Conformational, vibrational, NMR and DFT studies of N-methylacetanilide

Article abstract of DOI:10.1016/j.saa.2012.11.037

A detailed conformational, vibrational, NMR and DFT studies of N-methylacetanilide have been carried out. In DFT, B3LYP method have been used with 6-31G(d,p), 6-311++G(d,p) and cc-pVTZ basis sets. The vibrational frequencies were calculated resulting in IR and Raman frequencies together with intensities and Raman depolarisation ratios. The dipole moment derivatives were computed analytically. Owing to the complexity of the molecule, the potential energy distributions of the vibrational modes of the compound are also calculated. Isoelectronic molecular electrostatic potential surface (MEP) and electron density surface were examined. ¹H and ¹³C NMR isotropic chemical shifts were calculated and the assignments made are compared with the experimental values. The energies of important MO's of the compound were also determined from TD-DFT method.

Full text of DOI:10.1016/j.saa.2012.11.037

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Conformational, vibrational, NMR and DFT studies of N-methylacetanilide
b
c
d
e
V. Arjunan a, , R. Santhanam , T. Rani , H. Rosi , S. Mohan
⇑
a Department of Chemistry, Kanchi Mamunivar Centre for Post-Graduate Studies, Puducherry 605 008, India
b Research and Development Centre, Bharathiar University, Coimbatore 641 046, India
c Department of Chemistry, Rajiv Gandhi College of Engineering and Technology, Puducherry 605 402, India
d Department of Chemistry, Dr. S.J.S. Paul Memorial College of Engineering and Technology, Puducherry 605 110, India
e Department of Physics, Hawassa University, Main Campus, Hawassa, Ethiopia
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The FT-IR, FT-Raman, 1H, 13C and NMR spectral measurements of N-methylacetanilide and complete
assignments of the observed spectra have been elaborated. DFT calculations have been performed and
the structural parameters of the cis- and trans-isomers of the compound were determined from the opti-
mised geometry with 6-31G(d,p), 6-311++G(d,p) and cc-pVTZ basis sets and giving energies, harmonic
vibrational frequencies, depolarisation ratios, IR and Raman intensities. The geometric parameters,
harmonic vibrational frequencies and chemical shifts were compared with the experimental data of
the molecules. The influences of various factors regarding the stability and the conformational state of
"
"
"
"
"
FTIR, FT-Raman and NMR spectral
studies of N-methylacetanilide were
carried out.
The non-hydrogen atoms excluding
the phenyl group are almost
coplanar.
The CNC angle reveals less steric
hindrance and more stability in cis-
isomer.
the cis- isomer have been discussed.
A detailed natural bond orbital analysis was carried out.
The ELUMO ꢀ EHOMO orbital energy
gap in case of cis- isomer is found to
be 5.8753 eV.
The nN
stabilisation energy of
61.53 kJ molꢀ1
?
pꢁCO interaction has strong
.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2012
Received in revised form 6 November 2012
Accepted 8 November 2012
Available online 29 November 2012
A detailed conformational, vibrational, NMR and DFT studies of N-methylacetanilide have been carried
out. In DFT, B3LYP method have been used with 6-31Gꢁꢁ, 6-311++Gꢁꢁ and cc-pVTZ basis sets. The vibra-
tional frequencies were calculated resulting in IR and Raman frequencies together with intensities and
Raman depolarisation ratios. The dipole moment derivatives were computed analytically. Owing to the
complexity of the molecule, the potential energy distributions of the vibrational modes of the compound
are also calculated. Isoelectronic molecular electrostatic potential surface (MEP) and electron density sur-
face were examined. 1H and 13C NMR isotropic chemical shifts were calculated and the assignments made
are compared with the experimental values. The energies of important MO’s of the compound were also
determined from TD-DFT method.
Keywords:
FTIR
FT-Raman
NMR
Ó 2012 Elsevier B.V. All rights reserved.
N-methylacetanilide
DFT
NBO
⇑
Corresponding author. Tel.: +91 413 2211111, mobile: +91 9442992223; fax: +91 413 2251613.
E-mail address: varjunftir@yahoo.com (V. Arjunan).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.037

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
183
ABr), examined in benzene or dioxan, are defined by a planar,
cis- arrangement of the group CarANAC@O and dihedral angles of
38° 6° (standard deviation) between the amide group plane and
that of the aromatic ring, and that (b) in N-methylacetanilide the
C@O is trans to the phenyl group.
Introduction
Amides are of fundamental chemical interest as conjugation be-
tween nitrogen lone-pair electrons and the carbonyl
sults in distinct physical and chemical properties.
p
-bond, re-
A
planar
Many research works have been carried out on the spectro-
scopic and nonlinear properties of molecular crystals of N-phenyl-
acetamide derivatives [31–35]. The vibrational spectroscopic
analysis of acetanilide derivative particularly the N-methylacetan-
ilide has not been studied. Thus, owing to the vast biological signif-
icance and applications of N-methylacetanilide in an enormous
field and in continuation of earlier studies on N-(chloro substituted
phenyl)-2,2-dichloroacetamides [36] and N-(methylphenyl)-2,2-
dichloroacetamides [37–39], we have carried out an extensive
spectroscopic and quantum chemical studies on N-methylacetani-
lide in an effort to provide explanations for the conformational,
vibrational frequencies and to understand the effect of methyl
group on the frequencies of amide group.
structure of secondary amide group has been confirmed by mea-
surement with X-rays and the trans- configuration is the most sta-
ble [1–8]. Three different conformational forms of the amide group
are possible: trans, cis (planar) and non-planar. The ACOANHA
group adopts a planar ‘peptide-like’ conformation, as in the case
of formamide [9], methyl hydrazinocarboxylate [10], N-methylfor-
mamide [11], o-methyl acetanilide [12] and formanilide [13,14].
Many acetanilide derivatives exhibit fungicidal, herbicidal and
pharmacological activities which further stimulated the recent
interest in their chemistry. Anilide herbicides such as alachlor,
acetochlor, metolachlor, pretilachlor and butachlor are promising
weed control agents for a wide variety of economically important
crops including rice, cotton, potatoes and corns [14–17]. The chol-
oroacetanilide herbicide alachlor is one of the most extensively
used agro chemical [18,19]. Propanil (3,4-dichloropropioanilide)
is a selective contact anilide herbicide used for the control of broad
leveled and grass weeds. Acetanilide is used in medicine under the
name antifebrin, as a febrifuge and it has pain relieving properties
[20]. Acetanilide is a useful intermediate in various reactions of
aniline in which it is desirable to protect the amino group. The 3-
(N-phenylacetylamino)-2,6-piperidinedione, Antineoplaston A10,
is used as antitumour drug [21].
The stereochemistry of the amide bond is an important in bio-
logical and medicinal chemistry. Many experimental and theoreti-
cal studies on the cis–trans stereochemistry of the amide bond
have been made [22–24]. The stereochemistry is critical in deter-
mining the biological activity of the amide compounds. The rela-
tive stability of trans- and cis-amide conformation is affected by
N-methylation, without change of the apparent planar amide
structures. As amides are the simplest model for peptides and also
due to the fungicidal, herbicidal and several pharmacological activ-
ities of many acetanilide derivatives, their exact structure has been
the subject of many experimental and theoretical studies [25–27].
The amide structures of N-methylanilide and three of their
derivatives are cis- in the crystal, whereas those of the correspond-
ing NAH amide compounds (secondary amides) are trans [28,29].
The N-methylanilides with various aliphatic groups at the carbonyl
end, such as isopropyl, cyclopropyl, isopropenyl and t-butyl
adopted the cis- amide structure in the crystal [30]. Thus, it was
proved that cis-amide structure is more stable than the trans-
structure in N-methylacetanilide.
Experimental
A
chloroform solution containing acetyl chloride (1.57 g,
0.02 mol) was added drop wise to a solution of N-methylaniline
(2.14 g, 0.02 mol) and pyridine (2.37 g, 0.03 mol) in chloroform
(20 ml) under stirring on an ice-water bath. The reaction mixture
was stirred at room temperature for 3–5 h. A solid product was sep-
arated from the solution by suction filtration, purified by successive
washing with ordinary water, 0.5 mol/L HCl, 0.5 mol/L NaOH and
distilled water, respectively. The crude sample was allowed to dry
at room temperature. The yield of the product is about 69%. The
product was recrytallised with ethanol and dried in air. The melting
point of N-methylacetanilide is 101 °C. All elemental analysis (C, H,
N) were carried out on a Perkin Elmer 2400 CHN elemental ana-
lyser. The calculated (found)% for C9H11NO: C, 72.46 (72.48); H,
7.43 (7.41) and N, 9.39 (9.32). Scheme 1 depicts the preparation
of the compound under investigation.
The FTIR spectrum of the compound was recorded in the solid
phase by KBr disc method in a Bruker IFS66V spectrometer
equipped with a Globar source, Ge/KBr beam splitter and a TGS
detector in the range of 4000–400 cmꢀ1. The spectral resolution
was 2 cmꢀ1. The FT-Raman spectrum of the compound was also re-
corded with the same instrument with FRA 106 Raman module.
The Raman spectrum was obtained in the wavenumber range
4000–100 cmꢀ1 with a scanning speed of 30 cmꢀ1 minꢀ1. The light
scattering was excited using a low-noise diode pumped Nd:YAG la-
ser source operating at 1.064 lm with 200 mW power. A special
Thus, the stabilities of trans- and cis-amide models for N-methy-
lacetanilide were compared by using DFT studies. The conforma-
tion of secondary amides: derivatives of formanilide, acetanilide
and benzylamides were carried out [12]. The molar Kerr constants
of acetanilide and certain N- and p-substituted acetanilides are
analysed to indicate that (a) the preferred solute conformations
of the molecules XAC6H4ANHACOACH3 (X = AH, ACH3, ACl, or
(enhanced) liquid nitrogen cooled germanium detector was used.
The frequencies of all sharp bands are accurate to 2 cmꢀ1
.
Computational details
Since the amide group is a highly polarisable, the DFT study is
essential for the purpose of getting the structural parameters in
Scheme 1. The correlation diagram for the calculated and the experimental frequencies of cis-N-methylacetanilide.

184
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
precision with the standard 6-31Gꢁꢁ, high level 6-311++Gꢁꢁ and the
triple zeta cc-pVTZ basis sets incorporating both diffuse and polar-
isation functions. Thus, the gradient corrected density functional
theory (DFT) [40] with the three-parameter hybrid functional
Becke3 (B3) [41,42] for the exchange part and the Lee–Yang–Parr
(LYP) correlation functional [43], calculations have been carried
out in the present investigation, using 6-31Gꢁꢁ, 6-311++Gꢁꢁ and
cc-pVTZ basis sets with Gaussian-09 [44] program. The harmonic
vibrational frequency calculations were carried out resulting in IR
and Raman frequencies together with intensities and Raman depo-
larisation ratios. The dipole moment derivatives and polarizability
derivatives were computed analytically. Owing to the complexity
of the molecule, the potential energy distribution of the vibrational
modes of the compound are also calculated using Fuhrer et al. pro-
gram [45].
The NMR spectrum of the N-methylacetanilide molecules in
pyridine and dipole moment in dioxane solutions are in the confor-
mation with the N-methylacetamido group in the exo- or cis- con-
figuration (II) and with the plane of the group ACONRA orthogonal
to the benzene ring.
Isoelectronic molecular electrostatic potential surfaces (MEP)
and electron density surfaces [46] were calculated using 6-
311++G(d,p) basis set. The molecular electrostatic potential
(MEP) at a point ‘r’ in the space around a molecule (in atomic units)
can be expressed as:
In acetanilide the acetamido group is in the endo-configuration
(I), and the plane of the acetamido group and the benzene ring is
making an angle of 38°. But, the stable conformation of N-methy-
lacetanilide is completely different from the conformation of acet-
anilide. It is difficult to understand why the methylation of
acetanilide should completely reverse the relative stability of the
exo- and the endo- isomer. The angle between the acetamido
group and the benzene ring is larger when the acetamido group
is in the exo- configuration than in the endo-configuration. The
van der Waals radius of a carbonyl oxygen is about 1.5 Å, while
the van der Waals radius of a methyl group is 2.0 Å. Furthermore,
the methyl group is closer to the benzene ring as the CAC distance
is 0.3 Å longer than the CAO distance. The increase in the angle
from 38° to 90° must therefore be ascribed to larger van der Waals
interaction between the benzene ring and the exo-acetamido
group than between the benzene ring and the endo-acetamido
group. The carbon atom in the benzene ring closest to the carbonyl
oxygen is forced a little out of the ring plane due to van der Waals
interaction between these two atoms. Similarly, in N-methylace-
tanilide the bond between the benzene ring and the nitrogen atom
is bent about 6°, revealing the existence of strain in the molecule
induced by the van der Waals interaction between the methyl
group and the benzene ring. The non-hydrogen atoms excluding
the phenyl group are almost coplanar. The dihedral angle formed
between this plane and the benzene ring of cis-N-methylacetani-
lide is 85.47° and is well correlated with the 2-chloro-N-methyl-
N-phenylacetamide, 87.07° molecule [54].
Z
X
ZA
q
ðr’ !Þdr’
VðrÞ ¼
ꢀ
jRA ! ꢀr ! j
jr’ ! ꢀr ! j
A
where ZA is the charge on nucleus A, located at RA and
q
(r0) is the
electronic density function for the molecule. The first and second
terms represent the contributions to the potential due to nuclei
and electrons, respectively. V(r) is the resultant at each point r,
which is the net electrostatic effect produced at the point r by both
the electrons and nuclei of the molecule. The molecular electro-
static potential (MEP) serves as a useful quantity to explain hydro-
gen bonding, reactivity and structure–activity relationship of
molecules including biomolecules and drugs [47]. Structures result-
ing from the plot of electron density surface mapped with electro-
static potential surface depict the shape, size, charge density
distribution and the site of chemical reactivity of a molecule. Gauss
View 5.0.8 visualisation program [48] has been utilised to construct
the MEP surface, the shape of highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals.
The B3LYP method allows calculating the shielding constants
with accuracy. The 1H and 13C NMR isotropic shielding were calcu-
lated using the GIAO method [49,50] using the optimised parame-
ters obtained from B3LYP/6-311++G(d,p) method. The effect of
solvent on the theoretical NMR parameters was included using
the PCM model. The isotropic shielding constant values were used
to calculate the isotropic chemical shifts d with respect to tetra-
The conformational analysis was carried out through the poten-
tial surface scan the with B3LYP method using 6-31Gꢁꢁ basis set in
order to ascertain most stable geometry of NMA. During the scan,
methylsilane (TMS). diso(X) =
pic chemical shift and riso – isotropic shielding constant.
r
TMS(X) ꢀ
riso(X), where diso – isotro-
Results and discussion
Conformational analysis
The amide (ACONHA) group is planar, or nearly so [30,36,38,39,
51–53], giving rise to two isomers (cis- and trans-). The planes of
the amide-group and the benzene ring can make an arbitrary angle
with each other and on two main effects: p-electron communica-
tion and steric interactions between the benzene ring and the acet-
amido-group. It was found that the unmethylated acetamido-
groups were preferentially in the conformation with the carbonyl
oxygen (C@O) trans- to NAH, this conformation has been named
endo- or trans- (I). When the acetamido group is methylated, how-
ever, the other conformation, named exo- or cis- (1I) is found to be
the most stable.
Fig. 1. Potential energy surface of N-methylacetanilide.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
185
all the geometrical parameters were simultaneously relaxed while
the dihedral angle O9AC8AN7AC11 is varied in steps of 15°
ranging from 0° to 360°. For O9AC8AN7AC11 rotation a true local
minima of cis-N-methylacetanilide having energy ꢀ479.70168
Hartrees was determined. The potential energy barrier obtained
by the rotation of the amide group with the dihedral angle
O9AC8AN7AC11 is depicted in Fig. 1. The various conformers of
N-methylacetanilide are depicted in Fig. 2a–c. The compound
has two different conformers namely cis- and trans-N-methylace-
tanilide. The cis-N-methylacetanilide conformer is more stable
than the trans-N-methylacetanilide by 2.29 kcal molꢀ1
. The
O9AC8AN7AC11 dihedral angle of the most stable geometry of
cis-N-methylacetanilide determined by B3LYP/6-311++Gꢁꢁ method
is 0.1°. This confirms the existence of the amido group in the cis-
conformation.
Structural properties
The optimised molecular geometry and atom numbering
scheme of the compounds under investigation is shown in Fig. 3.
The optimised structural parameters bond lengths and the bond
angles for the thermodynamically preferred geometry of cis- and
trans-N-methylacetanilide conformers at B3LYP levels with 6-
31Gꢁꢁ, 6-311++Gꢁꢁ and cc-pVTZ basis sets are presented in Table 1.
It is observed that the mean CAC bond distance calculated between
the ring carbon atoms of cis- and trans-N-methylacetanilide by
B3LYP/6-311++Gꢁꢁ method is 1.395 Å, and thus the CAC and CAH
bond lengths are found to be not significantly deviated.
The bond distance of the compounds cis- and trans-N-methy-
lacetanilide determined at the DFT level of theory are in good
agreement with the structural parameters of 2-chloro-N-methyl-
N-phenylacetamide [54] and N-(3-hydroxyphenyl)acetamide [55]
where the mean bond distance of CAC (ring) is found to be
1.3798 Å. The C1AN7 bond distance increases in the order trans-
< cis-N-methylacetanilide. The bond length between the amide
nitrogen and the carbonyl group, N7AC8 of cis-N-methylacetani-
lide is 0.007 Å shorter than the trans- isomer. The angle at
C1AN7AC11of cis- isomer (120.6°) is more than the trans- isomer
(117.4°). This shows the more stability and less steric hindrance in
the cis- isomer. But the angle C8AN7ACl1 in cis- isomer (119.7°) is
less than the trans- isomer (121.6°). The dihedral angles
C1AN7AC8AO9 of cis- and trans- isomers are 179° and 4°,
respectively.
The energies and thermodynamic parameters of the cis- and
trans- conformers have also been computed at B3LYP methods
with 6-31Gꢁꢁ, 6-311++Gꢁꢁ and cc-pVTZ basis sets and are presented
in Table 2. The frequency calculations compute the zero point ener-
gies, thermal correction to internal energy, enthalpy, Gibbs free en-
ergy and entropy as well as the heat capacity for a molecular
system. The calculated SCF energy of the compounds clearly indi-
cates that the cis- isomer (ꢀ479.70168 Hartrees) is more stable
than trans- isomer (ꢀ479.69762 Hartrees). The larger total dipole
moment of cis- isomer (4.24 D) than trans- (3.76 D) molecule leads
to more charge separation.
Fig. 2. (a) cis-Amide conformation of N-methylacetanilide and (b) the plane of the
ring and the plane of amido group in cis-amide are perpendicular to each other and
(c) trans-amide conformation of N-methylacetanilide.
Analysis of molecular electrostatic potential
The molecular electrostatic potential surface (MEP) displays
electrostatic potential (electron + nuclei) distribution, molecular
shape, size and dipole moments of the molecule and it provides a
visual method to understand the relative polarity [56]. The total
electron density mapped with electrostatic potential surface of
cis-N-methylacetanilide constructed by B3LYP/6-311++Gꢁꢁ method
[57,58] is shown in Fig. 4.
Molecular electrostatic potential (MEP) mapping is very useful
in the investigation of the molecular structure with its physio-
Fig. 3. Optmised structure of the most stable cis-N-methylacetanilide.

186
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
Table 1
Structural parameters of the stable Cis-amide and Trans-amide conformations of N-methylacetanilide employing B3LYP method with 6-31Gꢁꢁ, 6-311++Gꢁꢁ and cc-pVTZ basic sets.
Structural parameters
trans-Amide
cis-Amide
Experimentala
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
Internuclear distance (Å)
C1AC2
C1AC6
C1AN7
C2AC3
C2AH12
C3AC4
C3AH13
C4AC5
C4AH14
C5AC6
C5AH15
C6AH16
N7AC8
1.403
1.402
1.431
1.393
1.081
1.396
1.086
1.394
1.086
1.395
1.086
1.085
1.392
1.464
1.223
1.524
1.094
1.089
1.096
1.098
1.090
1.091
1.396
1.396
1.429
1.387
1.077
1.391
1.082
1.388
1.082
1.390
1.082
1.081
1.386
1.462
1.217
1.519
1.090
1.085
1.091
1.094
1.086
1.087
1.399
1.398
1.433
1.391
1.080
1.395
1.084
1.392
1.084
1.394
1.084
1.084
1.388
1.466
1.219
1.521
1.093
1.088
1.094
1.096
1.089
1.089
1.4
1.4
1.393
1.393
1.431
1.39
1.082
1.39
1.082
1.39
1.082
1.39
1.082
1.082
1.378
1.46
1.219
1.517
1.09
1.397
1.397
1.435
1.394
1.084
1.394
1.084
1.394
1.084
1.393
1.084
1.084
1.381
1.464
1.222
1.519
1.092
1.093
1.088
1.094
1.088
1.094
1.384
1.380
1.437
1.379
0.93
1.381
0.93
1.373
0.93
1.382
0.93
0.93
1.345
1.454
1.220
1.509
0.97
1.433
1.395
1.086
1.396
1.086
1.396
1.086
1.395
1.086
1.086
1.384
1.462
1.225
1.521
1.094
1.094
1.089
1.096
1.089
1.096
N7ACl1
C8AO9
C8AC10
C10AH17
C10AH18
C10AH19
C11AH20
C11AH21
C11AC22
1.09
0.97
0.97
0.96
0.96
1.085
1.092
1.085
1.092
0.96
Bond angle (°)
C2AC1AC6
C2AC1AN7
C6AC1AN7
C1AC2AC3
C1AC2AH12
C3AC2AH12
C2AC3AC4
C2AC3AH13
C4AC3AH13
C3AC4AC5
C3AC4AH14
C5AC4AH14
C4AC5AC6
C4AC5AH15
C6AC5AH15
C1AC6AC5
119.1
121.3
119.6
119.9
119.7
120.4
120.9
119.1
120.0
119.2
120.4
120.4
120.3
120.3
119.3
120.5
120.2
119.3
120.9
117.3
121.2
122.7
116.9
120.5
112.6
106.9
111.6
109.3
107.7
108.7
112.9
110.5
108.9
108.3
108.4
107.8
ꢀ0.1
119.1
121.2
119.7
120.0
119.7
120.2
120.8
119.2
120.0
119.2
120.4
120.4
120.3
120.3
119.4
120.5
120.1
119.3
120.8
117.3
121.3
122.7
116.9
120.5
112.4
107.1
111.5
109.3
107.7
108.9
112.8
110.5
108.9
108.3
108.4
107.8
ꢀ0.1
119.3
120.9
119.7
120.0
119.7
120.3
120.7
119.2
120.0
119.3
120.3
120.3
120.3
120.3
119.5
120.4
120.1
119.5
120.4
117.4
121.6
122.5
116.8
120.7
112.3
107.2
111.4
109.3
107.8
108.8
112.7
110.5
108.7
108.4
108.5
107.8
ꢀ0.1
119.6
120.3
120.1
119.1
120.1
120.1
119.1
123.6
117.1
120.7
120.1
119.8
120.1
119.9
120.1
120.1
120.1
120.1
119.8
120.7
119.3
121.8
116.5
110.7
107.6
110.7
121.6
111.6
111.4
107.6
107.2
109.7
109.4
109.7
108.4
109.7
ꢀ0.7
119.5
120.3
120.2
119.2
120.2
120.2
119.2
123.6
116.9
120.6
120.1
119.8
120.1
119.8
120.1
120.1
120.1
120.1
119.8
120.5
119.5
121.9
116.7
110.6
108
119.6
120.3
120.2
119.3
120.1
120.2
119.3
123.5
116.8
120.6
120.1
119.8
120.1
119.8
120.1
120.1
120.1
120.1
119.8
120.6
119.7
121.8
116.7
110.4
108.1
110.6
121.5
111.8
111.1
107.7
107.3
109.7
109.2
109.6
108.6
109.5
ꢀ0.7
120.7
119.9
119.3
119.3
120.4
120.4
120.2
119.9
119.9
120.2
119.9
119.9
122.3
119.8
119.8
119.3
120.4
120.4
123.1
116.6
120.1
123.1
114.8
122.1
109.1
109.1
109.1
107.8
107.8
107.8
109.5
109.5
109.5
109.5
109.5
109.5
C1AC6AH16
C5AC6AH16
C1AN7AC8
C1AN7AC11
C8AN7ACl1
N7AC8AO9
N7AC8AC10
O9AC8AC10
C8AC10AHl7
C8AC10AH18
C8AC10AH19
H17AC10AH18
H17AC10AH19
H18AC10AH19
N7AC11AH20
N7AC11AH21
N7AC11AH22
H20AC11AH21
H20AC11AH22
H21AC11AH22
C6AC1AC2AC3
C6AC1AC2AH12
N7AC1AC2AC3
N7AC1AC2AH12
C2AC1AC6AC5
C2AC1AC6AH16
N7AC1AC6AC5
N7AC1AC6AH16
C2AC1AN7AC8
C2AC1AN7AC11
C6AC1AN7AC8
C6AC1AN7AC11
110.7
121.4
111.7
111.3
107.7
107.2
109.6
109.4
109.6
108.4
109.6
ꢀ0.8
ꢀ179.3
ꢀ178.2
2.6
ꢀ179.4
ꢀ178.4
2.3
ꢀ179.3
ꢀ178.5
2.3
179.4
ꢀ178.7
1.5
179.4
ꢀ178.7
1.4
179.4
ꢀ178.8
1.4
ꢀ0.3
ꢀ0.2
ꢀ0.3
0.6
0.7
0.6
178.9
177.8
ꢀ3.0
179.1
178.2
ꢀ2.5
179.1
178.1
ꢀ2.5
ꢀ179.6
178.6
ꢀ1.7
ꢀ179.6
178.7
ꢀ1.6
ꢀ177.9
178.6
ꢀ1.7
ꢀ43.5
144.6
138.4
ꢀ33.5
ꢀ46.1
141.9
135.6
ꢀ36.5
ꢀ49.9
138.6
131.7
ꢀ39.8
ꢀ88.2
91.6
ꢀ88.7
91.2
ꢀ85.7
92.5
93.9
ꢀ86.3
93.4
ꢀ86.7
96.5
ꢀ86.7

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
187
Table 1 (continued)
Structural parameters
trans-Amide
cis-Amide
Experimentala
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
C1AC2AC3AC4
0.4
0.2
0.4
0.4
0.4
0.4
C1AC2AC3AH13
H12AC2AC3AC4
H12AC2AC3AH13
C2AC3AC4AC5
C2AC3AC4AH14
H13AC3AC4AC5
H13AC3AC4AH14
C3AC4AC5AC6
C3AC4AC5AH15
H14AC4AC5AC6
H14AC4AC5AH15
C4AC5AC6AC1
C4AC5AC6AH16
H15AC5AC6AC1
H15AC5AC6AH16
C1AN7AC8AO9
C1AN7AC8AC10
C11AN7AC8AO9
C11AN7AC8AC10
C1AN7AC11AH20
C1AN7AC11AH21
C1AN7AC11AH22
C8AN7AC11AH20
C8AN7AC11AH21
C8AN7AC11AH22
N7AC8AC10AH17
N7AC8AC10AH18
N7AC8AC10AH19
O9AC8AC10AH17
O9AC8AC10AH18
O9AC8AC10AH19
180.0
179.6
ꢀ0.8
180.0
179.5
ꢀ0.7
179.9
179.6
ꢀ0.7
ꢀ179.7
ꢀ179.7
0.2
ꢀ179.7
ꢀ179.8
0.2
ꢀ179.7
ꢀ179.7
0.2
ꢀ0.3
ꢀ0.1
ꢀ0.2
0.0
0.1
0.0
179.8
ꢀ179.8
0.2
179.9
ꢀ179.8
0.2
179.8
ꢀ179.9
0.1
180.0
ꢀ179.9
0.1
179.9
ꢀ179.9
0.1
179.9
ꢀ179.9
0.1
ꢀ0.2
ꢀ0.2
ꢀ0.2
ꢀ0.1
0.2
0.1
ꢀ179.7
179.8
0.3
ꢀ179.7
179.8
0.3
ꢀ179.7
179.8
0.3
179.9
179.9
0.0
179.9
179.9
0.0
180.0
179.9
0.0
0.5
0.4
0.5
ꢀ0.2
ꢀ0.2
ꢀ0.2
ꢀ178.8
180.0
0.8
ꢀ179.0
179.9
0.6
ꢀ178.9
179.9
0.6
ꢀ179.9
179.7
0.0
ꢀ179.9
179.7
0.0
ꢀ179.9
179.7
0.0
4.9
3.9
4.0
ꢀ179.6
ꢀ179.7
ꢀ179.0
ꢀ175.4
176.5
ꢀ3.8
88.8
ꢀ149.8
ꢀ31.6
ꢀ83.1
38.3
ꢀ176.1
175.7
ꢀ4.4
90.0
ꢀ148.6
ꢀ30.4
ꢀ82.0
39.4
ꢀ176.2
175.2
ꢀ5.0
0.4
0.3
1.7
0.2
0.2
0.1
ꢀ179.7
ꢀ179.8
ꢀ176.0
90.8
59.3
59.0
60.1
ꢀ147.7
ꢀ29.5
ꢀ80.6
40.9
159.1
55.8
ꢀ179.7
ꢀ60.9
ꢀ118.9
0.9
120.8
ꢀ57.0
62.7
ꢀ177.4
122.9
ꢀ177.4
2.6
ꢀ178.9
ꢀ61.2
ꢀ118.7
1.2
121.1
ꢀ57.0
ꢀ62.8
ꢀ177.3
123.0
ꢀ117.2
2.6
ꢀ180.0
ꢀ60.0
ꢀ114.4
5.5
125.3
ꢀ59.6
ꢀ60.1
ꢀ179.8
121.3
ꢀ119.0
1.1
156.5
55.3
157.6
55.9
175.3
ꢀ66.0
ꢀ125.0
ꢀ5.0
176.0
ꢀ65.0
ꢀ124.1
ꢀ4.0
175.8
ꢀ65.2
ꢀ124.4
ꢀ4.4
113.7
115.0
114.6
a
Values taken from Ref. [54,55].
Table 2
The calculated thermodynamic parameters of the stable cis-amide and trans-amide conformations of N-methylacetanilide employing B3LYP method with 6-31Gꢁꢁ, cc-pVTZ and 6-
311++Gꢁꢁ basis sets.
Thermodynamic parameters (298 K)
trans-Amide
cis-Amide
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
B3LYP/6-31Gꢁꢁ
B3LYP/cc-pVTZ
B3LYP/6-311++Gꢁꢁ
SCF energy, (Hartrees)
ꢀ479.58091
121.68
119.90
115.20
31.10
ꢀ479.73854
121.18
119.40
114.67
39.31
ꢀ479.69762
120.94
119.17
114.38
39.12
ꢀ479.58465
121.52
119.74
114.72
39.34
ꢀ479.74234
121.05
119.27
114.26
39.30
ꢀ479.70168
120.83
119.06
113.98
39.47
Total energy (thermal), Etotal (kcal molꢀ1
)
Vibrational Energy, Evib (kcal molꢀ1
)
Zero point vibrational energy (kcal molꢀ1
Heat capacity, Cv (cal molꢀ1 Kꢀ1
Entropy, S (cal molꢀ1 Kꢀ1
)
)
)
99.63
100.53
99.99
34.52
105.71
107.95
Rotational constants (GHz)
X
Y
Z
2.44
0.75
0.61
2.46
0.75
0.61
2.44
0.74
0.61
2.29
0.71
0.67
2.30
0.71
0.68
2.29
0.71
0.67
Dipole moment (Debye)
lx
ly
lz
ltotal
ꢀ0.73
3.28
1.12
3.54
ꢀ0.71
3.30
1.20
3.58
ꢀ0.69
3.93
0.84
0.0
4.01
0.09
0.0
4.24
0.10
0.0
4.24
3.45
1.34
3.76
3.93
4.01
ELUMO+1 (eV)
ELUMO (eV)
EHOMO (eV)
EHOMOꢀ1 (eV)
ELUMOꢀHOMO (eV)
ꢀ0.4933
ꢀ0.6735
ꢀ6.3398
ꢀ6.9251
5.6663
ꢀ0.9143
ꢀ0.9881
ꢀ6.8633
ꢀ6.9033
5.8753
chemical property relationships [57–59]. The electrostatic poten-
tial contour map and the surface for positive and negative poten-
tials are shown in Figs. 5 and 6, respectively.
charge; light blue, slightly electron deficient region; yellow, slightly
electron rich region; green, neutral; respectively. It is obviously from
the Figs. 4 and 5 that the region around oxygen atoms linked with
carbon through double bond represents the most negative potential
region (red). The hydrogen atoms attached to the ends of methyl
group posses the maximum positive charge. The predominance of
green region in the ring surfaces corresponds to a potential halfway
between the two extremes red and dark blue colour. Thus, the total
The colour scheme for the MESP surface is red1, electron rich,
partially negative charge; blue, electron deficient, partially positive
1
For interpretation of colour in Figs. 4–6, the reader is referred to the web version
of this article.

188
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
Fig. 4. Total electron density mapped with electrostatic potential surface of cis-N-
methylacetanilide.
Fig. 6. Electrostatic potential surface of cis-N-methylacetanilide.
electrical transport properties [61]. The frontier orbital energy
gap (EHOMO ꢀ ELUMO) in case of cis- isomer is found to be 5.8753 eV.
Natural bond orbital (NBO) analysis
Fig. 5. Electrostatic potential contour surface of cis-N-methylacetanilide.
The atomic charges of cis-N-methylacetanilide calculated by
NBO analysis using the B3LYP 6-311++Gꢁꢁ method are presented
in Table 3. Among the ring carbon atoms C1 has a positive charge
while others have negative charge. The positive charge of C1 is due
to the attachment of highly electronegative nitrogen atom (N7) to
it. The negative charge of N7 is also due to the electron donating
nature of the methyl groups by means of hyper conjugative effect.
The very high positive charge on the amide carbon C8 is due to the
partial polar nature of C@O group. This also leads to a high negative
charge on the oxygen atom O9. The methyl group carbon atoms
C10 and C11 have high negative charge because of no bond
resonance.
The NBO method demonstrates the bonding concepts like atom-
ic charge, Lewis structure, bond type, hybridisation, bond order,
charge transfer and resonance possibility. Table 4 depicts the
bonding concepts such as type of bond orbital, their occupancies,
the natural atomic hybrids of which the NBO is composed, giving
the percentage of the NBO on each hybrid, the atom label, and a hy-
brid label showing the hybrid orbital (spx) composition (the
amount of s-character, p-character, etc.) of cis-N-methylacetanilide
determined by B3LYP/6-311++Gꢁꢁ method. The occupancies of
NBOs reflecting their exquisite dependence on the chemical envi-
ronment. The NBO energy values show the corresponding spatial
electron density surface mapped with electrostatic potential clearly
reveals the presence of high negative charge on the carbonyl oxygen
and in part of the ring while more positive charge around the ACH3
region.
Frontier molecular orbitals
Highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) are very important parameters
for quantum chemistry [60]. The energies of HOMO LUMO,
LUMO+1 and HOMO-1 and their orbital energy gaps are calculated
using B3LYP/6-311++Gꢁꢁ method and the pictorial illustration of
the frontier molecular orbitals are shown in Fig. 7. Molecular orbi-
tals provide insight into the nature of reactivity, conjugation, aro-
maticity, lone pairs and some of the structural and physical
properties of molecules. The region of HOMO spread over the en-
tire molecule while in the LUMO the ring part has more overlap-
ping. The frontier molecular orbital energies of trans- and cis-N-
methylacetanilide are presented in Table 2. The energy gap be-
tween HOMO and LUMO is a critical parameter in determining

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
189
Table 4
Bond orbital analysis of N-(methylacetanilide) by B3LYP/6-311++Gꢁꢁ method.
Bond
orbital
Occupancy Atom cis-N-methylacetanilide
Contribution from Atomic hybrid
parent NBO (%)
contributions (%)
C1@C2
C1@C2
C1AC6
C1AN7
C2AC3
C3AC4
C3@C4
C4AC5
C5AC6
C5@C6
N7AC8
1.9731
1.6668
1.9732
1.9832
1.9768
1.9794
1.6588
1.9794
1.9768
1.6593
1.9849
C1
C2
C1
C2
C1
C6
C1
N7
C2
C3
C3
C4
C3
C4
C4
C5
C5
C6
C5
C6
N7
C8
N7
C11
C8
O9
C8
O9
C8
C10
52.37
49.63
51.56
48.44
50.36
49.64
38.12
61.88
50.35
49.65
50.09
49.91
49.94
50.06
49.91
50.09
49.65
50.35
50.50
49.95
63.94
36.06
63.67
36.33
35.34
64.66
29.08
70.92
49.27
50.73
s(36.47) + p1.74(63.49)
s(35.76) + p1.80(64.20)
s(0.00) + p1.00(99.96)
s(0.00) + p1.00(99.95)
s(36.43) + p1.74(63.53)
s(35.75) + p1.80(64.21)
s(26.97) + p2.70(72.93)
s(33.90) + p1.95(66.06)
s(35.90) + p1.78(64.06)
s(35.67) + p1.80(64.28)
s(35.87) + p1.79(64.09)
s(35.69) + p1.80(64.27)
s(0.00) + p1.00(99.96)
s(0.00) + p1.00(99.96)
s(35.68) + p1.80(64.27)
s(36.85) + p1.79(64.10)
s(35.68) + p1.80(64.27)
s(35.90) + p1.78(64.06)
s(0.00) + p1.00(99.95)
s(0.00) + p1.00(99.96)
s(35.41) + p1.82(64.55)
s(30.88) + p2.24(69.02)
s(30.67) + p2.26(69.30)
s(23.57) + p 3.24(76.30)
s(32.76) + p2.05(67.07)
s(41.09) + p1.43(58.80)
s(0.00) + p1.00 (99.49)
s(0.00) + p1.00(99.88)
s(36.32) + p1.75(63.64)
s(27.36) + p2.65(72.57)
N7AC11 1.9806
C8AO9
C8@O9
1.9930
1.9896
C8AC10 1.9832
Table 5
Second order perturbation theory analysis of Fock matrix of cis-N-methylacetanilide
using NBO analysis using B3LYP/6-311++Gꢁꢁ method.
Donor (i)–acceptor (j)
interaction
E(2)a
E(j)–E(i)b
(a.u.)
F(i,j)e
(a.u.)
(kJ molꢀ1
)
cis-N-methylacetanilide
p
p
r
r
p
p
r
p
p
r
r
r
r
r
(C1AC2) ? pꢁ (C3AC4)
(C1AC2) ? pꢁ (C5 AC6)
(C2AC3) ? rꢁ (C1AN7)
(C2AH12) ? rꢁ (C1AC6)
(C3AC4) ? pꢁ (C1AC2)
(C3AC4) ? rꢁ (C5AC6)
(C5AC6) ? rꢁ (C1AN7)
(C5AC6) ? pꢁ (C1AC2)
(C5AC6) ? rꢁ (C3AC4)
(C6AH16) ? rꢁ (C1AC2)
(C8AC10) ? rꢁ (N7AC11)
(C10AH18) ? pꢁ (C8AO9)
(C10AH19) ? rꢁ (N7AC8)
(C11AH21) ? rꢁ (C1AN7)
19.87
20.49
4.28
0.29
0.29
1.11
1.10
0.28
0.28
1.11
0.28
0.28
1.10
0.97
0.54
0.96
0.91
0.82
0.82
0.28
0.65
0.65
0.70
0.63
0.068
0.069
0.062
0.064
0.069
0.068
0.062
0.070
0.068
0.064
0.060
0.047
0.060
0.055
0.069
0.069
0.0117
0.055
0.056
0.121
0.099
Fig. 7. Frontier molecular orbitals of cis-N-methylacetanilide.
4.66
20.97
20.51
4.26
21.59
20.40
4.65
4.63
4.54
4.49
4.19
6.37
6.33
61.53
4.99
5.24
Table 3
Atomic charges of the stable cis-amide and trans-amide conformations of N-methy-
lacetanilide by NBO analysis using B3LYP/6-311++Gꢁꢁ method.
Atom
trans-Amide
cis-Amide
C1
C2
C3
C4
C5
C6
N7
C8
0.1689
ꢀ0.1905
ꢀ0.1927
ꢀ0.2109
ꢀ0.1959
ꢀ0.2287
ꢀ0.5162
0.6940
0.1560
ꢀ0.2041
ꢀ0.1941
ꢀ0.1982
ꢀ0.1941
ꢀ0.2041
ꢀ0.5326
0.6950
n (LP(1)N7) ? rꢁ (C1AC2)
n (LP(1)N7) ? rꢁ (C1AC6)
n (LP(1)N7) ? pꢁ (C8@O9)
n (LP(1)N7) ? rꢁ (C11AH20)
n (LP(1)N7) ? rꢁ (C11AH22)
n (LP(2)O9) ? rꢁ (N7AC8)
n (LP(2)O9) ? rꢁ (C8AC10)
O9
ꢀ0.6160
ꢀ0.6650
ꢀ0.3581
0.2268
ꢀ0.6358
ꢀ0.6625
ꢀ0.3535
0.2135
25.55
18.66
C10
C11
H12
H13
H14
H15
H16
H17
H18
H19
H20
H21
H22
LP – lone pair.
0.2058
0.2050
0.2043
0.2062
0.2140
0.2342
0.2162
0.1852
0.2077
0.2074
0.2077
0.2136
0.2195
0.2201
0.2291
0.1870
a
Stabilisation (delocalisation) energy.
Energy difference between i (donor) and j (acceptor) NBO orbitals.
Fock matrix element i and j NBO orbitals.
b
e
symmetry breaking in the direction of unpaired spin. The Lewis
structure that is closest to the optimised structure is determined.
For example, the bonding orbital for N7AC8 with 1.9849 elec-
trons has 63.94% N7 character in a sp1.82 hybrid and has 36.06%
C8 character in a sp2.24 hybrid orbital. In the case of C8@O9 bond-
0.2032
0.2106
0.2351
0.1870

190
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
ing orbital with 1.9856 electrons has 29.08% C8 character in a sp1.0
hybrid and has 70.92% O9 character in a sp1.0 hybrid orbital. A
bonding orbital for N7AC11 with 1.9806 electrons has 63.67% N7
character in a sp2.26 hybrid and has 36.33% C11 character in a
sp3.24 orbital. The C1AN7 with 1.9832 electrons has 38.12% C1
character in a sp2.70 hybrid and has 61.88% N7 character in a
sp1.95 orbital. The CAC bonds of the aromatic ring posses more p
character than s character. This is clearly indicates the delocalisa-
sation is estimated from the second-order perturbation approach
[62] as given below
F2ði; jÞ
Eð2Þ ¼ q
i ej
ꢀ
ei
where qi is the donor orbital occupancy, ei and ej are diagonal ele-
ments (orbital energies) and F(i,j) is the off-diagonal Fock matrix
element.
tion of
p electrons among all the carbon atoms. Similarly C1AN7
bond has also posses more p character.
The Fock matrix analysis yield different types of donor–acceptor
interactions and their stabilisation energy. All lone pair-bond pair
interactions and bond pair–bond pair interactions with more than
4 kJ molꢀ1 stabilisation energies are listed in Table 5. In cis- isomer
Donor–acceptor interactions: Perturbation theory energy analysis
Strong electron delocalisation in the Lewis structure also shows
up as donor–acceptor interactions. This bonding–anti bonding
interaction can be quantitatively described in terms of the NBO ap-
proach that is expressed by means of second-order perturbation
interaction energy E(2) [58–61]. This energy represents the
estimate of the off-diagonal NBO Fock matrix element. The stabili-
sation energy E(2) associated with i(donor) ? j(acceptor) delocali-
of the molecule, the lone pair donor orbital, nN
between the nitrogen lone pair and the C8AO9 antibonding orbital
is seen to give a strong stabilisation, 61.53 kJ/mol. The nO
?
pꢁCO interaction
?
rꢁCN
stabilisation energy of lone pair of electrons present in the oxygen
atom (O9) to the antibonding orbital (rꢁ) of N7AC8 is equal to
25.55 kJ/mol. The bond pair donor orbital, pCC
between the C3AC4 bond pair and the C1AC2 antibonding orbital
?
pꢁCC interaction
Fig. 8. (a) The 1H NMR (b) 13C NMR spectra of cis-N-methylacetanilide.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
191
Table 6
The experimental and calculated 1H and 13C isotropic chemical shifts (diso, ppm) with respect to TMS and isotropic magnetic shielding constants (
r
iso) of cis-N-methylacetanilide.
Assignment
riso
(
13C)
Cal. (diso
)
Expt. (diso
)
Assignment
riso (1H)
Cal. (diso
)
Expt. (diso)
C1
C2
C3
C4
C5
C6
C8
C10
C11
31.31
48.82
48.61
50.14
48.40
48.79
8.67
153.22
135.71
135.92
134.39
136.13
135.74
175.86
25.21
144.59
127.03
129.70
127.65
129.70
127.03
170.32
22.37
H12
H13
H14
H15
H16
H17
H18
H19
H20
H21
H22
24.50
24.25
24.34
24.29
24.51
30.37
30.27
30.49
29.46
27.17
29.45
7.47
7.72
7.62
7.68
7.46
1.60
1.70
1.48
2.51
4.79
2.52
7.21
7.39
7.38
7.39
7.21
1.88
1.88
1.88
3.27
3.27
3.27
159.32
146.71
37.82
37.08
Fig. 10. Correlation of (a) 1H and (b) 13C experimental NMR chemical shift and
shielding constant of cis-N-methylacetanilide.
Fig. 9. Correlation of (a) 1H and (b) 13C experimental and theoretical NMR chemical
shift of N-methylacetanilide.
65]. The observed experimental chemical shift positions of ring
carbons atoms lie in the range 144.59–127.03 ppm. The ꢀI effect
of nitrogen (N7) reduces the electron density of the carbon atom
C1, thus its NMR signal is observed in the very downfield at
144.59 ppm. Due to the symmetry of the phenyl ring the carbon
atoms C2, C6 and C3, C5 are attributed to the downfield NMR sig-
nals 127.03 and 129.70 ppm, respectively. The carbonyl carbon
atom C8 is significantly observed in the downfield with chemical
shift value 170.32 ppm reveals the presence of partially positive
charge on the carbon atom. Comparing the chemical shift positions
of ring carbon atoms with that of methyl carbon atoms, C10 and
C11, the upfield chemical shift 22.37 and 37.08 ppm, respectively
give more stabilisation of 20.97 kJ molꢀ1 while between the C3AC4
bond pair and the C5AC6 antibonding orbital has 20.51 kJ molꢀ1
.
NMR spectral investigations
The observed 1H and 13C NMR spectra of the compound are gi-
ven in the Fig. 8a and b, respectively. The 1H and 13C theoretical
and experimental chemical shifts, isotropic shielding constants
and the assignments are presented in Table 6. Aromatic carbons
give signals with chemical shift values from 100 to 200 ppm [63–

192
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
1324 cmꢀ1 as strong to weak bands and at 1600 and 1586 cmꢀ1
in Raman. Similarly the very weak and medium intensity lines ob-
served in the infrared spectrum at 995 and 972 cmꢀ1 and the cor-
responding band in Raman spectrum is seen at 975 cmꢀ1 are
ascribed to the CACH3 and NACH3 stretching modes. In the present
work the bands occurring at 1007 and 720 cmꢀ1 in Raman spec-
trum are assigned to the CCC in-plane trigonal bending and ring
breathing vibrations, respectively. These frequencies appear in
the respective range [37–39]. The CCC in-plane bending and out
of plane vibrations are described as mixed modes as there are
about 12–24% PED contributions mainly from CAH in-plane bend-
ing and out of plane bending vibrations, respectively. The CAN in-
plane and out of plane bending modes also coupled to some extent
with that of C@O bending vibrations.
CAH vibrations
The CAH bond present in the phenyl group of the compound
gives medium and very weak bands at 3047, 3029 and
3012 cmꢀ1 in IR spectrum is attributed to the CAH stretching
vibrations of the phenyl group. The PED contribution of the aro-
matic CAH stretching modes indicates that these are also highly
pure modes as carbon–carbon stretching. The aromatic CAH in-
plane bending modes are observed at 1162, 1143 and 1086 cmꢀ1
in the IR spectrum. The CAH out of plane bending modes of the
present compound are observed in the said region [37–39] and
presented in the Tables 6 and 7. Since the alkyl CAH in-plane bend-
ing modes are observed in the same region of CAC stretching, sig-
nificant overlapping occurs. The aromatic CAH in-plane and out of
plane bending vibrations have substantial contribution from the
ring CCC in-plane and out of plane bending, respectively.
Fig. 11. (a) FTIR (b) FT-Raman spectra of cis-N-methylacetanilide.
is due to the electron donating effect of methyl groups. The peak
observed at 77 ppm belongs to the solvent CDCl3 peak.
Amide group vibrations
1H chemical shifts were obtained by complete analysis of the
NMR spectrum to quantify the possible different effects acting on
the shielding constant of protons. The hydrogen atoms present in
the benzene ring shows NMR peaks in the narrow range 7.21–
7.39 ppm. The C-methyl hydrogen atoms are observed in the up-
field at 1.88 ppm while the N-methyl hydrogen atoms shows the
peaks at 3.27 ppm shows that these protons are under high mag-
netic shielding. The calculated and experimental chemical shift
values are given in Table 6 shows a good agreement with each
other. The linear regression between the experimental and theo-
retical 1H and 13C NMR Chemical shifts are represented in Fig. 9.
The correlations of the experimental chemical shift with that of
the shielding constants are illustrated in Fig. 10.
The very strong doublet in IR spectrum observed at 1667/
1657 cmꢀ1 are assigned to the amide-I, the C@O stretching band
of the molecule. The NCA shows that the amide-I, band is to be
pure even though it has slightly mixed with the amide-III, CAN
stretching mode by about 10–15%. The CAN stretching mode, the
amide-III band is assigned to the wave number observed at 1352
and 1356 cmꢀ1 in the infrared and Raman spectra, respectively.
The amide-IV, C@O in-plane bending is found at 770 cmꢀ1 in the
IR spectrum. The amide-VI, C@O out of plane bending is observed
at 560 cmꢀ1 in IR.
Methyl group (ACH3) vibrations
The symmetric stretching,
theoretically as 2895 and 2871 cmꢀ1 by B3LYP/6-311++Gꢁꢁ meth-
od. The antisymmetric stretching a(CH3) are assigned to the wave-
ms(CH3) frequencies are determined
Vibrational analysis
m
The FTIR and FT-Raman spectra of cis-N-methylacetanilide are
shown in Fig. 11a and b. All the observed wavenumbers are as-
signed in terms of fundamentals, overtones and combination
bands. The observed and calculated frequencies along with their
relative intensities, probable assignments and potential energy dis-
tributions (PED) are presented in Table 7. The geometry of the mol-
ecule is considered by possessing C1 point group symmetry. The 60
fundamental vibrations of each N-methylacetanilide are distrib-
uted into the irreducible representations under C1 symmetry as
40 in-plane and 20 out of plane vibrations of ‘A’ species. All the
vibrations are active in both IR and Raman.
numbers 2969 and 2937 cmꢀ1. The methyl symmetric deformation
modes are assigned to the frequencies at 1421 and 1388 cmꢀ1 in IR
spectrum. The corresponding asymmetric deformation vibration is
ascribed to the band at 1453 cmꢀ1 in the IR spectrum. The presence
of the bands at 1075 and 1029 cmꢀ1 in infrared spectrum is as-
signed to the methyl rocking fundamentals. The calculated band
at 1274 and 1266 are assigned to the methyl wagging mode of
the compound. The assignments are agreed well with reported lit-
erature values [37–39]. All other observed fundamental modes are
given in Table 7.
Carbon–carbon vibrations
Scale factors
The aromatic carbon–carbon stretching vibrations of the mole-
cule are found in the IR spectrum at 1599, 1584, 1496 and
Computed harmonic frequencies typically overestimate vibra-
tional fundamentals due to basis set truncation and neglect of elec-

Table 7
The observed FTIR, FT-Raman and calculated frequencies determined by B3LYP method with 6-31Gꢁꢁ, 6-311++Gꢁꢁ and cc-pVTZ basis sets along with their relative intensities, probable assignments and potential energy distribution (PED)
of cis-N-methylacetanilide.a
Observed wavenumber (cmꢀ1
)
B3LYP/6-311++Gꢁꢁ calculated wavenumber (cmꢀ1
)
B3LYP/6-31Gꢁꢁ calculated wavenumber (cmꢀ1
)
B3LYP/cc-pVTZ calculated wavenumber (cmꢀ1
)
Assignment %PED
FTIR
FT-Raman
3058 m
Unscaled Scaled IR intensity Raman Activity Unscaled
Scaled
IR intensity
Unscaled
Scaled
IR intensity
3047 m
3029 vw
3193
3185
3179
3171
3165
3151
3151
3106
3073
3048
3022
1725
1637
1621
1527
1510
1501
1484
1477
1471
1440
1401
1378
1346
1314
1305
1193
1182
1164
1147
1101
1095
1051
1045
1019
1005
988
3034
3026
3020
3012
3006
2993
2993
2951
2919
2895
2871
1673
1588
1573
1481
1465
1456
1440
1433
1427
1397
1359
1336
1305
1274
1266
1157
1146
1129
1112
1068
1062
1020
1014
989
8.14
335.16
7.72
3211
3203
3198
3189
3182
3177
3176
3129
3092
3064
3037
1772
1655
1638
1540
1524
1514
1497
1489
1483
1445
1412
1387
1353
1326
1318
1197
1187
1171
1155
1108
1101
1052
1051
1019
1002
984
3035
3027
3022
3014
3007
3002
3002
2957
2922
2896
2870
1675
1588
1573
1479
1463
1453
1437
1429
1423
1387
1356
1331
1299
1273
1265
1149
1140
1124
1109
1063
1057
1010
1009
978
10.09
21.49
15.36
1.68
1.44
11.55
1.04
7.32
26.70
5.36
50.61
349.27
26.23
3.46
61.41
6.26
11.07
13.39
0.38
11.04
39.20
128.08
82.81
0.70
1.36
75.52
1.62
0.02
71.74
1.14
3195
3187
3181
3173
3166
3152
3151
3106
3069
3050
3023
1735
1640
1625
1534
1513
1503
1487
1483
1472
1442
1402
1378
1352
1314
1305
1197
1185
1167
1153
1102
1099
1054
1049
1025
1013
994
3035
3028
3022
3014
3008
2994
2993
2951
2916
2898
2872
1683
1591
1576
1488
1468
1458
1442
1439
1428
1399
1360
1337
1311
1275
1266
1161
1149
1132
1118
1069
1066
1022
1018
994
8.99
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
CH
CH
CH
CH
90mNH
93mCH
91mCH
94mCH
90mCH
92mCH
93mCH
91mCH
92mCH
95mCH
90mCH
95mC@O
89mCC
85mCC
86mCC
84mCC
87dCH3
85dCH3
86mCC
89dCH3
90dCH3
87mCN
85mCC
83mCC
16.77
11.69
1.20
1.53
2.80
18.25
12.20
1.07
1.62
4.69
73.57
113.90
20.98
78.74
67.82
29.90
65.91
133.54
152.73
16.16
29.71
8.92
2.49
10.31
8.03
4.60
1.67
5.93
5.93
3.01
18.21
0.55
0.59
23.23
3.30
2.68
6.67
3012 vw
CH
aCH3
aCH3
aCH3
aCH3
sCH3
sCH3
C@O
CC
9.85
6.80
8.05
6.88
2969 vw
2937 w
2937 w
25.47
5.75
48.32
442.18
40.73
3.05
66.52
6.90
13.93
15.97
0.27
17.09
25.31
114.03
117.08
0.39
1.78
72.07
1.30
0.07
72.89
1.23
27.52
6.76
3.67
12.30
0.03
0.002
0.07
24.83
5.71
47.75
377.25
33.88
3.56
60.96
9.12
11.36
12.33
2.16
14.86
25.77
102.71
115.23
0.39
1.69
74.20
1.12
0.04
69.45
1.38
26.26
6.00
3.34
8.34
0.06
0.01
0.03
1667/1657 vs
1599 s
1584 m
1600 m
1586 w
CC
CC
CC
1496 s
1453 m
daCH3
daCH3
mCC
dsCH3
dsCH3
m
m
m
1421 s
1388 s
1352 m
1324 w
1299 m
1356 w
1301 m
Cm
CC
CAph
x
x
CH3
CH3
60
62
x
x
CH3 + 22sCH3
CH3 + 18sCH3
1162 m
1143 m
1164 w
1155 w
bCH
bCH
bCH
bCH
bCH
61bCH + 21bCCC
70bCH + 14bCCC
67bCH + 16bCCC
65bCH + 15bCC
60bCH + 22bCC
1.30
4.16
0.32
0.13
11.93
37.45
0.08
1086 vw
1075 w
1029 m
1002 vw
995 vw
972 m
1087 w
24.42
4.87
4.62
7.95
0.91
0.01
22.80
0.84
q
q
CH3
CH3
62
q
CH3 + 18bCN
CH3 + 26bCN
1031 vw
1007 vs
60q
bCCC
57bCCC + 22bCH
59bCC + 18bCO
m
m
c
c
c
c
c
CACH3
NACH3
CH
CH
CH
975 m
924 m
975
958
962
944
937
902
983
964
950
918
68m
60c
56c
52c
48c
54c
NC + 14bCCC
CH + 18cCCC
CH + 24cCCC
CH + 28cCCC
CH + 26cCCC
CH + 20cCCC
0.04
945 w
924 w
979
938
949
910
27.55
1.44
3.76
1.65
976
939
979
946
27.45
0.49
3.22
929
901
7.10
5.30
932
895
4.46
931
903
7.68
CH
CH
854
828
0.02
0.87
859
824
0.03
861
835
0.03
777 m
717 m
706 s
790
722
714
634
766
700
692
615
13.81
1.65
49.02
0.24
0.84
13.85
0.19
790
724
716
634
759
695
687
608
11.03
2.35
28.12
0.25
797
724
720
636
773
702
698
617
10.80
5.92
38.33
0.15
bC@O
bCCC
bCCC
bCCC
67bC=O + 18bNH
53bCCC + 21bCH
57bCCC + 18bCH
55bCCC + 22bCH
720 s
626 w
623 m
4.27
631
612
6.22
4.42
632
606
6.90
632
613
6.31
c
CCC
54cCCC + 18cCH
592 w
560 s
594
567
453
576
550
439
4.64
28.50
5.13
0.26
0.69
1.45
588
568
454
564
545
435
5.59
21.40
6.84
595
572
454
577
555
440
4.25
27.22
5.87
bCN
55bCN + 20bCC
c
c
C@O
CN
60c
C=O + 21cNC
CN + 16cC=O
65c
422
421
401
410
408
389
0.16
1.63
5.78
0.51
0.10
5.82
425
418
402
408
401
386
0.03
2.40
5.65
426
424
403
413
411
391
0.14
1.51
5.59
bCAph
bCACH3
bCACH3
56bCC + 24bCH
52bCC + 20bCN
51bCC + 19bCO
403 vw
(continued on next page)

194
V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
tron correlation and mechanical anharmonicity [56]. To compen-
sate these shortcomings and to correlate the experimentally ob-
served and theoretically computed frequencies for each
vibrational modes of the compound under B3LYP methods, scale
factors are introduced. A better agreement between the computed
and experimental frequencies can be obtained by using different
scale factors for different types of fundamental vibrations. To
determine the scale factors, the procedure used previously [57–
74] have been followed that minimises the residual separating
experimental and theoretically predicted vibrational frequencies.
Fig. 12. The correlation diagram for the calculated and the experimental frequen-
cies of cis-N-methylacetanilide.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 182–196
195
The optimum scale factors for vibrational frequencies were deter-
mined by minimising the residual
(iv) The conformational state of the molecule depends on several
intramolecular factors: resonance effect between the amide
group and the aromatic ring, steric interaction between var-
ious substituents around the ANHACOA grouping in the
aromatic ring, conjugation between the carbonyl bond and
the nitrogen lone pair as well as direct field influences inside
the amide group.
(v) The non-hydrogen atoms excluding the phenyl group are
almost coplanar. The dihedral angle formed between this
plane and the benzene ring is 87.07°.
(vi) The C1AN7 bond distance increases in the order trans- < cis-
N-methylacetanilide. The bond length between the amide
nitrogen and the carbonyl group, N7AC8 of cis-N-methylace-
tanilide is 0.007 Å shorter than the trans- isomer.
(vii) The angle at C1AN7AC11of cis- isomer (120.6°) is more than
the trans- isomer (117.4°). This shows the more stability and
less steric hindrance in the cis- isomer.
N
ꢀ
ꢁ
X
2
D
¼
k
xTi heor
ꢀ
mEi xpt
i
where xiTheo and miExpt are the ith theoretical harmonic frequency
and ith experimental fundamental frequency (in cmꢀ1), respectively
and N is the number of frequencies included in the optimisation
which leads to
rffiffiffiffi
D
RMS ¼
N
The scale factors used in this study minimised the deviations be-
tween the computed and experimental frequencies. A uniform scal-
ing factor is recommended for all frequencies <1800 cmꢀ1 at the
B3LYP method with 6-31Gꢁꢁ/6-311++Gꢁꢁ basis sets and is adopted
in this study. Due to the large anharmonicities of CAH and NAH
stretching frequencies >2700 cmꢀ1 were scaled by two different
scale factors [72,73].
Initially, all scaling factors have been kept fixed at a value of 1.0
to produce the pure DFT calculated vibrational frequencies (un-
scaled) which are given in Table 7. Subsequently, the scale factors
0.95 and 0.945 for CAH and CH3 vibrations determined with 6-
311++Gꢁꢁ and 6-31Gꢁꢁ basis sets. The correction factors 0.945 for
CAH/CH3 vibrations with cc-pVTZ basis set. For all other vibrations
0.97 and 0.96 were used to produce the scaled wavenumber. The
resultant scaled frequencies are also listed in Table 7. These are
much closer to unity and thus the vibrational frequencies calcu-
lated by using the B3LYP functional with 6-311++Gꢁꢁ basis set
can be utilised to eliminate the uncertainties in the fundamental
assignments in infrared and Raman vibrational spectra. The corre-
lation diagram for the calculated and the experimental frequencies
are shown in Fig. 12.
(viii) The total electron density surface mapped with electrostatic
potential clearly reveals the presence of high negative
charge on the carbonyl oxygen and in part of the ring while
more positive charge around the ACH3 region.
(ix) The NBO analysis confirms the presence of resonance struc-
tures and reveals the exact s and p character of each bond.
The lone pair donor orbital, nN
?
pꢁCO interaction between
the nitrogen lone pair and C8@O9 antibonding orbital is seen
to give a strong stabilisation, 61.53 kJ/mol.
(x) The frontier orbital energy gap (EHOMO ꢀ ELUMO) in case of
cis- isomer is found to be 5.8753 eV. The nO
?
rꢁNC stabilisa-
tion energy of lone pair of electrons present in the oxygen
atom (O9) to the antibonding orbital (rꢁ) of N7AC8 is equal
to 25.55 kJ/mol.
(xi) The carbonyl carbon atom C8 is significantly observed in the
downfield with chemical shift value 170.32 ppm reveals the
presence of partially positive charge on the carbon atom. The
ꢀI effect of nitrogen (N7) reduces the electron density of the
carbon atom C1, thus its NMR signal is observed in the very
downfield at 144.59 ppm.
Conclusions
The structural parameters, thermodynamic properties and
vibrational frequencies of the optimised geometry of cis and trans-
amide form of N-methylacetanilide have been determined from
DFT-B3LYP methods. The effects of substituents (methyl and phe-
nyl group) on the amide vibrational frequencies were analysed.
1H and 13C NMR isotropic chemical shifts were calculated and
the assignments made are compared with the experimental values.
The energies of important MO’s of the compound were also deter-
mined from TD-DFT method. The total electron density and elec-
trostatic potential of the compound were determined by natural
bond orbital analysis.
(xii) The very strong doublet in IR spectrum observed at 1667/
1657 cmꢀ1 are assigned to the amide-I, the C@O stretching
band of the molecule.
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