOMO þ ELUMOÞ
ð8Þ
ð9Þ
1
v ¼ Àl ¼ À ðEHOMO þ ELUMOÞ
2
1
g ¼ 2 ðEHOMOÀELUMOÞ
ð10Þ
ð11Þ
1
S ¼ 2
g
2
l
x ¼ 2
ð12Þ
g
The chemical hardness is quite useful in explaining the chemical stability. The
molecules having a large HOMO–LUMO energy gap will be more stable and less
reactive than soft molecules having a small HOMO–LUMO energy gap. From
Table 8, the total energy of the NH form is lower than that of the OH form, while
chemical hardness of the NH form is greater than that of the OH one, which
indicates that the NH form of the title compound is more stable than its OH form in
the gas phase. In additional, in order to evaluate the solvent effect on the above-
mentioned properties of the title compound, we carried out calculations in four
kinds of solvent (Hexane, dichloromethane, Ethanol and acetonitrile) with the
B3LYP/6-311??G(2d,p) level using the PCM model, and the results are given in
Table 8. From Table 8, we can conclude that the total molecular energies obtained
by the PCM method decrease with the increasing polarity of the solvent. Solvent
effects improve the charge delocalized in the molecules, therefore inducing the
dipole moments to be raised.
According to these results, the stability of the title compound increases in going
from the gas phase to the solution phase.
The usefulness of this new reactivity quantity has recently been demonstrated in
understanding the toxicity of various pollutants in terms of their reactivity and site
123

S. A. Pourmousavi et al.
1
23