Synthesis, molecular structure, spectroscopic properties and stability of (Z)-N-methyl-C-2,4,6-trimethylphenylnitrone This paper is dedicated to our wonderful friend Dr. Samir Senior from Alexandria University (Egypt) who recently passed away.

Article abstract of DOI:10.1016/j.saa.2014.10.096

New N-methyl-C-2,4,6-trimethylphenylnitrone 1 has been synthesized starting from N-methylhydroxylamine and mesitaldehyde. The product was fully characterized using different spectroscopic techniques; FTIR, NMR, UV-Vis, high resolution mass spectrometry an

Full text of DOI:10.1016/j.saa.2014.10.096

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, molecular structure, spectroscopic properties and stability of
(Z)-N-methyl-C-2,4,6-trimethylphenylnitrone
a
c
a,b,
Jamal Lasri a, , Ali I. Ismail , Matti Haukka , Saied M. Soliman
⇑
⇑
a Department of Chemistry, Rabigh College of Science and Arts, King Abdulaziz University, P.O. Box 344, Rabigh 21911, Saudi Arabia
b Department of Chemistry, Faculty of Science, Alexandria University, Ibrahimia, P.O. Box 426, Alexandria 21321, Egypt
c University of Jyväskylä, Department of Chemistry, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New N-methyl-C-2,4,6-trimethylphenylnitrone has been synthesized and characterized using FTIR, NMR,
UV–Vis, high resolution mass spectrometry and X-ray diffraction. The relative stability and percentage
population of its two possible isomers (E and Z) have been calculated using the B3LYP/6-311++G(d,p).
ꢀ
ꢀ
ꢀ
New N-methyl-C-2,4,6-
trimethylphenylnitrone has been
synthesized.
Z-isomer is more stable than the
E-one both in gas phase and in
solution.
The IR, electronic and NMR spectra
were studied experimentally and
theoretically.
The studied nitrone has about five
times higher NLO than urea.
NBO analysis indicates the presence
of weak CAH---O non-conventional
interaction.
-
Me
Me
H
O
+
Me
Me
C
N
Me
Me
ꢀ
ꢀ
-
O
Me
+
C
N
H
Me
E
Z
a r t i c l e i n f o
a b s t r a c t
Article history:
New N-methyl-C-2,4,6-trimethylphenylnitrone 1 has been synthesized starting from N-methylhydroxyl-
amine and mesitaldehyde. The product was fully characterized using different spectroscopic techniques;
FTIR, NMR, UV–Vis, high resolution mass spectrometry and X-ray diffraction. The relative stability and
percent of population of its two possible isomers (E and Z) were calculated using the B3LYP/6-
311++G(d,p) method in gas phase and in solution. In agreement with the X-ray results, it was found that
Z-isomer is the most stable one in both gas phase and solution. The molecular geometry, vibrational fre-
quencies, gauge-including atomic orbital (GIAO), and chemical shift values were also calculated using the
Received 10 July 2014
Accepted 23 October 2014
Available online 31 October 2014
This paper is dedicated to our wonderful
friend Dr. Samir Senior from Alexandria
University (Egypt) who recently passed
away.
same level of theory. The TD-DFT results of the studied nitrone predicted a
p–
p⁄ transition band at
285.1 nm (fosc = 0.3543) in the gas phase. The rest of the spectral bands undergo either hyperchromic
or hypsochromic shifts in the presence of solvent. Polarizability and HOMO–LUMO gap values were used
to predict the nonlinear optical properties (NLO) of the studied compound. NBO analysis has been used to
determine the most accurate Lewis structure of the studied molecule.
Keywords:
Nitrone
B3LYP
X-ray single crystal
Electronic spectra
NBO
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding authors at: Department of Chemistry, Rabigh College of Science
and Arts, King Abdulaziz University, P.O. Box 344, Rabigh 21911, Saudi Arabia. Tel.:
+966 560247664 (J. Lasri). Tel:. +966 565450752 (S.M. Soliman).
E-mail addresses: jlasri@kau.edu.sa (J. Lasri), saied1soliman@yahoo.com
(S.M. Soliman).
Introduction
Nitrones, whose name comes from nitrogen ketone, have been
widely reviewed and illustrated in synthetic chemistry [1,2].
http://dx.doi.org/10.1016/j.saa.2014.10.096
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

1858
J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Nitrone derivatives have been used for the synthesis of many nat-
ural products of biological interesting materials [3]. These com-
pounds were found to act as an important key step in the
efficient incorporation of multiple stereocenters via cycloaddition
reactions [4]. The nitrone moiety has possessed pharmacological
activity that provides the effective center in the molecular struc-
ture of many drugs [5]. This makes nitrones a potential candidates
in many experimental animal models as biological active materials
[6,7] and as anti-cancer drugs [8]. A combination of nitrones and
anti-oxidants was found to have a synergistic effect to prevent
the acute acoustic noise-induced hearing loss [9]. The corrosion
of metals and alloys in organic acidic media was significantly low-
ered when treated by nitrones [10–13].
Inouye et al. [14] proposed that crystalline nitrones exist as
mainly Z-isomer. They also found that the E/Z isomerization
occurred when it dissolved. There are only few publications that
deal with the theoretical studies on the molecular structure and
spectral properties of nitrones [15,16].
In this work, the new N-methyl-C-2,4,6-trimethylphenylnitrone
1 is synthesized and characterized using FTIR, 1H, 13C NMR, mass
spectrometry and X-ray diffraction. We have studied the equilib-
rium between the E and Z isomers in gas phase and in various sol-
vents (with different polarity) to predict the effect of solvent on the
vibrational, electronic, and NMR spectra of the most stable isomer.
Frontier molecular orbitals (FMOs) and natural atomic charges
were calculated at the same level of theory. Natural bond orbital
(NBO) calculations were performed to predict the most accurate
Lewis structure of the studied nitrone. Also, the stabilization
energy of various intramolecular interactions was investigated.
was applied to the trapped peak at m/z 178.1233 with collision
energy (CID) set at 35 eV. The accuracy of the mass spectrometer
was enough to assign the fragmentation species as good as 5 ppm
as shown in Fig. S1 (Supplementary Information). Liquid chroma-
tography (Agilinet series 1260) was used to separate and measure
the UV/Vis spectrum for the nitrone compound in aqueous med-
ium. Five microliter of acetonitrile solution (1 mg of nitrone/mL)
was introduced into HPLC instrument that is coupled with UV/
Vis detector. Acetonitrile/water mixture (30:70) was used as
mobile phase at a flow rate of 1.0 mL/min for 2 min. After that,
the mixture was set at 100% water for another 10 min. Full scan
UV/Vis spectrum was measured for the separated nitrone peak
at retention time = 3.8 min, which means that the spectrum was
recorded in 100% water.
Synthesis of (Z)-N-methyl-C-2,4,6-trimethylphenylnitrone 1
Sodium carbonate (0.63 g, 5.98 mmol) was added to a solution
of N-methylhydroxylamine hydrochloride (1.00 g, 11.97 mmol) in
Experimental section
Material and instrumentation
Fig. 1. Thermal ellipsoid plot of 1. The thermal ellipsoids are drawn at 50%
probability.
All solvents and reagents were obtained from Sigma–Aldrich
and were used as received. 1H and 13C NMR spectra (in CDCl3)
were measured on Bruker Avance III HD 600 MHz (Ascend™ Mag-
net) spectrometer at ambient temperature. 1H and 13C chemical
shifts (d) are expressed in ppm relative to Si(Me)4. Infrared spec-
tra (400–4000 cmÀ1) were recorded on an Alpha Bruker FT-IR
instrument in KBr pellets. Mass spectra were carried out using
high resolution ion-trap time-of-flight mass spectrometry inter-
faced with electrospray source (MicrOTOF II mass spectrometry
from Bruker). The drying gas temperature of the mass spectrom-
etry was maintained at 200 °C. N2 was used as nebulizer gas at a
pressure of 20 psi. Positive mode scanning was performed at the
range of m/z 50–1500. Acetonitrile solution of the product com-
pound (nitrone) was continuously introduced into the mass spec-
trometer detector with a syringe pump at a flow rate of 20 lL/
min. The electrospray voltage and the collision cell RF were set
at 2.5 kV and 100 Vpp, respectively. Tandem mass spectrometry
Fig. 2. Packing of 1 viewed along crystallographic c-axis.
Me
Me
Me
_
O
O
Na2CO3
+
MeNHOH.HCl
C
N
Me
C
+
MeOH
Me
Me
H
H
r.t., 12 h
1
Me
Scheme 1. Synthesis of acyclic nitrone 1.

J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
1859
methanol (4 mL). The mixture was stirred for 5 min. Mesitaldehyde
Computational details
(1.61 g, 10.88 mmol) was then added and the mixture was stirred
at room temperature for 12 h. The precipitate formed during the
reaction was filtered off and the filtrate was evaporated to dryness
to provide further solid. The precipitate was then dissolved in
dichloromethane resulting in the precipitation of NaCl. The latter
was filtered off and the dichloromethane was then removed in
vacuo. The final yellow-white solid was washed with ether to
obtain pure (Z)-N-methyl-C-2,4,6-trimethylphenylnitrone 1 in c.a.
90% yield (Scheme 1).
A complete optimization was done on the two isomeric forms Z
and E by minimizing the energies with respect to all geometrical
parameters without imposing any molecular symmetry con-
straints. The DFT-B3LYP/6-311++G(d,p) method was used for the
complete optimization using Gaussian 03 program [21]. The struc-
tures of the optimized geometries were drawn by GaussView 4.1
[22]. The computational study was first carried out in gas phase.
After that the self-consistent reaction field (SCRF) theory [23], with
polarized continuum model (PCM), was used to predict the effect
of solvent on the stability of the studied isomers [24]. The stability
of the optimized geometry was confirmed by frequency calcula-
tions, which gave no imaginary values for the frequencies. The total
energy distribution (TED) of the vibrational modes of the mole-
cules was calculated with the VEDA program [25]. They were char-
acterized by their total energy distribution (TED%). UV absorption
energies of this compound were calculated by the TD-DFT method
in different solvents to predict the effect of solvent on the elec-
tronic spectra compared to the gas phase, also for visualizing
HOMO and LUMO states. The natural atomic charges are calculated
using NBO calculations as implemented in the Gaussian 03 package
[26] at the DFT/B3LYP level. The most accurate Lewis structure and
the different intermolecular charge transfer interaction (ICT) ener-
gies were investigated using the NBO calculations. The gauge
including atomic orbital (GIAO) method was used for the NMR cal-
culations. The 1H and 13C isotropic tensors referenced to the TMS
calculations were carried out at the same level of theory.
ÀO+N(Me)@C(H)(C6H2Me3-2,4,6)
Yield: 90%. IR (cmÀ1): 1610 (C@N). 1H NMR (CDCl3), d: 2.28 and
2.30 (two s, 6H and 3H, respectively, CH3Ph), 3.94 (s, 3H, CH3N),
6.90 (s, 2H, CHaromatic), 7.61 (s, 1H, C(H)@N). 13C NMR (CDCl3), d:
19.8 and 21.1 (CH3Ph), 53.2 (CH3N), 125.7 (Caromatic), 128.4
(CHaromatic), 135.7 (C(H)@N), 137.5 and 139.5 (Caromatic). DEPT-135
NMR (CDCl3), d: 19.8 and 21.1 (CH3Ph), 53.2 (CH3N), 128.4
(CHaromatic) and 135.7 (C(H)@N). ESI+-MS, m/z: 178.1233 [M+H]+.
Tandem MS (for 178.12), m/z: 178.1233, 160.1127, 146.0955,
131.0733, 119.0849, 105.0065, 91.0483. 73.068. UV/Vis, wave-
length maxima: 210, 224, 250 and 278 nm.
X-ray crystallography
The X-ray diffraction data of 1 (Figs. 1 and 2) was collected on
an Agilent Technologies Supernova diffractometer using Cu K
a
radiation (k = 1.5418 Å). The CrysAlisPro [17] program package
was used for cell refinements and data reductions. A multi-scan
absorption correction based on equivalent reflections (CrysAlisPro)
[17] was applied to the data. The structure was solved by direct
method using the SHELXL-97 [18] program with the Olex2 [19]
graphical user interface. Structural refinement was carried out
using SHELXL-97 [18]. Molecular graphics was created with the
UCSF Chimera package [20]. Hydrogen atoms were positioned geo-
metrically and constrained to ride on their parent atoms with
CAH = 0.93–0.96 Å and Uiso = 1.2–1.5 Ueq (parent atom). The crys-
tallographic details are summarized in Table 1. Selected bond
lengths and angles for 1 were tabulated in Table 2. For additional
information regarding other X-ray data see Supplementary
Information (Tables S1–S5).
Table 2
Selected bond lengths (Å) and angles (°) for the studied
nitrone 1.
O9AN8
N8AC7
N8AC10
C7AC1
O9AN8AC7
O9AN8AC10
C7AN8AC10
N8AC7AC1
1.290(2)
1.301(2)
1.471(2)
1.474(2)
125.00(14)
114.82(14)
120.13(14)
124.08(15)
Table 1
-
Crystal data for the studied nitrone 1.
Me
Me
H
C
O
+
1
Me
Me
N
Empirical formula
Fw
Temp. (K)
k (Å)
Crystsyst
Space group
a (Å)
C12H15NO
176.99
293(2)
Me
Me
Me
-
1.5418
Monoclinic
C2/c
25.6557(11)
5.0634(2)
15.7014(6)
102.554(4)
1990.92(14)
8
O
+
N
C
b (Å)
c (Å)
H
Me
b (deg)
E
Z
V (Å3)
Z
Scheme 2. The Z and E isomers of the studied nitrone 1.
qcalc (Mg/m3)
1.181
0.568
l(Mo K
a
) (mmÀ1
)
No. reflns.
Unique reflns.
GOOF (F2)
Rint
9680
2073
1.046
0.1273
0.0737
0.2028
Table 3
Effect of solvent on the population percentages (%) of the studied isomers.
Isomer
Pop. (%)
Gas
R1a (I P 2
wR2b (I P 2
r)
Cyclohexane
Chloroform
DMSO
Water
r)
Z
E
99.18
0.82
98.06
1.94
95.28
4.72
91.14
8.86
93.88
6.12
a
R1 =
wR2 = [
R
||Fo| À |Fc||/
R|Fo|.
b
2
R
[w(Fo À Fc2)2]/
R .
[w(Fo2)2]]1/2

1860
J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Table 4
Comparative experimental and angles of the Z-isomer in gas phase and in solvents of different polarities.
Parametera
Gas (
e
= 0)
Cyclohexane (
e
= 2.02)
Chloroform (
e
= 4.90)
DMSO (
e
= 46.70)
Water (
e
= 78.39)
X-ray
1.290
1.301
1.471
R(1-2)
R(2-3)
R(2-16)
1.281
1.317
1.482
1.285
1.314
1.481
1.290
1.313
1.480
1.296
1.310
1.479
1.297
1.309
1.479
R(3-4)
1.084
1.085
1.086
1.088
1.088
0.930
R(3-5)
1.466
1.468
1.469
1.471
1.471
1.474
R(5-10)
1.416
1.416
1.416
1.415
1.416
1.406
R(5-11)
1.420
1.419
1.419
1.417
1.418
1.415
R(6-7)
1.088
1.089
1.089
1.090
1.090
0.931
R(6-11)
1.395
1.396
1.397
1.398
1.398
1.393
R(6-20)
1.401
1.402
1.402
1.402
1.402
1.390
R(8-9)
1.088
1.088
1.089
1.090
1.090
0.930
R(8-10)
1.401
1.401
1.402
1.401
1.402
1.394
R(8-20)
1.396
1.397
1.398
1.399
1.399
1.390
R(10-12)
1.507
1.507
1.508
1.509
1.509
1.504
R(11-21)
1.513
1.513
1.513
1.513
1.513
1.506
R(12-13)
1.097
1.097
1.097
1.097
1.097
0.960
R(12-14)
1.093
1.093
1.093
1.093
1.093
0.961
R(12-15)
1.096
1.096
1.096
1.096
1.096
0.960
R(16-17)
1.091
1.091
1.092
1.092
1.092
0.961
R(16-18)
1.091
1.091
1.091
1.091
1.090
0.960
R(16-19)
1.092
1.092
1.092
1.092
1.092
0.961
R(20-25)
1.511
1.511
1.511
1.511
1.511
1.510
R(21-22)
1.096
1.096
1.096
1.096
1.096
0.959
R(21-23)
1.093
1.093
1.093
1.093
1.093
0.960
R(21-24)
1.098
1.098
1.098
1.097
1.097
0.959
R(25-26)
1.095
1.095
1.095
1.095
1.095
0.959
R(25-27)
1.094
1.094
1.094
1.094
1.094
0.960
R(25-28)
1.097
1.098
1.098
1.098
1.098
0.961
R(1---15)
A(1-2-3)
A(1-2-16)
A(3-2-16)
A(2-3-4)
2.207
2.228
2.258
2.290
2.289
2.305
125.987
114.140
119.873
114.648
125.611
107.316
111.223
107.497
119.609
122.554
117.646
119.725
118.666
121.968
119.351
121.137
118.896
119.297
121.803
119.496
117.995
120.773
118.404
119.174
122.418
119.333
121.214
111.121
110.822
110.650
111.712
110.687
111.933
107.925
106.780
109.416
110.995
109.071
110.608
111.365
111.388
111.115
107.922
107.107
107.257
125.737
114.246
120.016
114.753
125.478
107.414
111.081
107.558
119.670
122.358
117.751
119.811
118.695
121.857
119.292
121.103
118.895
119.316
121.786
119.589
118.053
120.763
118.478
119.194
122.325
119.417
121.167
111.174
110.835
110.620
111.684
110.695
111.877
107.942
106.830
109.314
110.934
109.151
110.581
111.348
111.382
111.079
107.942
107.083
107.343
125.507
114.369
120.123
114.864
125.281
107.520
110.903
107.611
119.782
122.207
117.842
119.866
118.733
121.788
119.248
121.097
118.893
119.332
121.772
119.639
118.109
120.739
118.537
119.224
122.235
119.444
121.135
111.205
110.825
110.669
111.699
110.682
111.806
107.976
106.811
109.225
110.892
109.259
110.545
111.337
111.380
111.031
107.968
107.033
107.442
125.209
114.405
120.386
115.145
124.796
107.650
110.723
107.661
120.018
121.786
118.106
120.021
118.748
121.588
119.157
121.071
118.890
119.369
121.739
119.757
118.194
120.790
118.629
119.257
122.109
119.637
120.997
111.220
110.817
110.654
111.695
110.699
111.722
107.938
107.064
109.023
110.824
109.360
110.522
111.365
111.374
110.956
107.979
106.957
107.582
125.231
114.434
120.334
115.064
124.939
107.654
110.673
107.675
119.945
121.890
118.003
120.014
118.741
121.630
119.160
121.072
118.895
119.368
121.736
119.755
118.203
120.771
118.618
119.265
122.113
119.596
121.006
111.202
110.799
110.705
111.713
110.681
111.687
107.959
107.020
109.028
110.828
109.375
110.536
111.360
111.374
110.942
108.004
106.944
107.608
124.994
114.819
120.139
117.933
124.087
109.453
109.447
109.433
117.980
122.886
117.084
119.975
118.897
121.924
118.836
121.259
119.138
119.057
121.805
119.893
118.506
120.980
119.016
119.074
121.910
119.145
120.509
109.442
109.515
109.462
109.430
109.489
109.535
109.416
109.488
109.504
109.550
109.549
109.396
109.506
109.471
109.495
109.464
109.429
109.481
A(2-3-5)
A(2-16-17)
A(2-16-18)
A(2-16-19)
A(4-3-5)
A(3-5-10)
A(3-5-11)
A(10-5-11)
A(5-10-8)
A(5-10-12)
A(5-11-6)
A(5-11-21)
A(7-6-11)
A(7-6-20)
A(11-6-20)
A(6-11-21)
A(6-20-8)
A(6-20-25)
A(9-8-10)
A(9-8-20)
A(10-8-20)
A(8-10-12)
A(8-20-25)
A(10-12-13)
A(10-12-14)
A(10-12-15)
A(11-21-22)
A(11-21-23)
A(11-21-24)
A(13-12-14)
A(13-12-15)
A(14-12-15)
A(17-16-18)
A(17-16-19)
A(18-16-19)
A(20-25-26)
A(20-25-27)
A(20-25-28)
A(22-21-23)
A(22-21-24)
A(23-21-24)

J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
1861
Table 4 (continued)
Parametera
Gas (
e
= 0)
Cyclohexane (
e
= 2.02)
Chloroform (
e
= 4.90)
DMSO (
e
= 46.70)
Water (
e
= 78.39)
X-ray
A(26-25-27)
A(26-25-28)
A(27-25-28)
108.057
107.204
107.516
108.122
107.186
107.531
108.213
107.153
107.538
108.343
107.176
107.438
108.361
107.166
107.449
109.499
109.479
109.378
a
Atom numbering are referred to Fig. 3.
Z-isomer
E-isomer
Fig. 3. The optimized molecular structure of the E- and Z-isomers calculated using the B3LYP/6-311++G(d,p) method.
abundance of the most stable species, Z is equal to 99.18% at
298 K in gas phase. This indicates that the Z-isomer is the predom-
inant in the gas phase. These results are in good agreement with
the experimentally measured X-ray structure.
Results and discussion
Stability and thermodynamic results
Solvent effects are important on the stability of isomers, since
polarity differences among isomers can make significant changes
in their relative energies in solution. We used PCM calculations
to analyze the effect of solvent on the stability of the studied iso-
mers. Table S6 (Supplementary Information) shows the total ener-
gies and thermodynamic parameters of the studied isomers in the
presence of solvents of different polarities. The effect of different
solvents on the calculated population percentage of the E and Z iso-
The studied nitrone 1 is capable of existing in two isomeric
structures; either the E or Z configuration. The possible isomers
of the studied compound are given in Scheme 2. The B3LYP/6-
311++G(d,p) calculated energy predictions of the studied isomers
are compared in Table S6 (Supplementary Information). The results
showed that the Z-isomer has the lowest energy value
(E = 557.9293 Hartrees) in the gas phase. The E-isomer is higher
in energy by only 2.86 kcal/mol. This suggests that nitrone isomers
capable of E/Z isomerization even at room temperature. The rela-
tive abundances of the Z- and E-isomers were calculated using
mers is shown in Table 3. In solution, the energy barrier (DE)
between the Z-isomer and the E-one decreased compared to the
gas phase. As expected, it is revealed that the population as well
as the stability of the more polar isomer (E) increases in presence
of solvent compared to the gas phase. Also, the stability and the
Gibbs free energy equation;
D
G = ÀRTlnK. (where
DG denotes the
difference between the Gibbs free energies of E-isomer relative to
the Z-one and K is the corresponding equilibrium constant). The

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J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Table 5
The calculated natural charges of the Z-isomer in the gas phase and in solution using
B3LYP/6-311++G(d,p) method.
Atom
Gas
Cyclohexane
Chloroform
DMSO
Water
O1
N2
C3
H4
C5
C6
H7
C8
À0.5445
0.0703
À0.0054
0.2234
À0.1281
À0.2508
0.2299
À0.2357
0.2317
0.0257
0.0294
À0.6816
0.2306
0.2293
0.2711
À0.4622
0.2459
0.2204
0.2446
À0.0162
À0.6772
0.2398
0.2380
0.2306
À0.6706
0.2369
0.2356
0.2390
À0.5707
0.0680
0.0056
0.2307
À0.1310
À0.2538
0.2353
À0.2403
0.2363
0.0224
0.0301
À0.6824
0.2321
0.2313
0.2671
À0.4630
0.2476
0.2258
0.2465
À0.0156
À0.6795
0.2414
0.2397
0.2334
À0.6727
0.2385
0.2369
0.2404
À0.5974
0.0652
0.0167
0.2400
À0.1345
À0.2570
0.2412
À0.2452
0.2415
0.0189
0.0303
À0.6829
0.2342
0.2336
0.2627
À0.4637
0.2485
0.2316
0.2478
À0.0155
À0.6821
0.2423
0.2416
0.2370
À0.6747
0.2400
0.2381
0.2419
À0.6267
0.0610
0.0304
0.2519
À0.1371
À0.2602
0.2475
À0.2501
0.2471
0.0146
0.0294
À0.6845
0.2380
0.2365
0.2569
À0.4642
0.2488
0.2379
0.2489
À0.0164
À0.6843
0.2426
0.2432
0.2412
À0.6765
0.2408
0.2395
0.2436
À0.6314
0.0601
0.0322
0.2548
À0.1389
À0.2606
0.2488
À0.2506
0.2483
0.0135
0.0296
À0.6845
0.2385
0.2369
0.2563
À0.4640
0.2487
0.2387
0.2489
À0.0162
À0.6847
0.2429
0.2434
0.2417
À0.6767
0.2410
0.2397
0.2438
H9
C10
C11
C12
H13
H14
H15
C16
H17
H18
H19
C20
C21
H22
H23
H24
C25
H26
H27
H28
Fig. 4. Comparison between the calculated and experimental bond distances of the
Z-isomer.
Fig. 5. Comparison between the calculated and experimental bond angles of the
Z-isomer.
percentage population of the E-isomer are higher in polar solvents
such as DMSO and water compared to the nonpolar medium
(cyclohexane).
Fig. 6. Molecular electrostatic potentials (MEP) mapped on the electron density
surface calculated by the DFT/B3LYP method for the most stable isomer (Z).
rone moiety is not coplanar with the benzene ring which agrees
with the X-ray structure analysis. The H15---O1 intramolecular
distance is predicted to be 2.207 Å which might indicate the pres-
ence of weak CAH---O interaction between the O-atom of the nit-
rone and the H-atom of the neighboring methyl group.
The solvent effect on the computed bond distances in solution is
almost insignificant in the nonpolar solvent such as cyclohexane.
In contrast, there is a noticeable change in the CAN and NAO bond
distances in polar solvents. These bonds are of high polarity and
can be involved in the solute–solvent interactions when using
polar solvents. [27]. In general, the CAN bonds are shortened
whereas the NAO bond distances are elongated due to solvent
effect in solution. The changes are found to increase with the
increase in the solvent polarity. Moreover, the CAH---O interac-
tion are of little importance in solution especially in solvents of
high polarity.
Molecular geometry
The optimized bond lengths and bond angles obtained for the
stable Z-isomer using the B3LYP method with 6-311++G(d,p) basis
set are given in Table 4 (the atom numbering of the optimized
structure are given in Fig. 3). The optimized geometry is compared
with the structural parameters obtained from the CIF file. The point
group of the most stable isomer (Z) is C1. Comparison between the
experimental and the calculated geometric parameters (bond dis-
tances and bond angles) is shown in Figs. 4 and 5. All bond lengths
are found to be overestimated except for the O1AN2 and C3AC5.
The maximum deviation of the predicted values from the experi-
mental data is 0.016 Å. These deviations are attributed to the phase
difference between the calculations and the experiment. The calcu-
lations refer to an isolated molecule in the gas phase, whereas the
experimental data are those of the molecule in the solid phase. The
maximum deviation of the calculated bond angles from the exper-
imental data is 1.524° (N2AC3AC5). The calculated angle between
the phenyl ring plane and the plane passing through the
C5AC3AN2AC16 is 51.82° (Fig. 3). The results indicate that the nit-
Natural atomic charges
The natural atomic charges (NAC) at different atomic sites of
the most stable isomer (Z), calculated using the DFT/B3LYP

J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
1863
Table 6
The calculated electronic spectra using the TD-DFT method.
Solvent
Gas
kmax
fosc
Major contribution
Solvent
kmax
fosc
Major contribution
285.1
244.1
217.2
0.3543
0.0494
0.0171
HOMO ? LUMO (74%)
Cyclohexane
288.4
242.7
211.7
0.4254
0.0644
0.0610
HOMO ? LUMO (78%)
HOMO ? L + 2 (73%)
H À 2 ? L + 1 (14%) + H À 1 ? L + 2
(10%) + HOMO ? L + 6 (67%)
H À 3 ? L + 1 (12%) + H À 2 ? L + 2
(52%) + H À 1 ? L + 2 (16%)
H À 1 ? LUMO (11%) + HOMO ? L + 2 (71%)
H À 2 ? L + 1 (92%)
205.9
0.1273
H À 2 ? L + 2 (27%) + H À 1 ? L + 2
(25%) + HOMO ? L + 8 (24%)
204.8
0.1220
Chloroform
287.2
240.8
0.4132
0.0624
HOMO ? LUMO (78%)
HOMO ? L + 2 (70%)
DMSO
284.4
238.4
0.3492
0.0491
HOMO ? LUMO (73%)
H À 1 ? L + 3 (14%) + HOMO ? L + 1
(62%)
208.0
205.2
0.0900
0.0739
H À 1 ? L + 2 (19%) + HOMO ? L + 6 (64%)
201.3
195.4
0.0532
0.3532
H À 3 ? L + 1 (10%) + H À 2 ? L + 2
(39%) + HOMO ? L + 6 (20%)
H À 2 ? L + 1 (24%) + H À 1 ? L + 3 (20%)
H À 2 ? L + 1 (10%) + H À 1 ? L + 2
(19%) + HOMO ? L + 6 (23%) + HOMO ? L + 7 (12%)
Water
283.5
238.1
200.6
194.6
0.3395
0.0465
0.0536
0.3329
HOMO ? LUMO (74%)
H À 1 ? L + 3 (14%) + HOMO ? L + 1 (65%)
H À 2 ? L + 2 (44%) + HOMO ? L + 6 (13%)
H À 2 ? L + 1 (25%) + H À 1 ? L + 3 (21%)
E
HOMO= -5.8872 eV
ELUMO=-1.3160 eV
ΔE = 4.5713 eV
Fig. 7. The ground state isodensity surface plots for the frontier molecular orbitals (MOs) contributed in the electronic transitions of the most stable isomer (Z).
Fig. 8. The calculated electronic spectra of the Z-isomer in gas phase and in different solvents using TD-DFT method.
method, are collected in Table 5. From the NAC values listed in
this table, we can observe the electropositive nature of hydrogen
atoms. All the hydrogen atoms have positive charges within range
from 0.2234 to 0.2711 in the gas phase. Of these H-sites, the H17
has the highest positive charge density probably due to the
CAH---O interaction. The highest negative charge densities occur
at the oxygen atom. Since the O-atom is more electronegative
than nitrogen so the latter has positive charge. The largest varia-
tion of the NAC values due to the solvent occurs at the O1 atom.
The presence of solvent increases the negative charge density at
O-atom. The change in the charge density at the O-site is higher
in polar solvents than in the nonpolar solvent. The O-atom locates
at terminal position in the molecule which makes them affected
most by the solvent.

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J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Table 7
with 6-311++G(d,p) basis set is used to predict the reactive sites
for electrophilic and nucleophilic attack. The negative (red) regions
of the MEP are related to electrophilic reactivity, whereas the posi-
tive (blue) regions are related to nucleophilic reactivity (Fig. 6). As
can be seen, negative regions are mainly localized over the oxygen
atom whereas the maximum positive region is localized on the
hydrogen atoms. The positive regions are associated with H4 and
H18 with values around +0.0354 and +0.0314 a.u., respectively.
Therefore, it is expected that the most preferred region for nucleo-
philic attack will be on H4 and H18. However, the most negative
region is localized on atom O1 (with a value of À0.0589 a.u.) which
will be the preferred site for electrophilic attack [30].
The calculated chemical shifts d (ppm) of Z-nitrone using GIAO method.
Atom dcalc (ppm)
Gas Cyclohexane Chloroform DMSO
7.98
dexp.
(ppm)
Water
H4
H7
H9
8.14
7.39
7.37
2.12
2.27
3.79
4.10
3.92
4.06
2.65
2.28
2.65
2.57
2.33
2.87
8.33
7.53
7.49
2.19
2.31
3.64
4.11
4.01
4.09
2.65
2.34
2.71
2.60
2.36
2.89
126.89
116.09
115.50
117.20
131.58
127.79
12.96
45.29
130.24
13.57
14.60
8.60
7.70
7.60
2.37
2.33
3.46
4.10
4.11
4.09
2.62
2.39
2.78
2.56
2.41
2.88
8.66
7.73
7.63
2.37
2.35
3.45
4.10
4.13
4.10
2.62
2.39
2.79
2.57
2.42
2.88
7.61
6.90
6.90
2.28
2.28
2.28
3.94
3.94
3.94
2.28
2.28
2.28
2.30
2.30
2.30
7.26
7.29
2.04
2.26
3.90
4.07
3.84
4.02
2.64
2.24
2.62
2.54
2.31
2.85
H13
H14
H15
H17
H18
H19
H22
H23
H24
H26
H27
H28
C3
C5
C6
C8
C10
C11
C12
C16
C20
C21
C25
Electronic absorption spectra and frontier molecular orbitals (FMOs)
In order to understand the electronic transitions of the titled
compound, the first twenty spin allowed singlet–singlet excita-
tions were calculated using TD-DFT calculations in the gas phase.
In these calculations, we have started from the gas phase opti-
mized geometry of the Z-isomer using the same level of theory.
Similar calculations were performed to get the excitation energies
of Z-isomer in different solvents using the polarized continuum
model (PCM). The results, which are shown in Table 6, represent
the calculated kmax values with their major contributions of
molecular orbitals to the formation of bands for the Z-isomer.
Of the twenty electronic transitions obtained from the current
calculations, the only four transitions which have the highest
oscillator strength (f) values are considered. These spectral bands
are calculated at 285.1, 244.1, 217.2 and 205.9 nm in the gas
phase. Experimentally, these transition bands are observed in
water at 278, 250, 224 and 210 nm, respectively. In view of calcu-
lated absorption spectrum, the maximum absorption wavelength
corresponds to the electronic transition from the HOMO to LUMO
with 74% contribution. Fig. 7 shows the isodensity surface plots of
122.69 124.66
115.92 116.00
114.38 114.92
117.14 117.14
131.75 131.64
125.90 126.87
13.11
45.04
128.28 129.29
129.63 130.02 135.7
116.26 116.14 137.5
116.19 116.29 128.4
117.13 117.24 128.4
131.41 131.35 139.5
128.55 128.71 137.5
13.31
44.79
12.65
45.32
12.71
45.37
19.80
53.20
131.16 131.31 139.5
13.80
14.45
13.68
14.53
13.33
14.59
13.34
14.60
19.80
21.10
Molecular electrostatic potential (MEP)
The molecular electrostatic potential (MEP) is used primarily for
predicting sites and relative reactivity towards electrophilic and
nucleophilic attacks, and in predicting the hydrogen bonding inter-
actions [28,29]. The MEP of the Z-isomer calculated using B3LYP
Fig. 9. Correlations between the experimental and calculated chemical shifts using the GIAO method.

J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
1865
Table 8
The second order perturbation energies E(2) (kJ/mol) of the most important charge transfer interactions (donor–acceptor) in the studied molecule using B3LYP method.
Donor NBO (i)
BD(2)C5AC10
Acceptor NBO (j)
E(2)a (kJ/mol)
E(j) À E(i)b (a.u.)
0.2
F(i,j)c (a.u.)
BD⁄(2)N2AC3
BD⁄(2)C6AC11
BD⁄(2)C8AC20
BD⁄(2)C5AC10
BD⁄(2)C8AC20
BD⁄(2)C5AC10
BD⁄(2)C6AC11
BD⁄(1)N2AC3
BD⁄(1)N2AC3
BD⁄(1)N2AC16
BD⁄(2)N2AC3
BD⁄(1)C12AH15
64.10
95.44
70.29
66.44
92.51
97.82
72.47
23.81
30.46
50.04
269.99
2.97
0.05
0.28
0.28
0.29
0.29
0.28
0.28
1.29
0.77
0.53
0.2
0.072
0.062
0.061
0.071
0.073
0.062
0.077
0.067
0.071
0.103
0.027
BD(2)C6AC11
BD(2)C8AC20
LP(1)O1
LP(2)O1
LP(3)O1
LP(1)O1
1.26
a
b
c
E(2) means energy of hyperconjucative interactions.
Energy difference between donor and acceptor i and j NBO orbitals.
F(i,j) is the Fock matrix element between i and j NBO orbitals.
molecular orbitals (MOs) contributed in this electronic transition.
It can be seen from this figure that the electron densities of these
lation is better for carbon-13 than that for proton since protons are
more affected by the solute intermolecular (solute–solvent) inter-
action than the carbons [33]. Also, the calculated chemical shifts in
the appropriate solvent for predicting the 1H NMR shifts is more
efficient than that in gas phase.
MOs are mainly localized on the
Therefore, the band calculated at 285.1 nm is attributed mainly
to a HOMO–LUMO transition that is predicted as
p⁄ transi-
p-system of the Z-nitrone.
p
?
tion. The calculated electronic spectra of the Z-nitrone in different
solvents compared to the gas phase are shown in Fig. 8. It can be
seen that, the calculated absorption band at 285.1 nm in the gas
phase showed only little spectral shift in all the studied solvents.
In contrast, the rest of the spectral bands underwent either hyp-
sochromic shift or hyperchromic effect in presence of solvent due
to the solute–solvent interactions. The spectral shifts and the
increase of absorption intensity are higher in polar solvents such
as DMSO and water compared to nonpolar solvents such as
cyclohexane.
Natural bond orbital analysis
The occupancy of electrons and p-character in significant NBO
natural atomic hybrid orbitals for the studied compound is given
in Table S7 (Supplementary Information). It is noted that, the hydro-
gen atoms have almost 0% of p-character for all CAH bonds. In con-
trast, 100% p-character was observed in atoms that have p-bonding
of CAC and CAN bonds. Similarly, almost 100% p-character was
observed in second and third lone pairs of O-atom.
It is well known that the ideal sp2 hybrid orbital has 66.67% p-
character. The BD(1)C5AC10 orbital with 1.9681 electrons has
50.92% C5 character (64.71%p) and 49.08% C10 character
(66.48%p). Also, the BD(1)C6AC11 orbital with 1.9719 electrons
has 49.67% C6 character (64.38%p) and 50.33% C11 character
(64.91%p). The BD(1)N2AC3 orbital with 1.9911 electrons has
63.27% N2 character (59.49%p) and 36.73% C3 character
(69.88%p). The low% p-character in these hybrids is due to the res-
onance effect with the phenyl group and the C@N bond. The two C-
atoms forming the CAC hybrids have almost the same %electron
density (%ED). Similarly, the BD(1)O1AN2 orbital with 1.9934 elec-
trons has 49.34% O1 character (78.14%p) and 50.66% N2 character
(71.41%p). As expected from the electronegativity values of C, N,
and O atoms, the valence hybrids analyses of NBO orbitals showed
that all the CAN and NAO bond orbitals are polarized towards the
nitrogen and oxygen respectively. The overall polarity of the stud-
ied compound was found to be 3.416 D.
Nonlinear optical effects
Nonlinear optical (NLO) materials have a great impact on infor-
mation technology and industrial applications. The understanding
of the relationships between the NLO activity and the electronic
properties of any given compound has great importance for the
development of new and more sophisticated materials with great
NLO properties [31]. The polarizability (a) and the ability of elec-
tronic transition are important factors that affect the NLO activity
of a compound. For comparative purposes, urea was used as refer-
ence for the NLO activity of the studied compound [32]. Our com-
pound has higher polarizability (
urea (34.06 Bohr3). It has also about five times higher NLO than
urea. In addition, the HOMO–LUMO energy gap ( E) indicates
the ease of the electronic transition and hence the ability of mole-
cule to absorb light in visible region. The E value of the studied
a
= 152.72 Bohr3) compared to
D
D
compound is calculated to be 4.5713 eV which is lower than that
for urea (6.7063 eV). Based on these theoretical data, the studied
compound could be used as NLO material even better than urea.
Second-order perturbation theory
NBO method provides significant information about interac-
tions in both filled and virtual orbital spaces which could enhance
the analysis of intra- and inter-molecular interactions. The stabil-
ization energy E(2) associated with delocalization is estimated
using the second-order perturbation theory [34]. The larger the
E(2) value, the more intensive is the interaction between electron
donors and electron acceptors [35]. Table 8 lists the calculated sec-
ond order interaction energies (E(2)) between the donor–acceptor
orbitals in the studied molecule.
NMR spectra
The isotropic magnetic shielding (IMS) values calculated using
the GIAO approach at the 6-311++G(d,p) level are used to predict
the 13C and 1H chemical shifts (dcalc) for the Z-isomer. The results
are correlated to the experimental NMR data (dexp) in CDCl3 sol-
vent. The solvent polarity effect is taken into account by the PCM
model using different solvent models. The experimental and theo-
retical values for 1H and 13C NMR chemical shift values are given in
Table 7. The correlation between the experimental and calculated
chemical shifts is shown in Fig. 9. The results show that the corre-
The most important interaction energy in this molecule, is the
electron donating from LP(3)O1 orbital to the antibonding p⁄(2)-
N2AC3 orbital which results in stabilization energy of 269.99 kJ/

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J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
Table 9
The calculated vibrational frequencies and intensities of the Z-isomer in gas phase and in solvents of different polarities using B3LYP/6-311++G(d,p) method.
No.
Gas
Cyclohexane
Chloroform
DMSO
Water
Y
Exp
S.F
Assignment
t
A
Y
A
t
A
t
A
A
1
2
3
4
5
6
7
8
9
3210
3170
3169
3166
3161
3121
3120
3119
3089
3089
3081
3071
3032
3032
3027
1654
1639
1603
1519
1511
1510
1497
1492
1492
1482
1474
1472
1458
1443
1431
1421
1418
1417
1340
1311
1281
1248
1216
1179
1140
1101
1067
1061
1060
1056
1035
1033
983
977
942
896
868
844
788
728
622
616
575
558
543
528
510
507
377
354
328
293
280
238
199
196
178
15.3
18.6
7.5
27.0
4.3
3200
3169
3159
3157
3154
3121
3119
3118
3088
3088
3082
3070
3031
3030
3028
1653
1641
1602
1518
1508
1507
1494
1489
1488
1480
1471
1470
1453
1442
1431
1420
1416
1415
1338
1311
1281
1248
1210
1178
1140
1103
1066
1060
1059
1056
1034
1032
981
975
942
897
868
845
797
728
621
616
574
558
542
528
509
507
376
354
329
290
278
237
197
195
177
13.4
8.0
3.4
3185
3168
3156
3141
3139
3120
3119
3117
3088
3087
3081
3068
3031
3029
3027
1651
1643
1601
1517
1506
1503
1491
1486
1485
1478
1469
1468
1449
1441
1430
1418
1415
1413
1335
1311
1281
1248
1203
1177
1140
1105
1065
1059
1058
1055
1034
1032
978
973
941
898
869
846
808
728
619
615
574
558
540
529
508
506
376
353
329
286
276
237
196
193
177
9.4
10.2
3.1
3171
3153
3150
3126
3125
3119
3118
3116
3087
3087
3081
3066
3030
3028
3028
1653
1646
1601
1517
1504
1499
1489
1484
1484
1478
1467
1466
1444
1439
1428
1417
1413
1411
1330
1313
1281
1248
1195
1176
1141
1106
1064
1059
1058
1055
1034
1031
974
970
941
899
870
847
816
726
618
615
573
559
538
529
506
505
374
352
330
282
273
236
196
186
177
1.8
3.0
3171
3153
3139
3123
3123
3119
3118
3116
3087
3087
3081
3066
3030
3028
3028
1653
1645
1600
1516
1504
1498
1488
1483
1483
1477
1466
1465
1443
1439
1428
1417
1413
1411
1330
1312
1280
1247
1194
1175
1141
1107
1064
1059
1057
1055
1034
1031
973
970
940
899
870
846
820
726
618
614
573
558
538
529
506
505
374
352
330
282
273
236
197
186
176
2.1
3.0
26.1
15.9
35.9
2.8
3.1
16.3
8.8
26.6
16.5
17.2
28.6
23.0
46.4
64.1
82.9
28.4
14.3
36.2
27.4
8.7
13.1
10.9
6.4
25.8
27.5
64.4
2.2
65.4
10.7
1.8
7.5
11.9
1.7
1.4
2.7
223.3
38.4
1.4
2.1
6.7
3.1
20.0
14.0
1.8
7.3
59.8
5.3
11.1
0.1
55.5
3.7
27.0
5.8
8.9
5.2
0.4
4.2
2.2
1.6
11.5
9.5
4.7
18.1
0.1
1.6
0.4
1.7
5.3
3054
3040
0.9513
0.9590
97tCH
91tCH
89tCH
94tCH
95tCH
96tCH
87tCH
90tCH
90tCH
94tCH
94tCH
100tCH
85tCH
84tCH
98tCH
80tCC
54tCN
22.6
11.4
28.3
11.2
2.7
15.5
23.7
20.5
14.4
18.8
14.2
19.5
16.8
24.5
27.8
40.1
38.7
58.6
45.9
25.1
9.9
11.9
20.0
19.9
12.5
21.5
14.1
20.5
16.8
20.9
29.5
36.9
38.0
67.7
58.1
28.0
10.5
36.1
28.2
6.0
16.0
8.6
6.5
33.0
26.5
69.9
3.3
35.3
7.2
18.7
14.3
16.1
13.4
18.6
16.7
28.2
25.8
40.6
37.1
45.6
38.2
21.3
8.5
23.0
4.9
2971
2947
2916
0.9617
0.9564
0.9617
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
30.6
16.7
17.1
29.0
25.6
45.0
64.3
81.1
28.7
13.7
36.7
26.1
8.5
14.2
10.3
6.3
26.8
25.2
63.4
2.7
67.2
11.1
1.7
9.1
10.8
1.7
1.2
1.9
219.2
30.4
1.3
1.8
6.6
2.8
19.7
13.4
1.8
6.8
59.4
0.7
9.4
0.1
53.9
2.7
27.6
4.6
9.6
5.5
0.4
4.3
1656
1639
1610
1510
1.0012
1.0003
1.0043
0.9939
54tCC
25dHCH
55dHCH
21dHCH
52dHCH
60dHCH + 11dCCC
77dHCH
71dHCH
31dHCH
63dHCH + 13sHCNC
22.9
25.4
3.3
13.3
8.4
32.3
24.5
4.4
15.0
8.5
1451
0.9694
7.3
5.9
35.7
20.1
38.7
1.8
10.8
2.9
2.0
7.4
5.2
1.3
0.6
0.6
96.4
2.1
1.0
0.5
4.0
0.4
12.3
0.9
1.6
2.1
10.3
0.2
2.9
0.1
22.6
0.6
16.6
1.3
9.9
2.2
0.2
2.9
1.3
0.7
7.2
4.0
1.5
32.1
26.3
58.3
2.5
20.3
4.7
2.0
7.0
7.6
1.4
0.7
1.1
136.7
5.2
1.1
0.8
5.1
1.2
15.1
1.7
1.8
3.6
19.1
0.2
4.1
0.1
29.6
0.8
19.7
2.0
10.5
3.0
0.2
3.4
1.4
0.9
9.0
5.4
2.2
11.1
0.4
11
14
t
t
NO + 17dHCC + 14dHCH
CC + 14dHCC
12dHCC + 56dHCH
63dHCH
61dHCH
73dHCH
1412
1371
0.9935
0.9676
2.5
6.1
10.1
1.4
0.9
47tCC
48t
11t
24t
42t
12t
CC + 23dCCC
CC + 45dHCC
CC + 13dCCC + 31dHCC
NO + 17
1.9
183.7
12.4
1.3
1.2
6.5
2.1
18.5
3.6
1.9
5.2
33.8
0.2
5.8
0.1
39.8
1.3
23.4
3.1
10.3
4.0
0.3
3.7
1.6
1.2
8.3
9.4
3.3
14.7
0.3
1.7
0.4
1.9
3.9
14.6
0.7
1.8
1181
1153
1126
1095
0.9709
0.9781
0.9875
0.9943
tCN + 18dHCC
CC + 34dHCC
29dHCH + 53sHCNC
10 CN + 14dHCH + 56sHCNC
t
11dHCH + 34sHCCC
52sHCCC
15dHCH + 47sHCCC
47sHCCC
1041
0.9857
11dHCH + 44sHCCC
10
16
t
t
CC + 48sHCCC
NO + 22tCN
956
930
0.9727
0.9872
50tCC
30
91
68
11
68
t
s
s
t
s
CC + 10
HCCC
HCCC + 10sCCCC(OUT)
CC + 15tCN
HCCC + 11sCCCN
tCN + 14dCCC
858
820
0.9882
0.9714
720
617
0.9883
0.9923
11dCCC + 39sCCCC
42sOCCN(OUT)
14
52
10
10
t
t
CN + 11dCCN + 21dCCC
CC
547
537
0.9517
0.9884
s
OCCN(OUT) + 26sCCCC(OUT)
2.1
1.5
9.4
11.4
4.9
17.1
0.1
1.6
0.4
1.6
tCC + 29dCCC + 12sOCCN(OUT)
57sCCCC(OUT)
66dCCC
14dCCC + 23dONC
12sCNCC + 21sCCCC(OUT)
13dCCC + 34dCNC
58dCCC
70dCCC
26dCCC + 12dCNC
51sCCCC
508
3054
0.9959
0.9513
7.7
0.5
1.8
0.3
1.2
6.7
1.7
0.4
1.9
0.4
1.6
7.3
5.4
0.5
1.6
+
18
sCCCC(OUT)
5.2
19.5
2.0
29 CCCC + 75sCCCC(OUT)
s
21.1
1.7
2.7
54sHCNC
43sCCCC
57sCCCC
163
1.4
158
153
142
3.8
142

J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
1867
Table 9 (continued)
No.
Gas
Cyclohexane
Chloroform
DMSO
Water
Y
Exp
S.F
Assignment
t
A
Y
A
t
A
t
A
A
74
75
76
77
78
142
104
89
64
22
2.6
141
103
88
64
15
3.5
1.6
10.5
2.1
137
102
87
65
17
4.5
2.5
13.1
2.4
139
100
83
65
46
3.8
3.9
16.9
3.1
137
100
85
65
46
4.4
4.8
16.7
3.1
55sCCCC
58sCCCN
12dCCC + 10
92sCCCC
66sONCC
0.9
8.5
1.8
0.2
sCCCC + 43sCNCC
0.2
0.3
0.3
0.3
mol. The
gation of respective
p
(CAC) ? p⁄(CAC) interactions are responsible for conju-
-bonds within ring leading to a maximum
stabilization energy of 97.82 kJ/mol. The
(CAC) ? p⁄(CAN) inter-
the CH3AN@C are predicted at 3169 and 3161 cmÀ1 whereas the
symmetric one is predicted at 3071 cmÀ1
p
.
p
The asymmetric and symmetric bending vibrations of methyl
groups normally appear in the region 1470–1440 cmÀ1 and
1390–1370 cmÀ1, respectively [41–44]. The calculations predicted
actions that stabilizes the molecule by up to 64.10 kJ/mol. The
charge transfer interactions are formed by the orbital overlap
between bonding (
in intramolecular charge transfer (ICT) leading to extra stabiliza-
tion of the system. NBO analysis clearly manifests the evidence
for the weak intramolecular charge transfer from LP(1)O1 to
p⁄(1)C12AH15 antibonding orbital. This confirms the presence of
non-conventional interaction (C12AH15---O1) that provides up
to 2.97 kJ/mol stabilization for the molecule.
p
) and antibonding (p⁄) orbitals, which results
them in the ranges; 1519–1458 cmÀ1 and 1431–1417 cmÀ1
respectively.
,
The rocking vibrations of the CH3 group appear as mixed vibra-
tions in the region 1170–1100 cmÀ1 [41–44]. Our calculations
revealed that CH3 rocking modes, which are coupled with other
vibration modes, were predicted at lower frequency region
(1140–1035 cmÀ1; except modes 43 and 45).
CAC and CAN vibrations
Analysis of the vibrational spectra
The ring C@C stretching vibrations usually occur in the region of
1625–1430 cmÀ1 [45]. In the present work, the calculated value for
benzene ring C@C vibrations were found at 1654 cmÀ1, 1603 cmÀ1
and 1443 cmÀ1. The C@N stretching vibration is calculated to be
1639 cmÀ1 which is exactly the same as that observed in experi-
mental FT-IR spectrum. These results correlate well with the
reported values in the literature [46,47].
The vibrational spectra of the most stable isomer (Z) of the stud-
ied compound has been calculated using the B3LYP/6-311++G(d,p)
method. The FTIR spectrum of the studied compound is shown in
Fig. S2 (Supplementary Information). The calculated vibrational
frequencies and intensities together with the scaling factors (S.F)
using the B3LYP/6-311++G(d,p) method are given in Table 9. The
assignment of these modes based on the total energy distribution
(TED) is given in the same table. The studied nitrone consists of
28 atoms. So, it has 78 normal vibrational modes. Herein we will
provide description of the fundamental IR characteristics of the
studied compound.
Effect of solvent on the vibrational spectra
The effect of different solvents on the IR vibrational spectra
(vibrational frequencies and vibrational intensities) is shown in
Table 9 and Fig. S3 (Supplementary Information). As can be seen,
the most significant variations in the vibrational frequencies and
vibrational intensities occur only for the C@N and the CAH stretch-
ing modes. Most of these stretching modes undergo stronger bath-
ochromic shifts in the presence of polar solvents such as water and
DMSO compared to non-polar solvent (cyclohexane). Generally, an
increase in the vibrational intensities in all solvent was noted com-
pared to the gas phase. The increase of IR intensities was found to
be dependent on the solvent polarity. The general trend for the IR
intensity was found to be: water > DMSO > chloroform > cyclohex-
ane > gas phase (Fig. S3, Supplementary Information). Most of the
other modes are almost unaffected by the solvent.
CAH vibrations
The studied nitrone has three C(sp2)AH bonds. Therefore, it
undergoes three CAH stretching vibrations that are observed in
the range of 3040–3054 cmÀ1. The B3LYP method predicted the
t
(CAH) mode of the HC@N stretching vibration at 3210 cmÀ1
.
The aromatic symmetric and asymmetrical stretching vibrations
are predicted at 3170 and 3166 cmÀ1, respectively. In aromatic
compounds, the CAH in-plane bending vibrations are observed in
the region 1300–850 cmÀ1 and are usually weak [36]. The CAH
out of plane bending modes [37–39] are usually of medium inten-
sity that arise in the region of 950–600 cmÀ1 [36]. The present DFT
calculations predicted the in-plane CAH bending in the regions
1458–1431 and 1281–1179 cmÀ1, whereas the out of plane CAH
bending are predicted in the regions 1067–1033 and 896–
Conclusions
In this work, the synthesis and characterization of the
N-methyl-C-2,4,6-trimethylphenylnitrone 1 is reported. This com-
pound has been characterized using different spectroscopic tools.
DFT calculations showed that the Z-isomer is the most stable in
the gas phase. The stability and population of the E-isomer is
higher in polar solvents than in nonpolar. The molecular structure
of the most stable isomer (Z) has been calculated and compared
with the X-ray structure data. The effect of solvent on the struc-
tural parameters of the studied compounds was reported.
Complete assignment of the vibrational modes has been performed
on the basis of total energy distribution (TED) analysis. The TD-DFT
showed that most of the spectral bands undergo either hypsochro-
mic shift or hyperchromic effect in the presence of the solvent
compared to the gas phase. The longest wavelength band at
788 cmÀ1
.
Methyl group vibrations
According to Pulay et al. [40], the methyl (CH3) group has five
types of vibrational frequencies namely: symmetric stretch, asym-
metric stretch, symmetric deformation, asymmetric deformation
and rocking. The studied molecule has four methyl groups; three
of them are directly attached to the phenyl ring and the fourth is
attached to the nitrogen of the N@C bond. The three methyl
(CH3) groups attached to the phenyl ring have nine CAH stretching
vibrations: six asymmetric
(3121–3081 cmÀ1) and three symmetric
the region (3032–3027 cmÀ1). The asymmetric CAH stretches of
t(CAH) modes occur in the region
t(CAH) modes occur in

1868
J. Lasri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1857–1868
[16] N.R. Sheela, S. Sampathkrishnan, M. Thirumalai Kumar, S. Muthu, Spectrochim.
Acta A 109 (2013) 272.
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2012.
[18] G.M. Sheldrick, Acta Crystallogr. Sec. A 64 (2008) 112.
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Meng, T.E. Ferrin, J. Comput. Chem. 25 (2004) 1605.
285.1 nm (fosc = 0.3543) in the gas phase showed a little spectral
shift in all the studied solvents. The studied nitrone has been pre-
dicted to have better nonlinear optical properties (NLO) than urea.
The second order perturbation theory has been used to predict the
different intramolecular charge transfer (ICT) interactions within
the studied molecules.
[21] M.J. Frisch, et al., Gaussian-03, Revision C.01, Gaussian Inc, Wallingford, CT,
2004.
Acknowledgment
[22] R. Dennington II, T. Keith, J. Millam, GaussView, Version 4.1, Semichem Inc.,
Shawnee Mission, KS, 2007.
[23] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
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University of Wisconsin, Madison, 1998.
This work has been partially supported by the King Abdulaziz
University (Rabigh Branch, Saudi Arabia).
Appendix A. Supplementary material
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