DFT-based molecular modeling, NBO analysis and vibrational spectroscopic study of 3-(bromoacetyl)coumarin

Article abstract of DOI:10.1016/j.saa.2011.07.013

The NIR-FT Raman and FT-IR spectra of 3-(bromoacetyl)coumarin (BAC) molecule have been recorded and analyzed. Density functional theory (DFT) calculation of two BAC conformers has been performed to find the optimized structures and computed vibrational wavenumbers of the most stable one. The obtained vibrational wavenumbers and optimized geometric parameters were seen to be in good agreement with the experimental data. Characteristic vibrational bands of the pyrone ring and methylene and carbonyl groups have been identified. The lowering of HOMO-LUMO energy gap clearly explains the charge transfer interactions taking place within the molecule.

Full text of DOI:10.1016/j.saa.2011.07.013

Spectrochimica Acta Part A 82 (2011) 118–125
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
DFT-based molecular modeling, NBO analysis and vibrational spectroscopic study
of 3-(bromoacetyl)coumarin
D. Sajana,b, Y. Erdogduc,∗, R. Reshmyb, Ö. Derelid, K. Kurien Thomasb, I. Hubert Joee
a Department of Physics, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India
b Department of Chemistry, Bishop Moore College, Mavelikara, Alappuzha 690110, Kerala, India
c Department of Physics, Ahi Evran University, 40040 Kirsehir, Turkey
d A. Keles¸og˘lu Education Faculty, Department of Physics, Selcuk University, Meram 42090, Konya, Turkey
e Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The NIR-FT Raman and FT-IR spectra of 3-(bromoacetyl)coumarin (BAC) molecule have been recorded and
analyzed. Density functional theory (DFT) calculation of two BAC conformers has been performed to find
the optimized structures and computed vibrational wavenumbers of the most stable one. The obtained
vibrational wavenumbers and optimized geometric parameters were seen to be in good agreement with
the experimental data. Characteristic vibrational bands of the pyrone ring and methylene and carbonyl
groups have been identified. The lowering of HOMO–LUMOenergy gap clearly explains the charge transfer
interactions taking place within the molecule.
Received 23 April 2011
Received in revised form 30 June 2011
Accepted 3 July 2011
Keywords:
3-(Bromoacetyl)coumarin
FT-IR and FT-Raman spectra
DFT
© 2011 Elsevier B.V. All rights reserved.
NBO
1. Introduction
[13], and [14], antibacterial [15] and antifungal agents [16], bio-
analytical reagents [17] or as plant growth regulating agents
Coumarin is a chemical compound which is found naturally in
some plants. Coumarin is found in a variety of plants such as tonka
bean, vanilla grass, woodruff, mullein, sweet grass, lavender, sweet
clover grass, and licorice, and also occurs in food plants such as
strawberries, apricots, cherries, and cinnamon. It is thought to work
by serving as a pesticide for the plants that produce it. It can be
synthetically produced as well. It has a distinctive odor, which has
led people to use it as a food additive and ingredient in perfume.
Due to concerns about coumarin as a potential liver and kidney
toxin, its use as a food additive is heavily restricted, although it is
perfectly safe to eat foods which naturally contain coumarin [1,2].
Coumarins are an important class of compounds because of
their applications in synthetic chemistry, medicinal chemistry and
photochemistry. Some coumarins are photoactive; they are also
widely used in organic solid chemistry [3,4], laser dyes [5,6], bio-
logical sensors [7] and molecular switches [8]. Coumarins exhibit
antiviral [9] and antimicrobial activities [10]. Coumarins gen-
erate strong interest stemming from their great physiological
and biological activities, which is further exemplified by their
roles as anticoagulants (dicoumarols and its derivatives, warfarin),
spasmolytics [11], chemotherapeutics [12], biological inhibitors
[18–22]. Coumarin and its derivatives have attracted much interest
due to their optical [23] and biological properties [24].
The vibrational spectroscopic techniques (IR and Raman spec-
troscopy) are powerful molecular structural techniques for the
investigation of biological systems. IR and Raman spectroscopy are
noninvasive methods yielding molecular fingerprint information;
thus, allowing a fast and reliable analysis of complex biological
systems such as bacterial or yeast cells.
The IR and Raman spectra serve as a “spectral fingerprint”
because it provides comprehensive chemical information for char-
acterization and identification of (cell-) biological systems on a
molecular level [25]. Both methods are characterized by a mini-
mal sample preparation and allow for a noninvasive investigation
of biological samples. Raman-spectroscopy is especially suited for
biological applications because it allows in contrast to IR absorp-
tion spectroscopy the analysis of aqueous samples as the Raman
signals of water exhibit low Raman intensities [26,27].
Vibrational assignments for the parent coumarin and its
derivatives such as dihydro-, 6-methyl and 7-methylcoumarins
have been reported based only on spectral correlations [28–31].
IR and Raman spectral investigations of 3-acetylcoumarin
[32], 6-methyl-4-bromomethylcoumarin [33] and 7-methyl-4-
bromomethyl coumarin [34] supported by ab initio density
functional theory studies have been reported. Though the crystal
structure of the title compound, 3-(bromoacetyl)coumarin (BAC)
∗
Corresponding author. Tel.: +90 3862114557; fax: +90 3862114500.
E-mail address: yusuferdogdu@gmail.com (Y. Erdogdu).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.07.013

D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
119
Fig. 1. The conformers and atomic numbering of 3-(bromoacetyl)coumarin.
Table 1
has been reported [35,36], the vibrational spectral features of BAC
crystal have not been subject of detailed analysis so far. The present
work describes the vibrational spectral investigations of BAC aided
by density functional computations to elucidate the correlation
between the molecular structure and biological activity, bonding
features, electron delocalization, and the intramolecular charge
transfer interactions. Furthermore, we interpreted the calculated
spectra in terms of total energy distributions (TEDs) and made the
assignment of the experimental bands.
The selected molecular parameters of conformers of BAC for B3LYP/6-311G(d,p)
level of theory.
Cis conformer
Trans conformer
Energy (a.u.)
−3223.37231972
0.000
−3223.35894185
8.390
Relative energy (kcal mol−1
EHOMO (eV)
)
−7.804
−7.804
ELUMO (eV)
ꢀEHOMO–LUMO (eV)
Dipole moment (Debye)
−5.873
−5.846
−13.677
6.627
−13.650
8.343
2. Experimental
Salicylaldehyde, ethylacetoacetate, piperidine, glacial acetic
acid, ethanol, bromine, and azoisobuteronitrile (AIBN) from
Sigma–Aldrich Chemical Company with a stated purity greater
than 99% were used without further purification to synthesize the
title compound. 3-Bromoacetylcoumarin was synthesized in a two
stage process, viz., Knoevenagel condensation and bromination. In
the first stage, a mixture of salicylaldehyde (0.01 mmol), ethylace-
toacetate (0.01 mmol), piperidine (4 drops), glacial acetic acid (2
drops) and ethanol (20 ml) was refluxed for 5 h and the reaction
mixture was poured into cold water, filtered, washed and dried
to yield 3-acetylcoumarin. The bromination of 3-acetylcoumarin
was carried out using bromine in glacial acetic acid (0.01 mmol)
and AIBN as catalyst at about 60–70 ◦C for 15 min. The crystals of
3-bromoacetylcoumarin formed were collected, washed with min-
imum quantity of glacial acetic acid, dried and purified by repeated
recrystallization from glacial acetic acid to obtain larger size crys-
tals of dimension 28 mm × 7 mm × 3 mm. Purity of the compound
was checked by thin layer chromatography. The infrared spectrum
of the sample was recorded between 4000 cm−1 and 400 cm−1
on a JASCO 400 FT-IR spectrometer that was calibrated using
polystyrene bands. The sample was prepared as a KBr disc. The FT-
Raman spectrum of the sample was recorded the 3500–50 cm−1
region on a Bruker RFS 100/S FT-Raman spectrometer using the
1064 nm excitation from a Nd:YAG laser and a liquid nitrogen
cooled Ge detector.
4. Results and discussion
4.1. Molecular geometries
The two anticipated stable conformers of BAC are shown in
Fig. 1(a) and (b). The energy calculations were carried out for the
two possible conformers, namely cis and trans, of the BAC molecule
by B3LYP method at 6-311G(d,p) levels and the results are tabulated
in Table 1. The energy obtained for cis conformer of BAC molecule
at B3LYP was found to be the global minimum.
For BAC molecule, the initial parameters were taken from the
work of Sparkes [36] and structure optimizations were performed
at the B3LYP levels at 6-311G(d,p) basis set. The optimized bond
lengths and bond angles of the cis conformer of the BAC molecule
at B3LYP level with 6-311G(d,p) as basis set is collected in Table 2.
These optimized geometric parameters of BAC are compared with
X-ray data of 3-(bromoacetyl)coumarin [36]. The DFT predicted
geometric parameters show very good agreement with the exper-
imental data. The averaged CH bond lengths of the coumarin part
˚
(C6H7, C8H9, C10H11, C12H13, C15H16) were determined at 1.085 A.
˚
˚
The C20H21 (1.089 A) and C20H22 (1.089 A) bond lengths for BAC are
slightly larger than those of the coumarin part.
4.2. NBO analysis
3. Computational details
By the use of the second-order bond–antibond (donor–acceptor)
NBO energetic analysis, insight in the most important delocaliza-
tion schemes was obtained. The change in electron density (ED)
in the (ꢁ*, ꢂ*) antibonding orbitals and E(2) energies have been
calculated by natural bond orbital (NBO) analysis [40] using DFT
method to give clear evidence of stabilization originating from
various molecular interactions. NBO analysis has been performed
on BAC in order to elucidate intramolecular hydrogen bonding,
intramolecular charge transfer (ICT) interactions and delocalization
of ꢂ-electrons of the coumarin ring. The hyperconjugative inter-
Gaussian 03 quantum chemical software was used in all calcu-
lations [37]. The optimized structural parameters and vibrational
wavenumbers for the 3-(bromoacetyl)coumarin molecule were
calculated by using B3LYP functional with 6-311G(d,p) as basis set.
The vibrational modes were assigned on the basis of TED analysis
using SQM program [38]. The calculated vibrational wavenumbers
were scaled with 0.967 for B3LYP/6-311G(d,p) in order to figure
out how the calculated data were in agreement with those of the
experimental ones [39].

120
D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
Table 2
Optimized geometric parameters of cis conformer of BAC for B3LYP calculations.
Bond angles (◦)
6-311G(d,p)
X-ray [36]
˚
Bond lengths (A)
6-311G(d,p)
X-ray [36]
Br1–C20
O2–C5
O2–C18
O3–C18
O4–C19
C5–C6
C5–C14
C6–H7
C6–C8
1.957
1.359
1.396
1.202
1.209
1.393
1.405
1.082
1.387
1.083
1.403
1.082
1.382
1.084
1.408
1.427
1.085
1.361
1.465
1.510
1.521
1.088
1.088
1.934
1.368
1.383
1.201
1.214
1.389
1.394
0.950
1.382
0.950
1.407
0.950
1.380
0.950
1.403
1.429
0.950
1.355
1.463
1.502
1.507
0.990
0.990
C5–O2–C18
O2–C5–C6
O2–C5–C14
C6–C5–C14
C5–C6–H7
C5–C6–C8
H7–C6–C8
C6–C8–H9
C6–C8–C10
123.6
117.8
120.8
121.2
119.1
118.7
122.0
119.2
121.0
119.6
119.9
119.8
120.2
120.8
120.3
118.8
118.7
117.3
123.9
120.1
122.1
117.7
119.9
117.3
122.7
116.7
116.0
127.2
119.2
122.8
117.8
112.9
107.6
107.6
110.7
110.7
106.9
122.9
117.3
121.1
121.5
120.6
118.7
120.6
119.5
120.9
119.5
120.2
119.6
120.2
119.8
120.3
119.8
118.8
117.3
123.8
119.0
121.9
119.0
119.9
117.5
122.5
115.9
116.6
127.4
119.1
122.2
118.5
112.4
109.1
109.1
109.1
109.1
107.8
C8–H9
C8–C10
H9–C8–C10
C8–C10–H11
C8–C10–C12
H11–C10–C12
C10–C12–H13
C10–C12–C14
H13–C12–C14
C5–C14–C12
C5–C14–C15
C12–C14–C15
C14–C15–H16
C14–C15–C17
H16–C15–C17
C15–C17–C18
C15–C17–C19
C18–C17–C19
O2–C18–O3
O2–C18–C17
O3–C18–C17
O4–C19–C17
O4–C19–C20
C17–C19–C20
Br1–C20–C19
Br1–C20–H21
Br1–C20–H22
C19–C20–H21
C19–C20–H22
H21–C20–H22
C
10–H11
C10–C12
C12–H13
C12–C14
C14–C15
C15–H16
C15–C17
C17–C18
C17–C19
C
19–C20
C20–H21
20–H22
C
action energy was deduced from the second-order perturbation
approach [41].
4.3. Vibrational analysis
The cis conformer of 3-(bromoacetyl)coumarin (Fig. 1) molecule
belongs to Cs symmetry. BAC molecule consists of 22 atoms, which
has 60 normal modes. The 60 normal vibrations are distributed as
40Aꢂ (in-plane) + 20Aꢂꢂ (out-of-plane). All the vibrations are active
in both IR and Raman spectra. The calculated frequencies together
with the experimental data of BAC molecule are presented in
Table 3. Theoretical and experimental (IR and Raman) spectra of
BAC are given in Figs. 2 and 3. The total energy distribution (TED)
was calculated by using the SQM program [38] and the fundamental
vibrational modes were characterized by their TED.
2
2
Fij
ꢀꢁ|F|ꢁꢁ
E(2) = −nꢁ
= −nꢁ
,
(1)
∗
εꢁ − εꢁ
ꢀE
where ꢀꢁ|F|ꢁ ꢁ 2 or Fi2j is the Fock matrix element between the i and
∗
j NBOs, εꢁ and εꢁ are the energies of ꢁ and ꢁ* NBOs, and nꢁ is the
population of the donor ꢁ orbital.
The Raman activities (Si) calculated with Gaussian 03 program
converted to relative Raman intensities (Ii) using the following rela-
tionship derived from the intensity theory of Raman scattering
[42,43]:
The most important interactions between Lewis and non-Lewis
orbitals with O lone pairs are the second order perturbation 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 O2 and the neighbor ꢂ*(C14–C5), ꢂ*(C18–O3) antibonding
orbital of the benzene ring. The p electrons of C14 and C18 toward the
n(LP2 O2) orbital and the overlap of the donor and acceptor orbitals
are notorious. The energy contribution of (LP2 O4) → ꢁ* (C17–C19),
(LP2 O2) → ꢁ* (C3–O3) and (LP2 O2) → ꢁ* (C5–C14) values are
21.12 kcal mol−1, 36.98 kcal mol−1 and 31.60 kcal mol−1, respec-
tively. This interaction is responsible for a pronounced decrease
of the lone pair orbital occupancy 1.98278 than the other occu-
pancy, and there is a possibility for hyperconjugation between O2
and the benzene ring. An important contribution for the molecu-
lar stabilization is further given by O3 through the overlap of its sp
1.38 lone pair n(LP1 O3) with the ꢁ*(C18–C17) orbital. The energy
f (ꢃ0 − ꢃi)4Si
Ii =
,
(2)
ꢃi[1 − exp(−((hcꢃi)/(kT)))]
with ꢃ0 being the exciting wavenumber in cm−1, ꢃi the vibrational
wavenumber of ith normal mode, h, c and k universal constants
and f is a suitably chosen common normalization factor for all peak
intensities. For visual comparison, the observed and simulated FT-
IR and FT-Raman spectra are presented in Figs. 2 and 3, respectively.
4.3.1. C–H vibrations
The aromatic structure shows the presence of the C–H stretching
vibrations in the region 3100–3000 cm−1 which is the char-
acteristic region for the identification of the C–H stretching
vibrations [47–49]. The C–H vibrations of the 3-acetylcoumarin
contribution of LP2 O3 → ꢁ* (C18–O2) value is 38.22 kcal mol−1
.

D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
121
Table 3
Observed and calculated vibrational modes of cis conformer of BAC (cm−1).
Mode no.
Symmetry
Theoretical
B3LYP/6-311G(d,p)
Experimental
Exp. IR
TED (%)d
Scaled freqa
IIR
IRaman
Exp. Raman
b
c
ꢃ60
ꢃ59
ꢃ58
ꢃ57
ꢃ56
ꢃ55
ꢃ54
ꢃ53
ꢃ52
ꢃ51
ꢃ50
ꢃ49
ꢃ48
ꢃ47
ꢃ46
ꢃ45
ꢃ44
ꢃ43
ꢃ42
ꢃ41
ꢃ40
ꢃ39
ꢃ38
ꢃ37
ꢃ36
ꢃ35
ꢃ34
ꢃ33
ꢃ32
ꢃ31
ꢃ30
ꢃ29
ꢃ28
ꢃ27
ꢃ26
ꢃ25
ꢃ24
ꢃ23
ꢃ22
ꢃ21
ꢃ20
ꢃ19
ꢃ18
ꢃ17
ꢃ16
ꢃ15
ꢃ14
ꢃ13
ꢃ12
ꢃ11
ꢃ10
ꢃ9
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
Aꢂ
Aꢂꢂ
Aꢂꢂ
3104
3093
3078
3074
3071
3053
3005
1747
1713
1596
1582
1537
1467
1431
1374
1345
1324
1275
1245
1233
1203
1145
1124
1124
1105
1013
1009
980
968
935
928
904
855
849
808
754
747
732
721
720
646
623
560
542
0.5
4.6
4.3
2.9
1.0
0.6
0.8
2.5
7.3
6.4
76
3109 w
3083 vw
3088 vw
3067 w
ꢃCH(99)
ꢃCH(100)
ꢃCH(99)
ꢃCH(98)
ꢃCH(98)
ꢃCH(100) CH2
ꢃCH(100) CH2
ꢃCO(85)
ꢃCO(86)
8a ꢃCC(67)
8bꢃCC(63)
19a ꢃCC(66) + ˇCCH(11)
ꢃCC(37) + ˇCCH(41)
19b ꢃCC(34) + ˇCCH(45)
ıCH2 (56)
ꢃCC(35) + ˇCCH(36)
14ꢃCC(84)
ꢃCC(20) + ωCCH(29)
ꢃCC(14) + ˇCCH(35) + ωHCBr(15)
ꢃCO(11) + ꢃCC(17) + ωCCH(23)
ꢃCO(22) + ꢃCC(28) + ˇCCH(26)
ꢃCC(12) + ꢄCCH(62)
ꢃCC(31) + ˇCCH(28)
ˇHCBr(32) + ꢄCCH(57)
ꢃCC(27) + ˇCCH(47)
ꢃCC(60) + ˇCCH(24)
ꢃCC(37) + ˇOCC(13)
CCH(40) + TCCCC(38)
CCH(66) + TCCCH(10)
CCH(43) + TCCCH(34)
ꢃCO(53) + ꢃCC(19)
ıCCC(43) + ˇCCH(12)
ꢅCH2 (10)(13) + CCH(33) + TOCCBr(10)
TOCCH(11) + CCH(40) ꢅCH2 (10)
ꢃCC(61) + ˇCCC(10)
TOCCC(36) + CCC(12) ꢅCH2 (10)
CCH(65)
ꢃCC(18) + ˇCCC(33) + ˇOCC(12)
ꢃCC(32) + ˇOCC(21)
TCCCC(40) + CCH(21) + TCCCO(11)
ꢃCC(10) + ꢃCBr(34) + ˇCCC(14)
ˇCCO(14) + ˇOCO(10) + ˇCCC(25)
ˇCCO(19) + ˇCCC(22)
1.8
0.7
0.3
0.0
0.1
2.9
13
100
27
3067 s
3026 s
2994 vw
2959 m
3046 m
2960 s
1746 w
1731 vs
1603 s
1577 vw
1556 vs
1486 m
1449 s
1719 m
1607 s
1599 s
1559 vs
1489 w
1453 m
1362 w
1335 m
1304 vw
1271 w
11
39
1.6
20
100
9.2
14
2.7
0.8
44
3.8
7.7
0.4
40
6.4
1.9
3.9
1.1
34
6.4
2.9
2.2
0.0
1.3
2.3
4.0
0.6
15
31
0.2
5.3
0.2
3.6
1.2
0.1
0.1
1370 m
1301 m
1269 vw
1226 vw
1216 vw
1205 vw
1153 m
1126 w
1216 s
1148 vw
1123 vw
1053 m
1030 vw
992 w
1059 s
1028 m
17
1.2
0.0
0.0
15
2.9
0.8
0.3
13
1.8
1.5
4.5
65
0.7
43
21
8.4
21
0.7
1.0
1.8
70
983 m
982 m
14
919 s
1.2
1.0
2.6
0.1
876 m
830 w
739 s
12
755 s
734 vw
0.0
0.9
0.0
0.3
10
0.2
2.2
670 s
633 w
573 m
553 s
672 s
635 w
568 vw
552 w
10
ꢃCBr(16) + ˇCCC(16) + ˇCCO(27)
ꢅCH (10) + TCCCC(42) + CCH(16)
542
530
454
442
373
366
319
270
228
221
168
101
94
94
61
20
0.0
0.1
0.8
0.1
0.0
0.6
0.8
0.2
0.4
0.2
0.4
0.3
0.9
0.3
0.4
0.3
ꢅCH2 (15) + TOCCC(21) + CCH(27)
2
452 w
410 vw
454 m
377 w
328 vw
238 w
TCCCC(36) + CCH(17)
ꢃCO(14) + ꢃCC(11) + ˇCCC(28) + ˇCCO(11)
CCH(15) + TCCCC(46) + ꢅCH2 (10)
ꢃCC(19) + ˇOCC(24) + ˇCCC(18)
ˇCCH(32) + ˇCCO(34)
21
9.2
18
0.0
0.3
0.2
0.7
0.4
1.6
0.7
0.9
6.9
TCCCO(27) + TCCCC(28)
ꢃ8
ꢃ7
ꢃ6
ꢃ5
ꢃ4
ꢃ3
ꢃ2
ꢃ1
ˇCCC(44) + ˇCCBr(13) + ˇCCO(13)
CCC(28) + TCCCO(27) + ꢅCH2 (13)
ꢃCC(21) + ꢃCBr(24) + ˇCCBr(20)
OCCC(19) + TCCCC(25) + TCCOC(25)
182 m
120 s
TOCCBr(18) + ꢅCH (16) + ꢄCCCH(13)
2
ˇOCC(10) + ˇCCC(51) + ˇCCBr(21)
OCC(54) + ꢅCH2 (14)
TCCCBr(20) + OCC(39) + TCCCH(15)
ꢃ: stretching, ꢅ: rocking, ω: wagging, : out-of-plane bending, ˇ: in-plane bending, ı: scissoring, ꢄ: twisting, T: torsion, vw: very week, w: week, m: medium, s: strong, vs:
very strong.
a
Scaling factor: 0.967.
b
Relative absorption intensities normalized with highest peak absorption equal to 100.
Relative Raman intensities calculated by Eq. (2) and normalized to 100.
Total energy distribution calculated B3LYP/6-311G(d,p) level, TED less than 10% is not shown.
c
d
are observed at 3079, 3068, 3046 and 3029 cm−1 in the FT-IR
spectrum and at 3081, 3070, 3047 and 3029 cm−1 in the FT-
Raman [34,50]. The observed bands in IR at 3067, 3088 and
3109 cm−1 attributed to C–H stretching vibrations of coumarin
part. DFT computations predict these modes at 3074, 3078 and
3104 cm−1 for B3LYP/6-311G(d,p) level of theory. The results
showed that the experimental and theoretical data were in good
agreement.

122
D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
Fig. 2. Theoretical (B3LYP with 6-311G(d,p) (a)) and experimental (b) (powder form) infrared spectra of 3-(bromoacetyl)coumarin.
The C–H stretching modes of acetyl group in the 3-
intense bands [47–49]. This scissoring mode is characterized by
the IR band at 1370 cm−1 and Raman at 1362 cm−1, which is com-
puted at 1374 cm−1. In BAC the wagging vibrations of CH2 are found
in both IR and Raman spectrum near 1270 cm−1 which is justified
by our DFT calculation also. The twisting and rocking vibrations
of CH2 group are observed at 1148 cm−1 and 830 cm−1 in Raman
spectrum respectively which is supported by results reported in
literature [47–49].
The in plane C–H bending of aromatic compounds vibra-
tions [47–49] appears in the region 1400–1050 cm−1. The C–H
in plane bending vibrations may be identified in BAC at 1335,
1148 and 1059 cm−1 in Raman spectrum are also supported by
computed results. These modes are observed in the IR spectrum
also in the same region. The absorption bands arising from C–H
out of plane bending vibrations [47–49] are usually observed
in the region from 1000 cm−1 to 675 cm−1. The out of plane
bending vibrations in BAC are observed as medium bands at
982 cm−1, 830 cm−1 and 739 cm−1 in Raman spectrum and the
corresponding IR bands are at 983 cm−1 and 734 cm−1, respec-
tively. Bromine compounds [48] show stretching vibrations in
the region 750–485 cm−1 due to C–Br. In the present case, the
band at 670 cm−1 in IR and 672 cm−1 in Raman is assigned
as the C–Br stretching mode. The C-Br deformations [49] are
found in the region 400–140 cm−1. The observed bands in Raman
at 182 cm−1 and 238 cm−1 are assigned to C-Br deformation
mode.
acetylcoumarin were observed at 2984, 2959 and 2931 cm−1 in the
FT-IR spectra [33,51]. The CH stretching vibrations of the acetyl
group were determined lower than those of the coumarin part.
The antisymmetric stretching (ꢃasCH2) and symmetrical stretch-
ing (ꢃsCH2) bands of methylene groups occur near 2926 and
2853 cm−1, respectively [47–49]. The qualitative interpretation
of intensities must rely upon the understanding of some basic
aspects of intramolecular charge distribution and on their effects
on infrared intensities. The methylene group hydrogen atoms in
BAC are subjected to the electronic effect of hyperconjugation
leading to the decrease of infrared intensities and blue shifting
of stretching wavenumbers, as discussed in the preceding methyl
vibrations section. The methylene antisymmetric stretching man-
ifests its characteristic band at 2994 cm−1 in Raman as weak band
and DFT computations predict this mode at 3053 cm−1. The sym-
metric stretching vibration is observed at 2960 cm−1 in IR spectrum
and at 2959 cm−1 in Raman. The changes in intensity of the CH2
stretching mode in infrared spectrum and shift the wavenumber to
the higher wavenumbers owing to the proximity of the high elec-
tronegative bromine atom resulting from hyperconjugation of the
methylene group with the aromatic ring system [52].
The four bending vibrations of C–H bonds in the methylene
groups are referred to as scissoring, twisting, wagging and rock-
ing and they are identified. The (ıs CH2) scissoring vibrations are
expected in the region 1455–1380 cm−1 and consist of medium
Fig. 3. Theoretical (B3LYP with 6-311G(d,p) (a)) and experimental (b) (powder form) Raman spectra of 3-(bromoacetyl)coumarin.

D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
123
4.3.2. Carbonyl vibrations
The acetyl carbonyl group bonded to the pyrone ring gives
rise to a series of characteristic bands corresponding to the car-
bonyl and methylene moieties. The intensity of these bands can
increase because of conjugation or formation of hydrogen bonds.
The increase of conjugation, therefore, leads to the intensification
of the Raman lines as well as increased infrared band intensities.
The carbonyl stretching wavenumber of the 3-acetylcoumarin is
observed sharp intense bands in IR spectrum at 1684 cm−1, which
is also observed in Raman at 1675 cm−1 as medium intensity bands
[34,50]. The sharp intense band in infrared spectrum at 1731 cm−1
is assigned to C19 O4 stretching mode, which is also observed
in the Raman spectrum at 1719 cm−1 as a medium intense band.
The results of computations give the wavenumber of this mode
as 1713 cm−1. It is evident that the C19 O4 stretching mode is blue
shifted by 25 cm−1 compared to 3-acetylcoumarin. This means that
in the acetyl part C O vibration is sensitive to the proximity of
the high electronegative bromine atom. The blue shifting of C
O
stretching wavenumber is further influenced by the hyperconjuga-
tion with the C18–C17 bond, which is confirmed by NBO analysis.
The carbonyl bands are most characteristic bands of the IR and
Raman spectrum [44–46]. The carbonyl stretching vibrations are
found in the region 1780–1700 cm−1 [47–49]. A weak IR absorp-
tion band at 1746 cm−1 corresponds to the carbonyl vibration in the
pyrone ring [32–34,50]; the corresponding theoretical band with
the comparable intensity at 1747 cm−1. The C18 O3 bond has in
the alpha position atom O2 and belongs to the ester group, there-
fore its wavenumber is higher than the wavenumber of the ketone
carbonyl group C19 O4. In BAC, the C O bond in the lactone part is
conjugated with C15 C17 and C14 C5 double bonds. In the present
study, the carbonyl vibrational wavenumbers in the lactone part
C18 O3 stretching have been lowered to different extents due to
conjugation, which is supported by NBO results also. The C18–O2
Fig. 4. Experimental UV spectrum of BAC (a) and TD-DFT//B3LYP/6-311G(d,p) level
UV spectrum of BAC (b).
substituent group vibrations. The ring in-plane, and out-of-plane
bending vibrations have been identified and presented in Table 3.
They are also supported by the literature [47–49]. The simultaneous
IR and Raman activation of the ring mode and carbonyl mode clearly
explains the charge transfer interaction between the electron-
donating group and the acceptor group through the ꢂ-conjugated
system. The ꢂ-electron cloud movement from the donor to the
acceptor can make the molecule highly polarized through the sin-
gle/double path, when it changes from the ground state into the
first excited state [52–54]. It is inferred that this mechanism plays
an important role in the biological activity of BAC.
and C5–O2 stretching bands are observed at 1216 and 919 cm−1
,
respectively, in IR spectrum as medium intensity bands whereas
the corresponding Raman bands are observed at nearby positions,
at 1216 cm−1, respectively. The C–O skeletal mode is less stable and
sensitive; it may be assigned to a weak Raman band at 971 cm−1
.
4.3.3. Ring vibrations
The C–C ring stretching vibrations have given rise to character-
istic bands in both the observed IR and Raman spectra, covering
the spectral range from 1610 to 1300 cm−1 [32–34,47–49]. The
observed bands at 1617, 1603, 1556, 1338, 1109, 807 and 748 cm−1
in both infrared and Raman spectra for acetyl coumarin have been
attributed to the ring stretching vibrations [34,50]. In BAC, the ring
mode manifests as strong bands in Raman spectrum at 1607 cm−1
and 1599 cm−1. The observed bands at 1603 cm−1 and 1577 cm−1
in IR correspond to ring mode, which are localized on the benzene
part of the molecule. These vibrations correspond to the eg mode
(8a) of benzene [51] at 1610 cm−1. The band at 1559 cm−1, 1489
and 1453 cm−1 in infrared spectrum contain C–C stretching and
aromatic C–H in plane bending character as well as minor contri-
butions from other modes [47–49]. These modes are observed in the
Raman spectrum also in the same region. Both modes are mainly
vibrations of the benzene ring and correspond to the e1u mode of
benzene at 1484 cm−1, which is calculated to be 1476 cm−1. The
intense band respectively for both in infrared and Raman spectrum
at 1301 cm−1 and 1304 cm−1 is predominantly a C–C stretching of
benzene, which is justified by PED results also. The infrared bands
identified at 633 cm−1 and 573 cm−1, for BAC have been designated
to ring in-plane bending modes, by careful consideration of their
quantitative descriptions. Small changes in wavenumber observed
for these modes are due to the changes in force constant/reduced
mass ratio, resulting mainly due to addition of bromoacetyl group
to coumarin and from different extents of mixing between ring and
4.4. Electronic spectra
The chemical structure of BAC is composed of a conjugated
system of double bonds and aromatic ring. Natural bond orbital
analysis indicates that molecular orbitals are mainly composed of
ꢂ atomic orbital, so above electronic transitions are mainly derived
from the contribution of bands ꢂ–ꢂ*. UV–vis absorption spectrum
of the sample in Methanol is shown in Fig. 4. The strong transitions
with ꢆmax at 363 and 293 nm have been observed are assigned to a
ꢂ–ꢂ* transition. The ꢂ–ꢂ* transitions with ꢆmax at 324 nm is mainly
localized in the pyrone ring [55]. The TD-DFT is particularly well
suited to low energy valence excited states that can be described
by combinations of single excitations. These are identified with
the calculated transitions at 270 and 188 nm. In this case, the first
energy transition is blue shifted from the experimental value by
90 nm because the theoretical spectrum was obtained in gas phase
without considering the solvent effect of the molecule. This elec-
tronic absorption corresponds to the transition from the ground to
the first excited state and is mainly described by one electron exci-
tation from the highest occupied molecular orbital (HOMO) to the
lowest unoccupied molecular orbital (LUMO).

124
D. Sajan et al. / Spectrochimica Acta Part A 82 (2011) 118–125
Fig. 5. HOMO (a1, a2) and LUMO (b1, b2) plots of cis and trans conformers.
4.5. HOMO, LUMO analysis
out their stability, the optimized geometry of the most stable con-
former and its vibrational spectrum. The energy calculations were
carried out for the two possible conformers namely cis and trans
of the BAC molecule. The energy obtained for cis conformer of
BAC molecule at B3LYP was found to be the global minimum.
The vibrational modes were assigned on the basis of TED analy-
sis and analyzed by using SQM program. The observed vibrational
wavenumbers and optimized geometric parameters were seen to
be in good agreement with the experimental data. The intramolec-
ular charge transfers (ICT), ꢂ-electron delocalization and its related
processes have been comprehensively investigated for the prospec-
tive BAC from vibrational spectroscopy and DFT computations. The
lowering of HOMO–LUMO energy gap clearly explains the charge
transfer interactions taking place within the molecule. It is pos-
sible to understand intramolecular charge transfer mechanisms
between the groups by comparing the charges on atoms.
The conjugated molecules are characterized by a small highest
occupied molecular orbital–lowest unoccupied molecular orbital
(HOMO–LUMO) separation, which is the result of a significant
degree of intramolecular charge transfer (ICT) from the end-
capping electron-donor groups to the efficient electron-acceptor
groups through ꢂ-conjugated path. The strong charge transfer
interaction through ꢂ-conjugated bridge results in substantial
ground state donor–acceptor mixing and the appearance of a
charge transfer band in the electronic absorption spectrum. Con-
sequently, an ED transfer occurs from the more aromatic part
of the ꢂ-conjugated system in the electron donor side to its
electron-withdrawing part mainly of quinonoid form. Furthermore,
the HOMO and LUMO topologies show certain overlap of two
orbitals in the middle region of the ꢂ-conjugated systems, which
is a prerequisite to allow an efficient charge transfer transition
[52–54]. The HOMO is located over acetyl carbonyl group, and the
HOMO → LUMO transition implies an electron density transfer to
the bromoacetyl group from the coumarin ring. The atomic orbital
compositions of the frontier molecular orbitals of cis and trans con-
formers of BAC are sketched in Fig. 5. The HOMO–LUMO energy gap
of BAC was calculated at the B3LYP/6-311G(d,p) level, which reveals
that the energy gap reflects the chemical activity of the molecule.
The LUMO as an electron acceptor represents the ability to obtain
an electron, and HOMO represents the ability to donate an elec-
tron. The energy gap (−13.677 eV) of HOMO–LUMO explains the
eventual charge transfer interaction within the molecule, which
influences the biological activity of the molecule. Consequently,
the lowering of the HOMO–LUMO band gap is essentially a con-
sequence of the large stabilization of the LUMO due to the strong
electron-acceptor ability of the electron-acceptor group.
Acknowledgements
We thank referees for their valuable suggestions. The author
(Y. Erdogdu) would like to thank Ahi Evran University Research
Fund for its financial support, Project Numbers: A10/2009. We also
thank Assoc. Prof. Dr. Mustafa Kurt for the Gaussian 03W program
package.
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