Vibrational spectra, structural conformations, scaled quantum chemical calculations and NBO analysis of 3-acetyl-7-methoxycoumarin

Article abstract of DOI:10.1016/j.saa.2012.07.084

The powder form NIR-FT Raman and FT-IR spectra of 3-acetyl-7- methoxycoumarin (3A7MC) have been recorded in the regions 4000-400 and 3500-100 cm^-1, respectively. The equilibrium geometry, vibrational frequencies, band intensities, NMR spectra, NBO analysis and UV-Vis spectral studies of the most stable conformer have been calculated by density functional B3LYP method with the 6-311G(d,p) basis set. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra, the calculated wavenumber and intensity of the vibrational bands and the potential energy distribution over the internal coordinates. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping the electron density isosurface with electrostatic potential surfaces (ESP). Natural bond orbital analysis has been carried out to understand the nature of different interactions responsible for the electron delocalization and the intramolecular charge transfer between the orbitals (n → π, n → σ, π → π).

Full text of DOI:10.1016/j.saa.2012.07.084

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Vibrational spectra, structural conformations, scaled quantum chemical
calculations and NBO analysis of 3-acetyl-7-methoxycoumarin
a
a,
⇑
c
a
b
c
Lynnette Joseph , D. Sajan , R. Reshmy , B.S. Arun Sasi , Y. Erdogdu , K. Kurien Thomas
a
Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India
Department of Physics, Ahi Evran University, 40040 Kirsehir, Turkey
Department of Chemistry, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India
b
c
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
"
FT-Raman and FT-IR spectra of 3-
acetyl-7-methoxycoumarin have
been recorded and analyzed.
Conformation analysis.
The computed vibrational
wavenumbers were seen to be in
good agreement with the
experimental data.
"
"
a r t i c l e i n f o
a b s t r a c t
Article history:
The powder form NIR-FT Raman and FT-IR spectra of 3-acetyl-7-methoxycoumarin (3A7MC) have been
recorded in the regions 4000–400 and 3500–100 cm , respectively. The equilibrium geometry, vibra-
Received 16 February 2012
Received in revised form 20 May 2012
Accepted 5 July 2012
ꢀ1
tional frequencies, band intensities, NMR spectra, NBO analysis and UV–Vis spectral studies of the most
stable conformer have been calculated by density functional B3LYP method with the 6-311G(d,p) basis
set. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman
spectra, the calculated wavenumber and intensity of the vibrational bands and the potential energy dis-
tribution over the internal coordinates. Information about the size, shape, charge density distribution and
site of chemical reactivity of the molecules has been obtained by mapping the electron density isosurface
with electrostatic potential surfaces (ESP). Natural bond orbital analysis has been carried out to under-
stand the nature of different interactions responsible for the electron delocalization and the intramolec-
Available online 23 August 2012
Keywords:
3
-acetyl-7-methoxycoumarin
FT-IR and FT-Raman spectra
PED
NBO
⁄
⁄
⁄
Electrostatic potential
ular charge transfer between the orbitals (n ?
p
, n ?
r
,
p
?
p
).
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
[3], antifungal agents [4], biological inhibitors [5], chemotherapeu-
tics [6,7] and as bio-analytical reagents [8]. They are useful antiox-
idants and show antitumour activity [9] and cytotoxicity [10–15].
Members of this group display a broad range of applications, as fra-
grances, pharmaceuticals, additives to food, cosmetics, agrochemi-
cals, optical brightening agents, dispersed fluorescent, tunable dye
lasers, and possess varied biological activities like anthelmintic,
hypnotic, insecticidal and anticoagulant properties [16–21]. The
presence of a methyl group or a halogen atom or both in the ring
system, often enhances the associated biological activity of
coumarin derivatives [22].
Coumarins are an important group of naturally occurring com-
pounds widely distributed in the plant kingdom such as Tonka
bean, lavender, sweet clover grass, licorice, strawberries, apricots,
cherries and cinnamon, and has been produced synthetically for
many years for commercial uses [1]. Coumarin derivatives have
been proven to function as anti-coagulants [2], antibacterial agents
⇑
Corresponding author. Tel.: +91 9495043765; fax: +91 4792303230.
E-mail address: dsajand@gmail.com (D. Sajan).
1
386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.07.084

L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
235
In order to elucidate the relationship between biological activity
and molecular structure, the knowledge of their conformational
distribution, electronic structure and spectral properties are partic-
ularly important. In the vibrational analysis of polyatomic complex
molecules, analysis of the assignment of vibrations is supported by
quantum chemical calculations, because there are many couplings
between vibrations. The vibrational spectra of 3-(bromoace-
tyl)coumarin [1], dihydro-, 6-methyl-, and 7-methylcoumarin,
reduced reaction time, energy saving, exclusion of hazardous sol-
vents etc. are extremely promising to the scientific as well as
industrial community.
NMR, IR and Raman spectra
Nuclear magnetic resonance (NMR) spectra were recorded to
confirm the presence of various types of protons and carbons for
the formation of the target compound. The NMR spectra of 3A7MC
grown from ethanol water system were recorded using Brucker
6
-methyl-4-bromomethylcoumarin,
coumarin, 6-chloro- and 7-chloro-4-bromomethylcoumarins,
-acetyl-6-bromocoumarin, 3-acetyl-6-methylcoumarin, 4-bro-
7-methyl-4-bromomethyl-
1
13
3
WM-400 (400 MHz) for H NMR and JEOL GSX (125 MHz) for
C
momethylcoumarin, 7-amino-4-trifluoromethylcoumarin [23–31]
molecules have been reported. According to previous literature,
electronic absorption spectra for the parent coumarin and its
derivatives have been reported by several authors [32–35]. The
aim of this work is to predict the structural, electronic and spectro-
scopic properties of 3-acetyl-7-methoxycoumarin (Fig. 1) by
applying experimental (NIR-FT Raman, FT-IR, NMR and UV–Vis
spectra etc.) and theoretical (DFT-B3LYP level of theory with the
use of standard 6-311G(d,p) basis set) methods. Though the crystal
structure of the title compound, 3-acetyl-7-methoxycoumarin
NMR spectrometers. The infrared spectrum of the sample was re-
ꢀ1
corded between 4000 and 400 cm on a JASCO 400 FT-IR spectrom-
eter, which was calibrated using polystyrene bands. The sample was
prepared as a KBr disc. The FT-Raman spectrum of the sample was
ꢀ1
recorded between 3500 and 100 cm regions on a Bruker RFS
100/S FT-Raman Spectrometer instrument using 1064 nm excita-
tion from an Nd:YAG laser. The detector is a liquid nitrogen cooled
Ge detector. The UV–Vis absorption spectrum of the synthesized
compound was recorded using SECOMOM ANTHELIE 70 M UV–Vis
Spectrophotometer in the wavelength range 200–900 nm.
(
3A7MC) has been reported [36], the vibrational spectral features
of 3A7MC crystal have not been subject of detailed analysis so
far. The UV–Vis spectroscopic behavior of 3A7MC was investigated
with the aim of analyzing the influence of the substituent in the
ring. Herein, the investigated results have been reported.
The experimental and theoretical results support each other, and
the calculations are valuable for providing a reliable insight into
the vibrational spectra and the molecular properties.
Computational details
The quantum chemical calculation has been performed using
the Becke-3-Lee–Yang–Parr (B3LYP) supplemented with the stan-
dard 6-311G(d,p) basis set, using the Gaussian 03 program [38]
to calculate the optimized geometry and vibrational wavenumbers
with their IR intensities and Raman scattering activities. The opti-
mized geometry corresponding to the minimum on the potential
energy surface have been obtained by solving the self-consistent
field (SCF) equations iteratively. Harmonic vibrational wavenum-
bers have been calculated using analytic second derivatives to con-
firm the convergence to minima on the potential surface and to
evaluate the zero-point vibrational energies without imposing
any molecular symmetry constraints. It is a well known fact that
ab initio calculations tend to overestimate the vibrational wave-
number with respect to the experimental ones. This is due to sev-
eral reasons, for instance, the use of finite basis set, the incomplete
implementation of the electronic correlation and the neglect of
anharmonicity effects in the theoretical treatment. To improve
the agreement between the predicted and observed wavenumber,
the computed harmonic wavenumbers are usually scaled for com-
parison. In this work the force field was scaled according to the
SQM procedure [39], the Cartesian representation of the force con-
stants were transferred to a nonredundant set of local symmetry
coordinates, chosen in accordance to the recommendations of
Experimental details
Synthesis of 3A7MC
Commercially available AR grade of m-methoxysalicylaldehyde
(
(
0.05 mol, 7.724 g), ethylacetoacetate (0.055 mol, 7 mL), piperidine
one drop), glacialaceticacid (two drops) were taken in a stoppered
flask and microwave irradiated for 15 s. The reaction mixture was
then poured into cold water containing ice and filtered, washed
with water and air-dried. Recrystallization from ethanol gave yel-
low needle shaped crystals. After about 5 days good quality crys-
tals appeared and these crystals were then grown to large size
crystals (yield 89%; m.p. 121 °C) of dimension 28 ꢁ 7 ꢁ 3 mm
within a period of 20 days by slow evaporation technique. Micro-
wave radiation enhances the kinetics of Knoevenagel condensation
by two to three folds over the conventional treatment [37]. The
outstanding advantages of the green chemical approach including
Fig. 1. Molecular structure and atomic numbering of 3-acetyl-7-methoxycoumarin.

2
36
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
Pulay et al. [40]. The scale factor calculation and characterization of
the normal modes using potential energy distribution (PED) were
done with the MOLVIB program 7.0 written by Sundius [41,42].
For the plots of simulated IR and Raman spectra, pure Lorentzian
[3H] could be due to the presence acetyl group. The peak integral
of multiplets in the aryl region d 6.993–7.762 ppm that accounts
for hydrogen atoms are well in agreement with suggested struc-
ture for the compound. The C NMR spectrum of the compound
is also in agreement with the number of carbon atoms present in
the compound.
1
3
ꢀ1
band shapes were used with a bandwidth (FWHM) of 10 cm
.
The prediction of Raman intensities was carried out by follow-
ing the procedure outlined below. The Raman activities (Si) calcu-
lated by Gaussian 03 and adjusted during scaling procedure with
MOLVIB were converted to relative Raman intensity (Ii) using the
following relation from the basic theory of Raman scattering [43]
Conformational analysis
The four conformers of 3A7MC molecule have been optimized at
B3LYP/6-311G(d,p) level of theory and are shown in Fig. 3. The zero
point corrected optimized energy of the conformers are predicted at
4
fð
m
o
ꢀ
m
i
Þ S
i
m
I
i
¼
ð1Þ
ꢀ
hc
kT
i
m
i
½1 ꢀ expð
Þꢂ
ꢀ
764.19653137 a.u. (conformer 1), ꢀ764.18604295 a.u. (conformer
), ꢀ764.19427090 a.u. (conformer 3) and ꢀ764.18334222 a.u.
conformer 4). The relative energy of the conformers are determined
at 0.0 kJ/Mol (conformer 1), 27.53 kJ/Mol (conformer 2), 5.93 kJ/Mol
conformer 3) and 34.62 kJ/Mol (conformer 4) for B3LYP/6-311
is the exciting frequency (in cmꢀ1 units),
m
is the vibra-
2
where
m
o
i
(
tional wavenumber of the ith normal mode, h c and k are universal
constants, and f is the suitably chosen common scaling factor for all
the peak intensities.
(
G(d,p) level of theory. As a result, the energy obtained for con-
former-1 of 3A7MC molecule was found to be the global minimum.
Isoelectronic molecular electrostatic potential surfaces (MEPSs)
and electron density surfaces were calculated using 6-311G(d,p)
basis set with (83,83,52) grid points with an average resolution
of 0.333 Bohr. The structure resulting from the plot of electron
density surface mapped with electrostatic potential surface depicts
the shape, size, charge density and the site of chemical reactivity of
a molecule. These surfaces were plotted by the computer software
GaussView (Gaussian Inc., Pittsuburgh, PA 15106, USA, 2003). Elec-
tronic transitions and oscillator strengths for 3A7MC molecule
were calculated by time-dependent density functional theory
The energy barrier of rotation around the O
bond was calculated for the B3LYP with 6-311G(d,p) basis sets.
The dihedral angles of C15–C14–O –C23 (around O –C14 bond) and
–C –C18–C19 (around C –C18 bond) were varied from 0° to 360°
by steps of 10°. The energy profile obtained from DFT method
Fig. 4) as a function of h [C15–C14–O –C23 and C –C
3 6
–C14 and C –C18
3
3
C
5
6
6
(
3
5
6
–C18–C19]
has shown a global minimum at 0° and global maximum at 90°
which predicts that the conformer 1 of 3A7MC is the most stable
one. This conformation and structure of 3A7MC obtained is used
for vibrational spectral analysis.
(
TD–DFT) using 6-311G(d,p) basis set and B3LYP functionals after
taking into account the configuration interaction between singly
excited states.
Molecular geometry
Results and discussion
The molecular structure and atomic numbering of conformer-1
of 3A7MC is shown in Fig. 1. Calculated geometric parameters and
definitions of the natural coordinates for the molecules are sum-
marized in Supplementary Table S1 (Supplementary data). Opti-
mized geometric parameters of 3A7MC are compared with XRD
data of the molecule of 3A7MC. Calculated RMS shows that hybrid
B3LYP functional gives quite good reproduction of experimental
parameters.
NMR spectra
The nuclear magnetic resonance (NMR) spectrum was recorded
to confirm the presence of various types of protons for the forma-
tion of the target compound. The recorded NMR spectrum is shown
in Figs. 2a and 2b. A singlet at 3.29 ppm [3H] indicates the presence
of methoxy linkage in the compound. Another singlet 1.63 ppm
Fig. 2a. Experimental 1H NMR spectrum of 3-acetyl-7-methoxycoumarin.

L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
237
Fig. 2b. Experimental 13C NMR spectrum of 3-acetyl-7-methoxycoumarin.
Conformer -1
Conformer -2
Conformer -4
Conformer -3
Fig. 3. Molecular structure of all conformers of 3-acetyl-7-methoxycoumarin.

2
38
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
Fig. 4. Dihedral angles–energy curve for 3-acetyl-7-methoxycoumarin (C
-311G(d,p) level of theory.
5 6 3
–C –C18–C19) for acetyl group (a and b) and C15–C14–O –C23 for methoxy group (c and d)) by B3LYP/
6
Table 1
Second order perturbation theory analysis of Fock matrix in NBO basis.
Lone pair
Occupancy
1.96089
Donor–acceptor interaction
Hybrid (% p character)
E(2)a (kJ molꢀ1
)
E(j)–E(i)b (a.u.)
F(i,j)c (a.u.)
⁄
SP1.97(66.30)
LP
1
O
1
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
n (LP
1
1
1
1
2
2
1
1
2
2
1
1
2
1
1
2
2
O
O
O
O
O
O
O
O
O
O
O
O
1
1
1
1
1
1
2
2
2
2
3
3
) ?
) ?
) ?
) ?
) ?
) ?
) ?
) ?
) ?
) ?
) ?
) ?
r
r
r
r
p
p
r
r
r
r
r
(C
(C
(C
5
5
9
–O
–C
–C17
4
)
)
)
10.54
20.24
29.65
3.27
137.0
135.3
8.06
1.16
0.99
1.07
1.09
1.05
0.35
1.10
1.08
0.68
0.66
1.08
1.10
0.34
0.98
1.13
0.55
0.70
0.048
0.062
0.078
0.026
0.052
0.096
0.042
0.039
0.104
0.098
0.024
0.081
0.101
0.025
0.056
0.129
0.097
⁄
⁄
⁄
6
(C15–C17
)
)
⁄
⁄
⁄
SP1.0(99.95)
SP0.74(42.65)
SP1.0(99.94)
SP1.6(62.34)
SP1.0(99.95)
LP
LP
LP
LP
2
1
2
1
O
O
O
O
1
2
2
3
1.73753
1.97736
1.88913
1.96244
(C
(C
5
–O
–C17
–C18
(C18–C19
(C –C18
4
)
9
)
)
(C
6
⁄
⁄
⁄
⁄
⁄
7.27
6
)
82.95
74.51
2.77
30.95
144.9
3.23
14.2
156.0
66.74
(C18–C19
(C14–C12
(C14–C15
)
)
)
r
⁄
LP
LP
2
1
O
O
3
4
1.81646
1.97895
O
3
) ?
p
(C14–C15
)
⁄
0.71
O
O
O
O
4
4
4
4
) ?
) ?
) ?
) ?
r
r
r
r
(C
5
(C
5
(C
5
(C
5
–O
1
)
SP (41.47)
⁄
⁄
⁄
–C
–O
6
)
LP
2
O
4
1.82836
1
)
SP99.99(99.89)
6
-C )
a
b
c
E(2) means energy of hyperconjugative interactions; cf. Eq. (2.0).
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.
NBO analysis
a measure of the delocalization or hyperconjugation [44]. The
main natural orbital interactions were analyzed with the NBO
5.0 program [45–47]. The hyperconjugative interaction
energy was deduced from the second-order perturbation
approach
The NBO analysis is already proved to be an effective tool for
chemical interpretation of hyperconjugative interactions and
electron density transfer from the filled lone pair electron.
DFT level computation is used to investigate the various
second-order interactions between the filled orbitals of one
subsystem and vacant orbitals of another subsystem, which is
2
2
h
r
jFj
r
i
F
ij
Eð2Þ ¼ ꢀn
r
¼ ꢀn
r
ð2Þ
e
r
ꢃ
ꢀ
e
r
DE

L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
239
Table 2
Definition of local symmetry coordinates (much like the natural internal coordinates) and the corresponding force constant (mdyne/Å) with scale factors used.
No
Symbol
Definition
Scale factors
Force constants (mdyne/Å)
Stretching
1
4
5
9
1
1
1
2
2
2
–3
CHr1
CHr2
CCr1
CC r2
r
r
r
1
2
0.923
0.926
0.989
0.765
0.924
0.930
0.918
0.829
0.944
0.844
5.534
5.719
8.032
7.638
5.378
5.321
5.235
12.397
4.393
13.266
3 4
, r r
–8
–13
5, 6 7 8
r , r , r
R
9
, R10, R11, R12, R13
p
p
3
4–15
6–17
8–19
0–22
3–24
5–27
CH
CH
CH
OC
CC
3
3
3
ss
ips
ops
(r14 + r15 + r16)/ 3, (r17 + r18 + r19)/
p
p
6
(2r14ꢀr15ꢀr16)/ 6, (2r17ꢀr18ꢀr19)/
p
p
2
(r15–r16)/ 2, (r18ꢀr19)/
Q
Q
Q
20, Q21, Q22
23, Q24
25, Q26, Q27
COr1
Bending
2
2
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
5
5
8
9
0
1
2
3
4
5–37
8
9
0
1
2
3–44
5–46
7–48
9–50
1–52
3
Rtrid1
Rasyd1
Rasydo1
Rtrid2
Rasyd2
Rasydo2
bCH1
(d28ꢀd29 + d30ꢀd31 + d32ꢀd33
)/p
6
0.711
0.869
0.714
0.504
1.369
1.218
0.875
0.850
0.699
1.279
0.960
0.572
1.009
0.926
0.899
0.944
0.778
0.759
0.975
1.502
1.553
1.425
1.487
1.528
1.440
0.515
0.530
1.236
1.235
1.366
1.309
1.157
0.693
0.576
0.582
0.858
0.892
1.397
p
(2d28ꢀd29ꢀd30 + 2d31ꢀd32ꢀd33)/
(d29ꢀd30 + d32ꢀd33)/2
6
6
(d34ꢀd35 + d36ꢀd37 + d38ꢀd39
)/p
6
p
(2d34ꢀd35ꢀd36 + 2d37ꢀd38ꢀd39)/
(d35ꢀd36 + d38ꢀd39)/2
p
(b40ꢀb41)/
2
p
)/p
)/p
bCH2
(b42ꢀb43)/ 2, (b44ꢀb45
2, (b46ꢀb47
2
p
CCOr
CCOr
CCb
1
(b48ꢀb49)/
(b50ꢀb51)/
(b52ꢀb53)/
2
2
2
p
p
2
p
COb
CCCb
(
a
54
ꢀ
a
55)/
2
(2b56
ꢀ
a
54
ꢀ
a
a
55
)/p
6
CH
CH
CH
CH
CH
3
3
3
3
3
sd
(a
57
+
a
58
+
59ꢀb63ꢀb64ꢀb65
)/p
6, (
a
a
60
+
a
61
+
a
62ꢀb66ꢀb67ꢀb68
)/p
6
p
p6
ipb
opb
ipr
opr
(2
a
58
57
ꢀ
a
a
58
ꢀ
a
59)/ 6, (2
a
62)/
60
ꢀ
a
2
61
ꢀ
62)/
p
p
(a
ꢀ
59)/ 2, (
a
61
ꢀ
a
p
)/p
6
(2b63ꢀb64ꢀb65)/ 6, (2b66ꢀb67ꢀb68
p
p
(b64ꢀb65)/ 2, (b67ꢀb68)/
2
OCb
b
69
Out-of-plane bending (wagging)
5
5
5
5
6
4
5
6
7–59
0–61
gCH r1
gCarO
gCC ar1
gCH ar2
gCO
x
70
x
71
x
72
x
73,
x
76,
0.541
0.621
0.567
0.803
0.938
1.328
0.716
0.474
0.465
0.712
x
x
74
, x
75
77
Torsion
6
6
6
6
6
7
7
7
2–63
4–65
6–67
8
9
0
1
2
Rpuck
Rasyt
Rasyto
tC–CH3
tC–O
tC–OCH3
tC–C
tC–O
(
(
s
s
78
78
ꢀ
s
s
79
80
+
+
s
80
ꢀ
ꢀ
s
s
81
+
s
82
ꢀ
s
83
)/p
6, (
s
84
87
+
s
85
ꢀ
s
86
+
s
87
ꢀ
s
88
ꢀ
s
89
)/p
6
1.027
0.997
1.334
1.056
1.022
1.168
0.955
0.780
0.377
0.366
0.543
0.063
0.036
0.012
0.512
0.021
ꢀ
s
81
83)/2, (
s
84
ꢀ
s
86
+
s
ꢀ
s
89)/2
p
)/p
12
(ꢀ
s
78 + 2
s
79
ꢀ
s
80
ꢀ
s81 + 2
s
82
ꢀ
s83)/ 12, (ꢀ 84 + 2
s
s
85
ꢀ
s
86
ꢀ
s
87 + 2
s
88
ꢀ
s
89
p
(s
(s
(s
(s
(s
90
92
95
+
+
+
s
s
s
91)/
2
+
s
s
+
94
)/p
3
93
+
97+
s
98
+
s
3
99
+
s
)/p
6
100
96
p
101
104
+
+
s
s
102
s103)/
105
+
106
s + s107)/2
2
2
⁄
where h
r
|F|
r
i or F is the Fock matrix element between the i and j
lone pair n(LP O ) with the
r
(C –C ) orbital. The energy contribu-
ij
1
4
5
6
⁄
⁄
ꢀ1
NBO orbitals,
e
r
and
e
r
ꢃ
are the energies of
r
and
r
NBO’s, and n
r
4
tion of LP2 O ? r (C19–H20) value is 29.4 kJ mol . These energy
is the population of the donor
r
orbital.
E(2) values are chemically significant and can be used as a measure
of the intramolecular C–Hꢄ ꢄ ꢄO hydrogen bonding interaction be-
tween the oxygen lone pair and the antibonding orbitals.
The Mulliken population analysis in the 3A7MC molecule was
calculated using the B3LYP/6-311G(d,p) level of theory. The charge
distribution structure and chart of 3A7MC are shown in Supple-
Table 1 shows the most important interactions between Lewis
and non-Lewis orbitals with O lone pairs, the second order pertur-
bation energy values, E(2), corresponding to these interactions, and
the overlap integral of each orbital pair. A very strong interaction
has been observed between the p-type orbital containing the lone
⁄
⁄
electron pair of O
1
and the neighbor
p
(C
9
–C17),
p
(C
5
–O
4
) anti-
mentary Fig. S1 (Supplementary Material). The oxygen (O ) and
3
bonding orbital of the benzene ring. This interaction is responsible
for a pronounced decrease of the lone pair orbital occupancy
carbon (C ) atom have large negative charges and behave as elec-
tron acceptor whereas the carbon atom C5 acts as electron donor
6
1
.73753, than the other occupancy, and there is a possibility for
hyperconjugation between O and the benzene ring.
The contributions of the stabilization energies for the n(LP
with large positive charge than the other carbon atoms. The result
suggests that the atoms bonded to the carbon atoms and all oxygen
atoms are electron acceptors and the charge transfer takes place
from C to O. The carbon atom C18 is more positive and C19 is more
negative than the other atoms due to electron-delocalization of
1
2
⁄
ꢀ1
⁄
O
1
) ?
74.51 kJ mol ] charge transfers have higher values than the other
delocalizations. An important contribution for the molecular sta-
bilization is further given by O through the overlap of its sp0.71
6 2 1
r (C –C18) [82.95 kJ mol ] and n(LP O ) ? r (C18–C19)
ꢀ1
[
p-electrons between the acetyl carbonyl and the pyrone, which in-
4
creases the double bond character of the bond joining them.

2
40
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
Vibrational spectral analysis
3
in IR is attributed to the CH asymmetric bending mode. The weak
ꢀ1
band around 1381 cm in IR corresponds to symmetric bending
mode. The enhancement of IR intensity is due to the presence of
carbonyl next to methyl in the acetyl group [56]. This is also in
agreement with the calculated results. The relatively large value
of IR intensity of the symmetric bending mode suggests a large po-
sitive charge localized on the hydrogen, which further supports the
The vibrational spectral assignments of 3A7MC have been car-
ried out with the help of normal coordinate analysis. Internal coor-
dinates have been described in Supplementary Table S2 according
to Pulay’s commendations [40]. The computed wavenumbers are
selectively scaled according to the SQM procedure comprising a
set of 11 transferable scale factors (Table 2) suggested by Rauhut
and Pulay [48]. The detailed vibrational assignments of fundamen-
tal modes along with the calculated IR and Raman intensities and
normal mode description (characterized by PED) are reported in
Table 3. For visual comparison, the observed and simulated FT-IR
and FT-Raman spectra are presented in Figs. 5 and 6, respectively.
occurrence of hyperconjugation. The rocking vibrations of the CH
group in 3A7MC appear as mixed vibrations. These modes usually
3
ꢀ1
appear in the region 1070–1010 cm [52–54]. The intense band in
ꢀ1
IR spectra at 1185 cm is attributed to the CH
3
rocking mode,
which is coupled with aromatic C@O stretching mode. The infrared
ꢀ
1
band around 929 cm is due to the superposition of aromatic C–C
stretching and CH twisting mode, which is in agreement with the
3
Acetyl group vibrations
calculated results also.
The acetyl group bonded to the pyrone ring gives rise to a series of
characteristic bands corresponding to the carbonyl and methyl moi-
eties. The carbonyl stretching wavenumber is very sensitive to the
factors that disturb the nature of the carbonyl group and its precise
wavenumber is a characteristic of the type of the carbonyl com-
pound being studied. Particularly detailed correlations have been
made for the carbonyl bond stretching wavenumber. The carbonyl
stretching wavenumber has been most extensively studied by infra-
Methoxy group vibrations
The methyl vibrational band positions are most consistent
when this group is attached to other carbon atoms. When the
CH
ing and bending bands can shift position due to electronic effects
[56]. This causes the O–CH stretching bands to be spread over a
larger region than that of the C–CH group. For the aryl methoxy
group, the methyl stretching bands occur in the region 3000–
3
group is directly attached to an oxygen atom, the C–H stretch-
3
3
d+-
red spectroscopy. This multiply bonded group is highly polar (>C
dꢀ
ꢀ1
ꢀ1
@O
) and therefore gives rise to an intense infrared absorption
–p bond-
2815 cm [56–59]. The observed bands at 3017 cm in the Ra-
ꢀ1
band. The carbon oxygen double bond is formed by the p
p
p
man spectrum and 3016 cm in the IR are assigned to methyl
asymmetric stretching vibrations. The symmetric stretching mode
of methoxy group of 3A7MC is observed as a weak band in IR and
Raman spectra around 2855 and 2853 cm
asymmetric stretching and symmetric stretching modes of meth-
ing between carbon and oxygen. Because of the different electro
negativities of carbon and oxygen atoms, the bonding electrons
are not equally distributed between the two atoms. The following
two resonance forms contribute to the bonding of the carbonyl
ꢀ1
respectively. The
+
ꢀ
ꢀ1
group >C@O M C –O . The lone pair of electrons on oxygen also
oxy group vibrations are calculated to be 3020 cm (42%) and
ꢀ
1
determines the nature of the carbonyl group. The position of the
2924 cm (90%) respectively. The lowering of the IR intensity of
the CH stretching and the blue-shifting of methyl stretching
@
C
O stretching vibration is very sensitive to various factors such
3
@
as the physical state, electronic effects by substituents, ring strains
49]. Consideration of these factors provides further information
modes is due to electronic effects resulting from induction of the
methyl group with the aromatic ring system from lone pair oxygen
atoms to the antibonding
[
⁄
about the environment of the C@O group. The carbonyl stretching
r
(C–H) bonds [56].
vibrations generally occurs as a strong absorption in the region from
The asymmetric bending vibrations of methoxy groups nor-
ꢀ
1
ꢀ1
1
730–1645 cm . This portion of the infrared spectrum is most use-
mally appear in the region 1460 cm [52–54]. The medium band
ꢀ
1
ꢀ1
ful because the position of the carbonyl absorption is quite sensitive
to substitution effects and the geometry of the molecule. The sharp
at 1466 cm in IR and strong band at 1474 cm in Raman are as-
signed to the methoxy asymmetric bending mode. The weak band
ꢀ
1
ꢀ1
ꢀ1
intense band in IR at 1683 cm and weak band at 1683 cm in Ra-
man corresponds to acetyl carbonyl stretching vibrations. Reduction
in intensity in the Raman band of the acetyl carbonyl stretching
vibrations is probably caused by the conjugation of carbonyl bond
with the ring [50]. The C@O in plane and out of plane bending modes
are coupled with other modes (Table 3).
around 1445 cm in Raman spectra is due to the symmetric bend-
ing mode. The higher wavenumber corresponds to methyl in phase
umbrella mode and the lower one arises from the out of phase um-
brella mode of CH
results also. The rocking vibration of the CH
appearing as mixed vibrations. The mixing of C–O stretch and
CH rock, as predicted by computations, result in the weak band
3
group. This is in agreement with the calculated
3
group in 3A7MC is
Methyl groups are generally referred to as electron donating
substituents in the aromatic ring system [51]. The asymmetric
3
ꢀ1
at 1128 cm in Raman, which is also strongly coupled with aro-
matic C–H in-plane bending mode. The intensity enhancement is
3
and symmetric stretching modes of CH group normally appear
ꢀ1
at 2965 and 2880 cm [52–54]. The asymmetric stretching mode
is observed as a weak band as out of phase vibration (indicated as
owing to increased conjugation with the ring-system. The calcu-
⁄
lated hyperconjugative interactions between LP
1
(O
3
) ?
r
(C23
–
ꢀ
1
⁄
CH
2
3
ops in Table 3) at 2980 cm
in IR and a weak band at
H
24/C23–H25) and LP
2
(O
3
) ?
r
(C23–H24/C23–H25) are 12.19 and
21.67 kJ mol respectively, show the magnitude of conjugation
ꢀ1
ꢀ1
ꢀ1
974 cm in Raman. The weak Raman band at 2924 cm and a
ꢀ1
weak band in IR at 2930 cm are assigned to methyl symmetric
stretching mode. The calculated wavenumbers of the above modes
with the aryl ring.
ꢀ1
ꢀ1
are 2978 cm (100%) and 2930 cm (99%) respectively for methyl
asymmetric and symmetric modes. The lowering of the CH
Ring system vibrations
3
The aromatic C–H stretching [23–31,54–56] vibrations are gen-
ꢀ1
stretching intensity in IR and the blue shifting of methyl stretching
erally observed in the region 3000–3100 cm . In this region, the
bands are not affected by the nature of the substituents [57]. Hence
wavenumber is due to the hyperconjugation interaction between
⁄
ꢀ1
ꢀ1
the
r
(C–H) and
r
(C18–O
2
) bond. The NBO analysis clearly shows
in the present investigation, the IR bands at 3120 cm , 3051 cm ,
ꢀ1
ꢀ1
ꢀ1
that there are 4.6, 21.4 and 7.1 kJ mol
of energy involved in
and Raman bands at 3084 cm , 3067 cm have been assigned to
C–H stretching vibrations. Most of the C–H stretching modes are
found to be weak which is due to the charge transfer from the
hydrogen atoms to the carbon atoms. DFT calculated wavenumbers
are in good agreement with the experimental values. A comparison
with the results from the normal mode of coumarin shows that the
⁄
⁄
r
(C19–H20) ?
r
(C18–O
(C18–O
2
),
r
(C19–H21) ?
r
2
(C18–O ) and r(C19–
⁄
H22) ?
r
2
) hyperconjugative interactions.
The asymmetric and symmetric bending vibrations of methyl
groups normally appear in the region 1465–1410 and 1390–
ꢀ1
ꢀ1
1
370 cm respectively [52–55]. The medium band at 1433 cm

Table 3
Vibrational assignment of 3A7MC by normal mode analysis based on SQM force field calculations.
Calculated wavenumbers (cmꢀ1)
Observed wavenumbers (cm
ꢀ1
)
IR intensity
Raman intensity
Characterization of normal modes with PED (%)
Mode no.
Unscaled
scaled
FTIR
Raman
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
1
3228
3206
3180
3174
3146
3146
3107
3080
3048
3016
1815
1752
1656
1628
1576
1537
1504
1495
1482
1466
1456
1451
1403
1397
1379
1313
1293
1278
1226
1223
1191
1170
1144
1097
1055
1049
1003
994
3106
3085
3061
3051
3036
3020
2978
2965
2930
2924
1731
1683
1608
1576
1554
1540
1495
1474
1440
1428
1412
1381
1355
1303
1282
1254
1229
1218
1190
1141
1122
1091
1073
1058
1037
1022
994
965
928
904
893
870
841
810
787
779
720
691
678
641
615
584
542
3120w
–
–
3051w
–
3016w
2980w
2956w
2924w
2855w
1731w
1683vs
1604s
1572w
–
1540vs
–
1466m
–
1433m
–
1381w
1354w
1296w
1279w
1248vs
–
–
0.31
0.50
0.71
0.15
3.99
6.23
0.84
7.27
0.41
4.32
0.64
100
1.60
4.00
1.80
0.90
3.00
4.50
1.30
2.60
4.90
1.90
0.20
4.90
1.20
34.0
16.0
0.80
100
0.01
1.20
4.80
1.80
4.10
2.40
0.90
2.60
64.0
5.90
13.0
1.40
8.20
4.30
8.00
0.10
4.30
1.20
1.80
1.60
18.0
0.60
3.80
0.10
9.70
23.0
13.0
0.10
42.0
0.70
0.50
18.0
0.10
15.0
6.40
5.80
m
m
m
m
CHR2(99)
CHR2(99)
CHR2(99)
CHR1(96)
2
3084w
3067w
–
–
3
4
5
CH3ips1(61), CH3ops2(26)
CH3ops2(42),CH3ips1(39), CH3ips2(14)
CH3ops1(100)
CH3ips2(74), CH3ops2(25)
CH3ss1(99)
6
3017w
2974w
–
2930w
2853w
1729m
1683w
1604vs
1570vs
–
7
8
9
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
CH3ss2(90)
m
m
m
m
m
COR1 (80)
CO(74)
CCR2(59), dCHR2(15), Rasyd(11)
CCR1(42),
CCR1(42),
28.6
84.5
43.3
3.98
87.6
3.21
9.29
4.63
10.2
1.99
2.95
3.38
6.03
26.2
5.19
12.2
8.08
34.1
1.51
87.0
0.93
1.28
0.40
0.83
3.75
0.90
2.11
17.6
0.08
7.72
0.07
17.8
1.66
0.26
1.44
1.09
0.18
2.94
2.14
9.79
0.10
m
m
CCR2 (28)
CCR2 (22)
–
dCHR1 (33), mCCR2 (30), mCCR1(14)
1474s
–
1445w
–
1410w
–
1355s
–
1282w
–
1232w
1206m
–
1140w
1128w
1095w
1075w
–
CH3asydi2(33), CH3asydo2(21)
CH3asydo2(73), CH3asydi2(23)
CH3syd2(73)
CH3asydo1(93)
m
CCR2(44), dCHR2(19)
H3asydi1(82)
CCR1(24),
CH3syd1(61),
CCR2(34), CCR1(31), dCHR1(11)
dCHR1(11), Rtrid2(15),
CCR2(21), CCR1(18),
dCHR2(37), Rtrid1(12),
CH3Roko1(32),
dCHR1(23), CCR2(15),
dCHR2(30), CH3Roko2(21),
CH3Roki2(69), CH3Roko2(24)
dCHR2(53), CCR2(16)
CCR1(26), COR1(15), dCHR2(13)
CH3Roko1(36),OCwag (30)
CO(68), CCR2(17)
CCwag(36), tCOC(29)
trid2(25), COR1(19),
CH3Roki1(40), CC (25)
wag((78),CCHwag((12)
COR1(42), Rtrid1(14), dCO R1 (13)
wag((62),R puck2(19)
CCR1(26), CCR2(24),
wag((76), Rasyto(10)
trid1(23), COR1(18), Rtrid2(14)
m
mCCR2(21), CH3syd1(11)
mCCR1(13)
m
m
m
m
CCR2(14),
m
CCR1(10),
mCOR2(10)
m
m
mCOR2(15),
mCC(13)
–
CCR2(11),
1185s
1137w
–
1089w
1073w
–
1036m
–
999m
957w
929w
–
m
COR1(7), CH3Roki1(10)
CC(11)
COR1(12)
m
m
m
m
m
m
–
–
m
m
995m
973m
–
R
m
mCCR2(18), dCHph2(10)
988
965
931
858
838
828
823
783
738
732
654
648
627
596
592
m
–
R
m
R
m
R
R
–
884w
–
–
–
–
774w
715w
–
861w
–
m
mCC(14)
804w
787w
768w
718m
692w
665w
–
m
OCwag(38), Rpuck1(33), CCwag(28)
CCR1(26), Rasyto(14),Rtrid2(10)
puck(58)
COwag(40), Rasyto2(19)
asydo2(41), dCHR!(23)
dCCO (23), Rtrid1(14),
OCCwag(45), CH3Roko1(21)
m
R
–
641m
620w
-
R
–
mCCR1(11),
596w
–
–
R
asydo1(33), dCCO (12), Rasydi2(12)
(continued on next page)

2
42
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
aromatic C–H stretching vibrations around 3050 cm 1 remain
unaltered by methoxy and acetyl substitution [23–31]. The C–H
in-plane bending vibrations appear in the region 1300–
ꢀ
ꢀ
1
1
000 cm and C–H out of plane bending vibrations in the range
ꢀ1
of 1000–675 cm . The bands corresponding to the C–H in-plane
bending vibrations are identified in 3A7MC at 1248, 1137,
ꢀ
1
ꢀ1
1
073 cm in infrared spectra and 1206, 1140, 1075 cm in Ra-
man spectra. The corresponding calculated modes are dominated
by C–H in plane bending and coupled with C–C stretching. The
C–H out-of-plane bending vibrations and ring breathing and ring
puckering vibrations are shown in Table 3.
The carbonyl stretching vibrations are found in the region 1750–
ꢀ
1
ꢀ1
1
700 cm [52–54]. The weak IR band at 1731 cm and the medium
ꢀ1
band 1727 cm in Raman spectrum corresponds to the carbonyl
vibration in the pyrone ring [23–31]. The corresponding theoretical
band with the comparable intensity is at 1731 cm . In 3A7MC, the
C@O bond in the lactone part is conjugated with C @C and C
ꢀ
1
7
6
9
@C17
double bonds. The increase in single bond character due to conjuga-
tion lowers the wavenumber of carbonyl and double bond absorp-
tions. In the present study, the carbonyl vibrational wavenumbers
in the lactone part C
5 4
@O stretching have been lowered to different
extents due to conjugation. The orbital interaction energy for n(LP
2
⁄
ꢀ1
⁄
O
4
) ?
r
(C
5
–O
1
) is 156.0 kJ mol
and n(LP
2
O
1
) ?
p
(C
5 6
–C ) is
ꢀ
1
6
6.74 kJ mol which are higher values than the other delocaliza-
tions. These charge transfer interactions of 3A7MC can be responsi-
ble for biological properties [58]. The medium Raman band at
ꢀ
1
9
73 cm is attributed to C–O stretching mode, which is coupled
with C–C stretching and C–H in plane bending mode. The C–C ring
stretching vibrations have given rise to characteristic bands in both
the observed IR and Raman spectra, covering the spectral range from
ꢀ
1
1
610 to 1300 cm . In 3A7MC, the ring mode manifests as a strong
ꢀ1
band in Raman and a weak band in infrared spectra at 1604 cm
,
which is localized on the benzene part of the molecule. These vibra-
ꢀ
1
tions corresponds to the e
g
mode (8a) of benzene [57] at 1610 cm .
ꢀ1
The infrared band at 1572 and 1540 cm is attributed to C–C
stretching. The weak IR bands at 1354 and 1279 cm are predomi-
nantly a C–C are predominantly a of benzene. The C–C stretching
vibration at 1410 cm is coupled with the C–C–H bending modes,
which can be inferred from results of PED calculations. The very
strong band at 1248 cm in IR spectra is attributed to ring stretch-
ing mode of the benzene part. The ring stretching, in-plane, and out-
of-plane bending vibrations have been identified and presented in
Table 3. They also are in agreement with the literature [23–31].
ꢀ1
ꢀ1
ꢀ1
Low wavenumber vibrations of hydrogen bonds
The attractive interaction between the hydrogen donor group
and the acceptor moiety leads to the occurrence of new vibrational
degrees of freedom, the so called hydrogen bond modes [59]. Such
modes are connected with elongations changing the Xꢄ ꢄ ꢄY distance
and/or the relative orientation of the hydrogen bonded groups.
Thus, they provide direct insight into the structure of hydrogen
bonds and into processes of bond formation and cleavage. As such
modes are characterized by a high reduced mass of the oscillator
and a small force constant determined by the comparably weak
attractive interaction along the hydrogen bond, hydrogen bond
modes occur at low wavenumbers in the range between 50–
ꢀ
1
3
00 cm . The low wavenumber bands of the hydrogen bond
vibrations are generally found to be weak, broad and asymmetric
in the Raman spectrum. The low wavenumber degrees of freedom
such as librations as well as interaction-induced give rise to addi-
tional absorption and Raman bands that frequently overlap with
the bands of the hydrogen bond modes. In addition, a substantial
spread of vibrational wavenumber occurs for liquids with multiple
hydrogen bonding geometries, resulting in a pronounced inhomo-
geneous broadening of the vibrational bands. The band in Raman at
ꢀ
1
8
2 cm
has been attributed to a translational motion of the

L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
243
ꢀ
1
Fig. 5. (a) Experimental FT-IR spectra of 3-acetyl-7-methoxycoumarin in the range 4000–400 cm and (c) simulated IR spectra at the B3LYP/6-311G(d,p) level of theory.
ꢀ
1
Fig. 6. (a) Experimental FT-Raman spectra of 3-acetyl-7-methoxycoumarin in the range 3500–100 cm and (b) simulated Raman spectra at B3LYP/6-311G(d,p) level of
theory.
hydrogen bonded molecules including some bending component,
and oscillator strengths, time-dependent DFT (TD–DFT) calcula-
tions involving configuration interaction between the singly ex-
cited electronic states were conducted on the molecule. Position
and absorbance of the experimental peaks (Supplementary
Figs. S2 and S3) together with the calculated transition energies,
optical strengths, main configurations and mixing coefficients of
the singlet ground and excited states and spectral assignments
are given in Table S3. A close agreement has been obtained be-
tween the experimental and calculated values of transition
energies.
ꢀ1
the 182 cm band to a translational motion of the hydrogen bond.
The lattice vibrations of rotatory type are generally stronger in
intensity than the translatory type. The lattice modes in 3A7MC
are found to be very intense in Raman compared with other modes
in the high wavenumber region.
UV–Vis spectral studies
The UV–Vis spectra of 3A7MC molecule recorded in methanol
solution are given in Supplementary S2. In an attempt to under-
stand the nature of electronic transitions in terms of their energies
TD-DFT calculations predict three transitions in the near ultra-
violet region for 3A7MC molecule. The strong transitions at 274.4

2
44
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
HOMO
LUMO
ELUMO=2.501 eV
EHOMO=-6.852 eV
HOMO-1
LUMO+1
EHOMO-1= -7.015 eV
ELUMO+1= -0.936 eV
HOMO-2
LUMO+2
EHOMO-1= -7.370 eV
ELUMO+1= -0.356eV
Fig. 7. HOMO and LUMO plot of 3-acetyl-7-methoxycoumarin by B3LYP/6-311G(d,p) level of theory.
(
2
0.5681) and 179.48 (0.3989) nm have been observed at 300.8 and
ational definition of LUMO and HOMO in the context of DFT can be
written as:
⁄
39.0 nm, respectively, and are assigned to a
p–p
transition
(
Supplementary Table S3). The numbers in parentheses represent
EA ¼ ꢀnLUMO
ð3Þ
oscillator strengths. On the basis of TD-DFT calculations, this shift
may be explained as arising out of a larger decrease in the energy
of the LUMO orbital (2.501 eV) than the HOMO (ꢀ6.852 eV) and
the HOMO-1 (ꢀ7.015 eV) orbitals, relative to their positions in
IP ¼ ꢀnHOMO
ð4Þ
where nHOMO and nLUMO correspond to the Kohn Sham [66] one
electron eigen values associated to the frontier molecular orbital:
HOMO and LUMO, respectively. IP and EA refer to ionization poten-
tial and electron affinity of the system respectively. The HOMO–
LUMO energy values of 3A7MC are shown in Fig. 7. HOMO–LUMO
energy gap is used as a quantum chemical descriptor in establishing
correlations for chemical and biochemical systems [67]. A large
HOMO–LUMO gap implies high stability for the molecules in the
sense of its lower charge transfer in complexes. Polarizability is an-
other characteristic property which is related to HOMO–LUMO en-
ergy gap. Soft molecules with small energy gap will be more
polarizable than hard molecules. The comparatively lower value
of the ELUMO-EHOMO (Fig. 7) of 3A7MC shows that the molecule is
polarizable and biologically active [68]. The HOMO is located on
3
A7MC molecule.
The energy of the LUMO is directly related to the electron affin-
ity and characterizes the susceptibility of the molecule towards at-
tack by nucleophiles [60]. An electron affinity (EA) refers to the
capability of a legend to accept precisely one electron from a do-
nor. However, to influence the interacting systems, covalent or
hydrogen bonding may take place which can result in a partial
charge transfer [61]. A current approximation is to use Koopman’s
theorem to express these quantities in terms of the frontier one
electron energy levels: HOMO and LUMO [62–65]. The energy of
the HOMO is directly related to the ionization potential (IP) and
characterizes the susceptibility of the molecules towards attack
by electrophiles. Hard nucleophiles have a low HOMO energy
while hard electrophiles have a high LUMO energy [60]. The oper-
2
the C18@O bonds of the acetyl group. The LUMO in 3A7MC,

L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
245
however, populates on the lactone part, with only minor population
on the O and C atoms .The population of LUMO on the bonding be-
forms antibonding orbitals. Minor population can be
located on the O and C atoms. The difference between the orbital
energies corresponding to HOMO-1(ꢀ7.015 eV) and HOMO-2
1
6
@
tween C
5 4
O
@
1
6
(
ꢀ7.370 eV) is much higher than 0.05 eV indicating that the
HOMO-1 and HOMO-2 are nondegenerate in 3A7MC. Similar con-
clusion can be drawn from the LUMO + 1 and LUMO + 2 orbital en-
ergy calculations.
Molecular electrostatic potential
The molecular electrostatic potential (ESP) at a point r in the
space around a molecule is (in atomic units)
Fig. 8a. Molecular electrostatic potential mapped on the
q(r) = 0.0004 a.u. isoden-
Z
0
sity surface for 3-acetyl-7-methoxycoumarin in the range from ꢀ5.644E-2 (red) to
X
Z
A
q
jr
ðrÞdr
+5.644E-2 (blue).
VðrÞ ¼
ꢀ
ð5Þ
0
ꢀr
jR
A
ꢀ rj
j
A
Z
A
is the charge on nucleus A located at R
A
and
q(r) is the electron
density. The first term in the expression represents the effect of
the nuclei and the second represents that of electrons. The two
terms have opposite signs and therefore opposite effects. V(r) is
their resultant at each point r; it is an indication of the net elec-
trostatic effect produced at the point r by the total charge distri-
bution (electrons + nuclei) of the molecule. Electrostatic potential
correlates with dipole moment, electronegativity, partial charges
and site of chemical reactivity of the molecule. It provides a visual
method to understand the relative polarity of a molecule. While
the negative electrostatic potential corresponds to an attraction
of the proton by the concentrated electron density in the mole-
cule (and is colored in shades of red on the ESP surface), the po-
sitive electrostatic potential corresponds to repulsion of the
proton by atomic nuclei in regions where low electron density ex-
ists and the nuclear charge is incompletely shielded (and is col-
ored in shades of blue). By definition, electron density isosurface
is a surface on which molecule’s electron density has a particular
value and that encloses a specified fraction of the molecule’s elec-
tron probability density. The electrostatic potential at different
points on the electron density isosurface is shown by coloring
the isosurface with contours. The graphical representation of the
molecular electrostatic potential surface (MEP or ESP), as de-
scribed by Politzer and Truhlar [69] is a series of values repre-
senting the evaluation of the interaction energy between
positively charged (proton) probe and points on a solvent accessi-
ble surface as defined by Connolly [70–72].
a
Fig. 8b. Molecular electrostatic potential mapped on the
isodensity surface for 3A7MC.
q(r) = 0.0004 a.u.
The electron density isosurface onto which the electrostatic po-
tential surface has been mapped is shown in Figs. 8a–8c for
3
A7MC. Such surfaces depict the size, shape, charge density and
site of chemical reactivity of the molecules. The different values
of the electrostatic potential at the surface are represented by dif-
ferent colors; red represents regions of most negative electrostatic
potential, blue represents regions of most positive electrostatic
potential and green represents regions of zero potential. Potential
increases in the order red < orange < yellow < green < blue. Two
projections of the ESP surfaces – one in molecular plane and in per-
pendicular planes – in the case of 3A7MC are shown in Figs. 8a and
8
b. These projections clearly show a nucleophilic region sandwich-
ing the -system, leaving a more electrophilic region in the plane
p
of the hydrogen atoms. In the case of 3A7MC molecule (Figs. 8a
and 8b), which contain polar oxygen atoms, the shape of the elec-
trostatic potential surface is influenced by the stereo structure and
charge density distributions in the molecules with sites close to the
oxygen atom of the acetyl group and the carbonyl group showing
regions of most negative electrostatic potential. The ED plot for
molecule shows a uniform distribution (Fig. 8c).
Fig. 8c. Electron density of 3-acetyl-7-methoxycoumarin molecule calculated at
the B3LYP/6-311G(d,p) level of theory (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.).

2
46
L. Joseph et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 234–247
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Acknowledgments
The author Lynnette Joseph would like to thank the University
Grants Commission, New Delhi for its financial support, Project
number: MRP(S)-899/10-11/KLKE002/UGC-SWRO. One of the
authors (Y. Erdogdu) would like to thank Ahi Evran University Re-
search Fund for its financial support, Project numbers: FBA-11-09.
[
[
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[
Appendix A. Supplementary data
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