3
4
A. Raj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 28–36
0
0
1
.2201–1.2441 ÅA and CAN length as 1.4544 ÅA . The experimental
SERS in silver colloid
0
values of NAO bond lengths are 1.222–1.226 ÅA and CAN lengths
0
in the range 1.442–1.460 ÅA [60]. Sundaraganesan et al. [46] re-
In the SERS spectrum of the title compound, the aromatic CAH
stretching vibrations are observed at 3070 and 3060 cm as weak
0
ꢃ1
ported CAN bond lengths as 1.453, 1.460 ÅA (DFT) and NAO bond
0
lengths in the range 1.228–1.248 ÅA . The CNO angles are reported
bands for the phenyl ring PhI, which suggests that the ring PhI may
be in a position close to the perpendicular to the silver surface [70–
72], possibly a tilted position since it is a weak band. It has also
been documented in the literature [73] that when a benzene ring
moiety interacts directly with a metal surface, the ring breathing
[
46] in the range 117.4–118.7° where as for the title compound,
the range is 117.4–117.3°. The DFT calculations give shortening
of angle C AC AS18 by 1.2° and increase of angle C AC AS18 by
.6° from 120° at C position and this asymmetry of exocyclic an-
4
3
2
3
5
3
ꢃ1
gles reveal the repulsion between carboxylic group and the phenyl
ring II [61]. DFT calculations give the shortening of the angle C1-
mode has to be red-shifted by ꢅ10 cm along with substantial
band broadening in the SERS spectrum. Neither a substantial red
shift nor significant band broadening was identified in the SERS
spectrum of the title compound implying that the probability of
AC
20° at C
the hydrogen bonding with H
ment in the angle C AC AC
angles are nearly the same (119.4, 119.7°).
For the carboxylic group, the bond angles O12AC11AO13 = 122.3,
12AC11AC = 112.7, O13AC11AC = 124.9, C11AO12AH32 = 106.0°
2
AN10 by 6.3° and the increase of angle C
position and this asymmetry of exocyclic angle, reveal
, which is evident from the enlarge-
by 3.3°. At C position the exocyclic
3 2
AC AN10 by 2.7° from
1
2
7
a direct ring p-orbital to metal interaction should be low, in accor-
dance with a tilted position of the ring PhI.
In the SERS spectrum of 2-amino,5-nitropyrimidine [74], the
2
symmetric NO stretching mode corresponds to the most intense
1
2
3
6
O
4
4
band, which appears broad and significantly downshifted from
ꢃ1
which are similar to the reported values [62]. For the title com-0
pound C11AO13 = 1.2132, C11AO12 = 1.3545, O12AH32 = 0.9754 ÅA 0
and the corresponding reported values are 1.203, 1.361, 0.965 ÅA
1344 to 1326 cm , suggesting a binding to the silver through
the lone pairs of the oxygen atom. Carrasco et al. [75] observed
ꢃ1
the
tasNO
2
band in the SERS spectrum at ꢅ1500 cm with med-
[
63]. The carbon–oxygen distance unambiguously define the single
ium intensity, which demonstrates the importance of nitro group
and double bonds in the carboxylate group and are in agreement
with the values given by Ng et al. [64] For the carboxyl group, Kad-
in regard to the interaction with the metal. Further, they observed
the enhancement of tPh modes revealing that the molecule is ori-
4
uk [65] reported C AC11 = 1.481 and the in the present case the
ented perpendicular to the metal surface, whereas the changes that
corresponding value is 1.4906 Å.
occur in the nitro group indicate that the interaction occurs
The sulfanyl moiety is twisted from the phenyl ring II and Phe-
through O atoms of the nitro moiety. The interaction induces a p
nyl ring I as is evident from the torsional angles, C
ꢃ170.9, AC AS18AC19 = 151.3, AC AS18AC19 = ꢃ24.2,
AC AC AS18 = 168.5 and C23AC21AC19AS18 = 85.4, C25AC23AC21-
5
AC
4
AC
3
AS18 = -
electronic redistribution primarily around both the nitro group
and the aromatic portion in the vicinity of the substitution site.
Gao and Weaver [76] observed broadening and downshift of the
corresponding band of nitrobenzene adsorbed on gold via the nitro
C
4
3
C
2
3
C
1
2
3
AC19 = 179.8,
ꢃ94.2°. At C
C
24AC22AC21AC19 = ꢃ180.0,
C22AC21AC19AS18 = -
4
position, C AC AC11 is increased by 0.8° and
AC11 is reduced by 2° from 120° which show the interaction
3
4
group. In the SERS spectrum of p-nitroaniline, the
tPh vibration at
ꢃ1
C
5
AC
4
1597 cm is very strong, indicating the interaction between ben-
zene ring and the metal surface. Also, a strong enhancement is ob-
between O13 and S18 atoms.
Analysis of organic molecules having conjugated
systems and large hyperpolarizability using infrared and Raman
p
-electron
served [77] for the symmetric stretching mode of NO
2
at 1336 cm
ꢃ1
and the wagging mode of NO
2
at 865 cm
.
spectroscopy has been evolved as a subject of research [66]. The
For the title compound the symmetric stretching mode of NO
2
is
ꢃ1
first hyperpolarizability (b
0
) of this novel molecular system is cal-
seen at 1319 cm in the SERS spectrum which is absent in the nor-
ꢃ1
culated theoretically, based on the finite field approach. In the
presence of an applied electric field, the energy of a system is a
function of the electric field. First hyperpolarizability is a third rank
tensor that can be described by a 3 ꢂ 3 ꢂ 3 matrix. The 27 compo-
nents of the 3D matrix can be reduced to 10 components due to the
Kleinman symmetry [67].
The components of b are defined as the coefficients in the Taylor
series expansion of the energy in the external electric field. When
the electric field is weak and homogeneous, this expansion becomes
mal Raman spectrum, whereas the computed value is 1332 cm . A
charge transfer from the oxygen atoms of the NO
tal is evidenced by the marked downshift of the symmetric stretch-
ing of the NO group as is detected by the SERS spectrum [74–77].
Interaction through the NO
2
group to the me-
2
2
group was also supported by the pres-
ꢃ1
ence of modes at 1575, 1554, 1353, 780, 736, 703, 532, 509 cm in
the SERS spectrum. According to surface selection rule, vibrations
involving atoms that are close to the silver surface will be en-
hanced [78,79]. The in-plane bending modes dCH of the aromatic
ꢃ1
ring PhI are observed at 1432, 1172, 1066 cm in the SERS spec-
X
X
X
X
1
2
1
6
1
24
i
i
j
i
j
k
i j k l
E ¼ E
0
ꢃ
liF ꢃ
a
ijF F ꢃ
bijkF F F ꢃ
cijklF F F F þ::::
trum. The presence of these modes suggests that the benzene ring
i
ij
ijk
ijkl
PhI is oriented tilted to the silver surface [78,79]. Also
t
PhI modes
i
ꢃ1
where E
0
is the energy of the unperturbed molecule, F is the field at
observed at 1502, 1432, 1172, 1066 cm in the SERS spectrum
supports this fact. According to the surface selection rules
[80,81], the presence of in-plane vibrational modes at 1172,
the origin, li
, aij, bijk and cijkl are the components of dipole moment,
polarizability, the first hyper polarizabilities, and second hyperpo-
ꢃ1
larizibilities, respectively. The calculated first hyperpolarizability
1066, 996 cm and of the out-of-plane vibrational modes at 836,
ꢃ30
ꢃ1
of the title compound is 8.86 ꢂ 10
esu, which is comparable with
462 cm in the SERS spectrum of the title compound suggest that
the reported values of similar derivatives [68] and experimental
evaluation of this data is not readily available. We conclude that
the title compound is an attractive object for future studies of non-
linear optical properties.
there is a certain angle between the ring PhI and the surface of the
silver particle. The substituent sensitive in-plane and out-of-plane
modes are also detected at the same time for the ring PhI, suggest-
ing a tilted orientation of the molecule [49,82]. In the case of SERS
spectrum of thymine molecules on silver [83], the deformation
band of the methyl group attached with the phenyl ring is present
RMS values of wavenumbers were evaluated using the follow-
ing expression [69].
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢃ1
at 1355 cm with the position of the methyl group close to the
X À
Á
1
n
2
calc
i
exp
i
RMS ¼
t
ꢃ
t
metallic surface. In 2-methyl pyridine, Bunding et al. [84] noticed
significant shift and broadening of the CH modes in the SERS spec-
3
trum. They explained this in terms of the interaction of the methyl
group with the metal surface. In the present study, the CH
i
n ꢃ 1
The RMS error of the observed Raman bands is 33.31, 10.13 and that
for IR bands is 33.57, 11.08, respectively, for HF and DFT calculations.