IR, Raman and SERS spectra of 2-phenoxymethylbenzothiazole

Article abstract of DOI:10.1016/j.saa.2009.05.022

The FT-IR and FT-Raman spectra of 2-phenoxymethylbenzothiazole were recorded and analyzed. The surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational wavenumbers of the compound have been computed using the Har

Full text of DOI:10.1016/j.saa.2009.05.022

Spectrochimica Acta Part A 74 (2009) 132–139
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
IR, Raman and SERS spectra of 2-phenoxymethylbenzothiazole
a,∗
b
c
d
e
C. Yohannan Panicker , Hema Tresa Varghese , Asha Raj , K. Raju , Tugba Ertan-Bolelli ,
e
e
f
f
Ilkay Yildiz , Ozlem Temiz-Arpaci , Carlos M. Granadeiro , Helena I.S. Nogueira
Department of Physics, TKM College of Arts and Science, Kollam, Kerala, India
Department of Physics, Fatima Mata National College, Kollam, Kerala, India
Department of Physics, Government Polytechnic College, Attingal, Kerala, India
Department of Physics, University College, Trivandrum, Kerala, India
Ankara University, Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Tandogan 06100, Ankara, Turkey
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
a
b
c
d
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
The FT-IR and FT-Raman spectra of 2-phenoxymethylbenzothiazole were recorded and analyzed. The
surface enhanced Raman scattering (SERS) spectrum was recorded in a silver colloid. The vibrational
wavenumbers of the compound have been computed using the Hartree–Fock/6-31G* basis and compared
Received 22 December 2008
Received in revised form 8 May 2009
Accepted 18 May 2009
−1
with the experimental values. The appearance of the Ag–O stretching mode at 237 cm in the SERS
spectrum along with theoretically calculated atomic charge density, leads us to suggest that the molecule
is adsorbed through the oxygen atom with the molecular plane tilted on the colloidal silver surface. The
direction of charge transfer contribution to SERS has been discussed from the frontier orbital theory.
Keywords:
IR
Raman
SERS
©
2009 Elsevier B.V. All rights reserved.
Benzothiazole
Hartree–Fock ab initio calculations
1
. Introduction
drugs. Benzothiazoles constitute an important class of compounds
with profound interest to medicinal/industrial chemists as com-
pounds bearing the benzothiazolyl moiety. They exhibit diverse
biological properties such as antitumour [15,16], antimicrobial [17],
antiglutamate/antiparkinson [18], broad spectrum Ca2+ channel
antagonist [19], inhibition of enzymes such as aldose reductase
[20], monoamine oxidase [21], lipoxygenase [22], cyclooxyge-
nase [23], acetylcholine esterase [24], thrombine [25], proteases
[26], H –K ATpase [27], carbonic anhydrase [28], HCV helicase
[29], plant growth regulation [30] and have industrial applica-
tions such as antioxidants [31], and vulcanization accelerators
[32]. 2-Phenoxymethyl benzothiazole [33,34] was resynthesized
for evaluating its biological activity. This structure was found to
be very potent either against Gram positive bacteria Staphylococcus
aureus with a MIC value [34] of 3.12 ␮g/ml or as an eukaryotic Topoi-
somerase II inhibitor [35]. Even, 2-phenoxymethylbenzothiazole
(IC50 value of 11.4 ␮M) was found to be more active than the ref-
erence drug etoposide (IC50 value of 21.8 ␮M). Considering the
enormous biological importance, we present here the detailed
experimental and theoretical normal Raman spectroscopy, SERS
and FT-IR spectra of the title compound.
Surface enhanced Raman scattering (SERS) spectroscopy is a
well-established and highly effective technique of observing Raman
scattering from species present at trace concentrations [1,2]. It
is a useful tool in surface chemistry because of its high sen-
sitivity and potential in providing useful information regarding
metal–adsorbate interactions [3,4]. The compounds containing a
thiazole ring have shown useful biological properties and have been
developed as fungicides, herbicides, or plant growth regulators [5].
The biological importance of thiazole derivatives was emphasized
during the period 1941–1945, when research on the structure of
the antibiotic penicillin showed the presence of a thiazolidine ring
in this important therapeutic agent [6]. The 2-aminobenzothiazole
molecule is known for its local anaesthetic action and has numer-
ous applications in human and veterinary medicine [7]. It is a
metabolite of methabenzthiazuron [8] and is reported to form
the main fraction of soil-bound residues [9]. Anthelmintics exert
their chemotherapeutic effect by interfering with some biochem-
ical or physiological processes essential for the survival of the
parasite in the host [10]. Several substituted benzimidazoles and
benzothiazoles [11–14] have been identified as potent authelmintic
+
+
2. Experimental
∗ Corresponding author.
E-mail address: cyphyp@rediffmail.com (C.Y. Panicker).
The FT-IR spectrum (Fig. 1) was recorded on a Pye Unicam SP-
1025 spectrometer with KBr pellets. Raman spectra (Figs. 2 and 3)
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.05.022

C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
133
was induced by addition of an aqueous solution of MgCl (1 drop of
2
−
3
a 2 mol dm solution). Polyvinylpyrrolidone was then used to sta-
bilize the colloid (1 drop of 0.1 g/10 cm aqueous solution). The final
3
colloid mixture was placed in a glass tube and the Raman spectrum
was recorded.
3. Synthesis of 2-phenoxymethylbenzothiazole
The general synthetic procedure was employed to prepare the
compound 2-phenoxymethylbenzothiazole involving the reaction
of the phenoxyacetic acid with 2-aminothiophenol by refluxing
in the presence of dehydrating agents in one step procedure. For
the preparation of the title compound trimethylsilyl polyphosphate
ester (PPSE) was used as the cyclodehydration reagent in the ring
closure reactions [38].
Fig. 1. IR spectrum of 2-phenoxymethylbenzothiazole.
A
mixture of phenoxyacetic acid (5 mmol) and 2-
aminothiophenol (6.9 mmol) was heated under reflux with
◦
stirring for 3 h at 140 C in 15 ml PPSE. At the end of the reaction
period, the mixture was taken to 30 ml dichloromethane and
neutralized with 50 ml 1N NaOH solution. The organic layer was
separated and the aqueous solution extracted with 3× 25 ml
portions of CH Cl . The combined extracts were dried on Na SO ,
2
2
2
4
filtered and the solvent was removed with rotary evaporator. The
residue was purified by flash chromatography, eluting with CHCl3
and the obtained product was recrystallized using CH Cl /hexane.
2
2
◦
Yield 72%. Melting point 77–78 C [34].
.00–7.80 (m, 2H, C-5H(H9) and C-2H(H8)), 7.40–6.90 (m, 7H,
C-6H(H10), C-1H(H7) and phenyl protons), 5.47 (s, 2H, CH (C-14)).
8
2
4. Computational details
Calculations of the title compound were carried out with Gaus-
Fig. 2. Raman spectrum of 2-phenoxymethylbenzothiazole.
sian03 program [39] using the HF/6-31G* basis set to predict
the molecular structure and vibrational wavenumbers. Molecular
geometry was fully optimized by Berny’s optimization algo-
rithm using redundant internal coordinates. Harmonic vibrational
wavenumber values were calculated using the analytic second
derivatives to confirm the convergence to minima on the poten-
tial surface. The wavenumber values computed at the Hartree–Fock
level contain known systematic errors due to the negligence of elec-
tron correlation [40]. We therefore, have used the scaling factor
value of 0.8929 for HF/6-31G* basis set. Parameters correspond-
ing to optimized geometry of the title compound (Fig. 3) are
given in Table 1. The absence of imaginary values of wavenumbers
on the calculated vibrational spectrum confirms that the struc-
ture deduced corresponds to minimum energy. The assignment of
the calculated wavenumbers is aided by the animation option of
MOLEKEL program, which gives a visual presentation of the vibra-
tional modes (Fig. 4) [41,42].
were recorded on a Bruker RFS 100/s FT instrument (Nd:YAG laser,
1
064 nm excitation). The 1H NMR spectra were obtained with a
Bruker AC 80 MHz spectrometer and TMS was used as an internal
standard. Elemental analysis was carried out with a Perkin-Elmer
model 240-C apparatus. The results of the elemental analysis (C,
H, N) were within ± 0.4% of the calculated amounts. The aque-
ous silver colloid used in the SERS experiments was prepared by
reduction of silver nitrate by sodium citrate, using the Lee–Meisel
method [36,37]. Solutions of the compound were made up in
3
ethanol (0.1 mmol in 1 cm of solvent) and transferred by a microsy-
3
ringe into the silver colloid (10 ␮l in 1 cm of the colloid) such that
−3
−3
the over all concentration was 10 mol dm . Colloid aggregation
5. Results and discussion
5.1. IR and Raman spectra
The observed IR and Raman bands with their relative intensi-
ties, calculated values and assignments are given in Table 2. The
N stretching skeletal bands [43–46] are observed in the range
C
−
1
−1
1672–1566 cm . Saxena et al. [44] reported a value 1608 cm for
polybenzodithiazole and Klots and Collier [47] reported a value of
−
1
1
[
517 cm for benzoxazole as ꢀC N stretching mode. Yang et al.
−
1
48] reported a band at 1626 cm
in the IR spectrum as ꢀC N
for the oxazole ring. For the title compound, the HF calculations
−
1
give ꢀC N mode at 1622 cm and experimentally no bands are
observed.
Fig. 3. SERS spectrum of 2-phenoxymethylbenzothiazole.

1
34
C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
Table 1
Optimized geometrical parameters of 2-phenoxymethylbenzothiazole, atom labeling is according to Fig. 4.
◦
◦
Bond lengths (Å)
Bond angles ( )
Dihedral angles ( )
C1–C2
C1–C6
C1–H7
C2–C3
C2–H8
C3–C4
C3–S11
C4–C5
C4–N12
C5–C6
1.3846
2.3972
1.0727
1.3851
1.0718
1.3921
1.8095
1.3888
1.4011
1.3816
1.0714
1.0724
1.8181
1.2704
1.4895
1.0825
1.0825
1.4307
1.3901
1.3846
1.3846
1.3874
1.0721
1.3874
1.0721
1.3884
1.0727
1.3884
1.0727
1.0726
A(2,1,6)
A(2,1,7)
A(6,1,7)
A(1,2,3)
A(1,2,8)
A(3,2,8)
A(2,3,4)
A(2,3,11)
A(4,3,11)
A(3,4,5)
A(3,4,12)
A(5,4,12)
A(4,5,6)
A(4,5,9)
A(6,5,9)
A(1,6,5)
120.9
119.5
119.6
118.2
120.7
121.1
121.2
129.1
109.7
120.4
114.9
124.8
118.7
119.4
121.9
120.7
119.5
119.8
86.7
113.3
115.4
121.5
123.1
109.7
109.7
108.1
108.6
110.4
110.4
118.8
119.5
119.5
121.0
119.3
119.4
121.2
119.3
119.4
121.2
120.2
119.7
120.1
120.2
119.7
120.1
119.8
120.1
120.1
D(6,1,2,8)
D(7,1,2,3)
D(2,1,6,10)
D(7,1,6,5)
D(1,2,3,11)
D(8,2,3,4)
D(2,3,4,12)
D(11,3,4,5)
D(2,3,11,13)
D(3,4,5,9)
D(12,4,5,6)
D(5,4,12,13)
D(4,5,6,10)
D(9,5,6,1)
D(3,11,13,14)
D(4,12,13,14)
D(11,13,14,15)
D(11,13,14,16)
D(12,13,14,15)
D(12,13,14,16)
D(12,13,14,17)
D(13,14,17,18)
D(16,14,17,18)
D(16,14,17,18)
D(14,17,18,19)
D(14,17,18,20)
D(17,18,19,21)
D(17,18,19,22)
D(20,18,19,21)
D(20,18,19,22)
D(17,18,20,23)
D(17,18,20,24)
D(19,18,20,23)
D(19,18,20,24)
D(18,19,21,26)
D(22,19,21,25)
D(22,19,21,26)
D(18,20,23,27)
D(24,20,23,25)
D(24,20,23,27)
D(19,21,25,23)
D(19,21,25,28)
D(26,21,25,23)
D(26,21,25,28)
D(20,23,25,21)
D(20,23,25,28)
D(27,23,25,21)
D(27,23,25,28)
180.0
−180.0
−180.0
180.0
180.0
−180.0
−180.0
−180.0
180.0
−180.0
180.0
−180.0
−180.0
180.0
−180.0
180.0
−120.4
120.4
59.6
C5–H9
C6–H10
S11 –C13
N12 –C13
C13– C14
C14 –H15
C14 –H16
C14 –O17
O17 –C18
C18 –C19
C18 –C20
C19 –C21
C19 –H22
C20–C23
C20–H24
C21 –C25
C21 –H26
C23–C25
C23–H27
C25–H28
A(1,6,10)
A(5,6,10)
A(3,11,13)
A(4,12,13)
A(11,13,12)
A(11,13,14)
A(12,13,14)
A(13,14,15)
A(13,14,16)
A(13,14,17)
A(15,14,16)
A(15,14,17)
A(16,14,17)
A(14,17,18)
A(17,18,19)
A(17,18,20)
A(19,18,20)
A(18,19,21)
A(18,19,22)
A(21,19,22)
A(18,20,23)
A(18,20,24)
A(23,20,24)
A(19,21,25)
A(19,21,26)
A(25,21,26)
A(20,23,25)
A(20,23,27)
A(25,23,27)
A(21,25,23)
A(21,25,28)
A(23,25,28)
−59.6
180.0
180.0
−60.1
60.1
90.9
−90.9
178.6
−0.4
0.4
−178.6
−178.6
0.4
−0.4
178.6
−179.7
178.9
−0.8
179.7
−178.9
0.8
−0.3
−179.9
179.4
−0.2
0.3
179.9
−179.4
0.2
at 2873 cm 1. The bands observed at 2905 cm (IR), 2909 cm
−1
−
−1
The vibrations of the CH group, the asymmetric stretch ꢀasCH2,
2
−
1
−1
symmetric stretch ꢀsCH , scissoring vibration ıCH , appear in
(Raman) and 2840 cm (IR), 2846 cm (Raman) are assigned as
asymmetric and symmetric CH modes, respectively. In the present
case, the band observed at 1497 cm
2
2
−
1
the region 2945 ± 45, 2885 ± 45 and 1445 ± 35 cm , respectively
2
−
1
−1
[
49,50]. The HF calculations give ꢀasCH at 2920 cm and ꢀsCH2
in the IR spectrum and
2
Fig. 4. Optimized geometry of 2-phenoxymethylbenzothiazole.

C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
135
Table 2
Calculated vibrational wavenumbers, measured infrared and Raman bands positions and assignments of 2-phenoxymethylbenzothiazole.
(HF) (cm 1
−
)
ꢀ
−1
(IR) (cm
)
ꢀ
−1
(Raman) (cm
)
ꢀ
(HSERS (cm
−1
)
Assignments
ꢀ
3
3
037
035
3088 w
3052 w
–
3029 w
–
3099 w
3068 vvs
3033 sh
–
3060 s
–
ꢀCH II
ꢀCH I
ꢀCH II
ꢀCH I
ꢀCH I
ꢀCH II
ꢀCH I
ꢀCH II
ꢀCH I
3
3
3
3
031
029
021
017
–
–
–
–
–
–
–
–
3
3
009
004
2999
2920
2873
2985 w
2905 w
2840 w
–
2987 vw
2909 m
2846 m
–
2994 w
2935 wbr
ꢀ
ꢀ
asCH2
sCH2
1
622
612
599
594
578
497
485
462
455
452
372
326
289
245
230
223
214
169
163
153
144
118
064
053
024
022
016
009
007
005
ꢀC N
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
–
–
ꢀPh I
ꢀPh I
ꢀPh II
ꢀPh II
ꢀPh I
ıCH2
ꢀPh II
ꢀPh I
ꢀPh II
ωCH2
ꢀPh I, CN
ıCH I
ꢀPh II
ꢀC18–O17
ıCH II
␶CH2
ıCH I
ꢀCC
ıCH II
ıCH I
ıCH II
1601 s
1587 s
1559 w
1522 s
1497 s
–
1454 m
1436 m
1357 s
1315 m
1294 w
1278 vw
1259 vvs
–
1588 m
1586 sh
1560 m
1523 s
–
1575 w
1554 w
–
1444 sh
1436 w
1347 vw
1315 w
1276 w
1247 sh
1238 m
–
1432 w
1319 m
1233 wbr
1172 w
–
–
1188 s
1168 s
–
1140 w
1126 w
1079 s
1053 s
–
1177 vw
1168 w
1153 sh
–
1126 w
1064 w
–
1120 w
1066 w
ıCH I
ıCH II
–
–
ꢀC14–O17
ıCH I
Ring breath II
ꢁCH II
ꢂCH2
Ring breath I
ꢁCH I
ꢁCH I
ꢁCH II
ꢁCH I
ꢁCH II
ꢁCH I
ꢁCH II
ꢁCH I
ꢁCH I
–
–
–
–
1016 m
–
1016 m
–
–
1
1000 w
–
9
9
9
90
60
20
996 m
–
996 m
–
939 w
882 w
865 w
839 w
–
–
939 w
877 w
8
74
865 w
–
836 w
–
856
822
798
786
836 w
–
–
780 vw
736 sh
725 m
7
7
6
77
11
91
750 vvs
731 vs
689 s
663 m
–
750 vw
709 s
686 sh
663 w
–
ꢁCH II
ꢁPh I
ꢀC–S
703 wsh
6
6
80
24
ıPh I
6
14
613 w
–
596 w
515 w
508 w
–
477 w
458 w
434 m
419 m
–
–
ıPh(X) II
ꢁPh(X) I
ꢁPh(X) II
ꢁPh(X) I
ꢁPh(X) II
ıPh(X) II
ıPh(X) II
ꢁPh II
ꢁPh I
ıPh(X) I
ıPh(X) II
ꢁPh(X) I, ıPh(X) II
ꢁPh(X) II
ıPh(X) I, II
ꢀAg–O
597
597
521
601 vw
596 w
–
507 w
–
588 vw
509 s
5
18
4
4
4
93
83
49
462 w
–
462 w
416 vw
360 w
427
–
–
3
3
84
83
375 m
–
291 vw
233 w
3
2
00
96
–
–
–
216
237 vvs
1
1
1
93
29
18
–
–
–
213 w
134 sh
96 s
ꢁPh(X) II
ꢁPh(X) I
tCH2
158 s
ꢀ
, stretching; ı, in-plane bending; ꢁ, out-of-plane deformation; ꢂ, rocking; t, torsional; ω, wagging; s, strong; w, weak; m, medium; v, very; br, broad; I, mono-substituted
phenyl ring; II, disubstituted phenyl ring; X, substitutent sensitive; Ph, phenyl ring; subscript: as, asymmetric; s, symmetric.

1
36
C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
−
485 cm (HF) are assigned as the scissoring mode of CH . Bands
2
1
Table 3
1
−30
esu) values of
Calculated all ˇ components and ˇtot (×10
of hydrocarbons due to CH2 twisting and wagging vibrations, are
the title compound.
−1
observed in the region 1180–1390 cm [45,50]. These bands are
ˇ components
HF/6-31G*
generallyappreciablyweakerthanthoseresultingfromCH scissor-
2
ing vibrations. The CH wagging and twisting modes are assigned at
ˇxxx
ˇxxy
ˇxyy
−28.793
0.016
20.080
−0.015
6.310
2
−1
−1
−1
−1
1
357 cm (IR), 1347 cm (Raman), 1372 cm (HF) and 1214 cm
−
1
(
HF), respectively. The band calculated at 1007 cm is assigned as
ˇyyy
the rocking mode ꢂCH . The CO stretching vibration [49] absorbs
2
ˇxxz
ˇxyz
−
1
moderately to strongly in the range 1045 ± 45 cm and the band
0.010
−
1
calculated at 1024 cm is assigned as C14–O stretching mode.
ˇyyz
22.979
50.614
−0.003
−4.608
0.420 × 10
17
ˇxzz
Since the identification of all the normal modes of vibration of
large molecules is not trivial, we tried to simplify the problem by
considering each molecule as a substituted benzene. Such an idea
has already been successfully utilized by several workers for the
vibrational assignments of molecules containing multiple homo-
and heteroaromatic rings [51–55]. In the following discussion, the
mono-substituted and disubstituted phenyl rings are designated as
rings I and II, respectively. The modes in the two phenyl rings will
differ in wavenumber and the magnitude of splitting will depend
on the strength of interactions between different parts (internal
coordinates) of the two rings. For some modes, this splitting is
so small that they may be considered as quasi-degenerate and for
the other modes a significant amount of splitting is observed. Such
observations have already been reported [51–53,56].
ˇyzz
ˇzzz
−30
ˇtot (esu)
calculated value is 1326 cm 1. This mode is not pure but contains a
significant contribution from the ꢀPh mode.
−
5.2. Hyperpolarizability and geometrical parameters
Analysis of organic molecules having conjugated ␲-electron
systems and large hyperpolarizability using infrared and Raman
spectroscopy has evolved as a subject of research [62]. The poten-
tial application of the title compound in the field of nonlinear optics
demands the investigation of its structural and bonding features
contributing to the hyperpolarizability enhancement, by analyzing
the vibrational modes using the IR and Raman spectra. The ring
stretching bands at 1601, 1522, 1454, 1436 cm
have their counterparts in Raman at 1588, 1523, 1444, 1436 cm
respectively, and their relative intensities in IR and Raman spectra
are comparable.
The existence of one or more aromatic rings in a structure
is normally readily determined from the C–H and C C–C ring
−
1
related vibrations. The C–H stretching occurs above 3000 cm
−
1
and is typically exhibited as a multiplicity of weak to moder-
ate bands, compared with the aliphatic C–H stretch [57]. In the
present case, the ab initio calculations give ꢀC–H modes in the
observed in IR
−
1
,
−
1
range of 2999–3037 cm . The bands observed at 2985, 3029, 3052,
3
−1
−1
088 cm in the IR spectrum and at 2987, 3033, 3068, 3099 cm
The first hyperpolarizability (ˇ ) of this novel molecular system
0
in the Raman spectrum are assigned as the ꢀC–H modes of the
benzene ring.
is calculated using HF/6-31G* basis set, 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 × 3 × 3 matrix. The
27 components of the 3D matrix can be reduced to 10 components
due to the Kleinman symmetry [63].
The components of ˇ are defined as the coefficients in the Tay-
lor series expansion of the energy in the external electric field.
When the electric field is weak and homogeneous, this expansion
becomes
The benzene ring possesses six ring stretching vibrations, of
which the four with the highest wavenumbers (occurring respec-
−1
tively near 1600, 1580, 1490 and 1440 cm ) are good group
vibrations. In the absence of ring conjugation, the band near
−
1
−1
1
1
580 cm is usually weaker than that at 1600 cm . The fifth ring
−
stretching vibration is active near 1355 ± 35 cm , a region which
overlaps strongly with that of the CH in-plane deformation and
the intensity is in general, low or medium [49,58]. The sixth ring
stretching vibration or ring breathing mode appears as a weak band
ꢀ
ꢀ
ꢀ
1
2
1
6
i
i
j
i j k
−1
near 1000 cm
observed at 1601, 1587, 1559, 1522, 1454, 1436, 1315, 1278 cm
in the IR spectrum and at 1588, 1586, 1560, 1523, 1444, 1436, 1315,
in mono-substituted benzenes [49]. The bands
E = E0 −
ꢃiF −
˛ijF F −
ˇijkF F F
−
1
i
ij
ijk
ꢀ
1
−
1
i
j
k l
1
247 cm in the Raman spectrum are assigned as ꢀPh ring stretch-
− 2
ꢁijklF F F F + ...
4
ing modes. As seen from Table 2, the ab initio calculations give these
ijkl
−1
modes [49] in the range 1612–1245 cm
.
i
The CH out-of-plane deformations [49] are observed between
where E0 is the energy of the unperturbed molecule, F is the field
at the origin, ꢃ , ˛ , ˇ and ꢁijkl are the components of dipole
−1
1
000 and 700 cm . Generally, the C–H out-of-plane deforma-
i
ij
ijk
tions with the highest wavenumbers have a weaker intensity
than those at lower wavenumbers. The out-of-plane CH defor-
moment, polarizability, the first hyper polarizabilities, and second
hyperpolarizibilites, respectively. The calculated first hyperpolar-
−
1
izability of the title compound is 0.42 × 10−30 esu. We conclude
mation at 750 cm
6
and the out-of-plane ring deformation at
−
1
that the title compound is an attractive object for future studies
of nonlinear optical properties (Table 3).
89 cm in the IR spectrum for a pair of strong bands character-
istics of mono-substituted benzene derivative [49,59]. In the case
of 1,2-disubstituted benzenes, a strong absorption in the region
To the best of our knowledge, no X-ray crystallographic data of
this molecule has yet been established. However, the theoretical
results obtained are almost comparable with the recently reported
structural parameters of the parent benzothiazole molecule
[64]. From Table 1, it is clearly seen that the dihedral angles
−
1
7
55 ± 35 cm
is observed and is due to ꢁCH, which is assigned
−
1
at 731 cm in the IR spectrum [51]. For benzothiazole derivatives
60] ꢀC–S is reported at 648 cm (Raman) and at 660 cm (HF). In
the present case the band at ∼663 cm in both spectra is assigned
as ꢀC–S mode. Primary aromatic amines with nitrogen directly on
the ring absorb strongly at 1330–1260 cm due to stretching of the
phenyl carbon–nitrogen bond [50]. Sandhyarani et al. [61] reported
− −1
1
[
−1
◦
C6–C5–C4–N12 and C4–N12–C13–C14 are 180 . This indicates that
−1
the benzene ring II and the thiazole ring moieties of the title com-
pound are planar as in the case of 2-amino-6-methylbenzothiazole
[65]. For 2-amino-6-methylbenzothiazole [65] the bond lengths
C –C , C –C , C –C , C –N12, C13–S , C –S are respectively,
−1
ꢀ
CN at ∼1318 cm for 2-mercaptobenzothiazole. For the title com-
−1
pound ꢀC N is observed at 1315 cm in both spectra and the
4
5
4
3
3
2
4
11
3
11
4
12

C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
137
Fig. 5. Correlation graph.
1
.4145, 1.4333, 1.4066, 1.4110, 1.3167, 1.8807, 1.8316 Å (DFT cal-
culations). The corresponding values for the title compound are
.3888, 1.3921, 1.3851, 1.4011, 1.2704, 1.8181 and 1.8095 Å. The ab
initio calculations give the bond angles C –C –N , C –N12–C13
In the SERS spectrum of the title compound, the aromatic
C–H stretching vibration of ring I is observed as a strong band at
−
1
−1
1
3060 cm and as a weak band at 2994 cm , which suggest that the
phenyl ring I may be in a position close to the perpendicular orien-
tation [69,70]. But for the phenyl ring II, the aromatic CH stretching
vibrations are absent in the SERS spectrum, which means that the
ring II may be somewhat flat on the silver surface [69,70].
,
3
4
◦
12
4
◦
◦
◦
C3–S11 –C13, N12–C13–S11 as 114.9 , 113.3 , 86.7 and 115.4 for the
title compound. These values are in agreement with the bond
angles reported for 2-amino-6-methylbenzothiazole [65] (116.1 ,
◦
◦
◦
112.0 , 115.7 ) and for 2-amino-4-methylbenzothiazole [60]. For 2-
It has also been documented in the literature [71] that when a
benzene ring moiety interacts directly with a metal surface, the ring
amino-4-methylbenzothiazole [60] the bond lengths C –C , C –S11 ,
C13–N12, C –N are 1.3916, 1.8046, 1.2962, 1.4173 Å, respectively
and the bond angles C5–C –C , C –C –S , C –S11 –C13, are 120.2 ,
3
4
3
−
1
breathing mode is red shifted by 10 cm along with substantial
band broadening in the SERS spectrum. In the present case, the ring
breathing mode of ring I is absent in the SERS spectrum while that
4
12
◦
4
3
4
3
11
3
◦
◦
108.8 , 88.1 , respectively.
−
1
In order to investigate the performance and vibrational
of ring II is present at 1016 cm without any wavenumber shift.
Neither a substantial red shift nor significant band broadening was
identified in the SERS spectrum of the title compound implying that
the probability of a direct ring ␲-orbital to metal interaction should
be low, in accordance to a tilted position of the ring.
wavenumbers of the title compound root mean square (RMS) and
correlation coefficient between calculated and observed wavenum-
bers were calculated (Fig. 5). RMS values of wavenumbers were
calculated using the following expression [66].
−1
ꢁ
ꢂ
ꢂ
ꢃ
The presence of CH2 mode at 2953 cm in the SERS spectrum
indicates the closeness of CH2 group with metal surface and inter-
action of the silver surface with phenoxy group. This is supported
n
ꢀ
1
(ꢀ ic alc − ꢀ ie xp)2
RMS =
n − 1
−
1
by the strong band at 237 cm which may be due to Ag–O stretch-
ing vibration [72–74]. This band may be due to Ag–O/Ag–N/Ag–S
stretching vibration. Apart from the interaction of the phenoxy
group in the adsorption process, the fuzed thiazole moiety of the
molecule can also bind to the silver surface through the lone pair
of electrons of either the nitrogen or sulfur atoms or through both
of them. A more favoured adsorption site can be enumerated theo-
retically by estimating the partial atomic charges on each of these
probable active sites [75,76]. The higher the negative charge density
on the atom the higher the probability of it acting as an adsorbative
site for the silver substrate. Theoretical results estimated from HF ab
initio calculations show that the partial atomic charges on the nitro-
gen, sulfur and oxygen atoms determined by natural population
analysis are −0.514, 0.592 and −0.789, respectively. The negative
charge density is thus observed to be more appreciable on the oxy-
gen atom than others, thereby indicating the active involvement
of the oxygen atom on the adsorption process. The appearance of a
i
The RMS errors between the observed and scaled wavenumbers
are found to be 18.52 and 18.35 for IR and Raman wavenumbers,
respectively. Slight variation is found in the experimental and cal-
culated vibrational modes. It must be due to the fact that hydrogen
bond vibrations present in the crystal lead to strong perturbation
of the infrared wavenumbers and intensities of many other modes.
The experimental results belong to solid phase and theoretical cal-
culations belong to gaseous phase.
5.3. SERS spectrum
SERS is already regarded as a valuable method because of its
high sensitivity, which enables the detection and spectroscopic
study of even single molecules [2]. The vibrational information
contained in the SERS spectrum provide the molecular specificity
required to characterize the adsorbate–surface interactions, specif-
ically, the orientation of the adsorbed species on the metal surface.
The relative intensities from the SERS spectra are expected to differ
significantly from normal Raman spectra owing to specific sur-
face selection rules [67]. The surface selection rule suggests that
for a molecule adsorbed flat on the silver surface, its out-of-plane
vibrational modes will be more enhanced when compared with its
in-plane vibrational modes and vice versa when it is adsorbed per-
pendicular to the surface [67,68]. It is further seen that vibrations
involving atoms that are close to the silver surface will be enhanced.
When the wavenumber difference between Raman bands in normal
−
1
strong band at 237 cm in the SERS spectrum ascribed to the Ag–O
stretching indicates that the phenoxy group of the title compound
is indeed adsorbed through the oxygen atom. The presence of a
−
1
weak broad band at 1233 cm due to ꢀCO in the SERS spectrum
supports the above argument.
The in-plane bending modes ıCH of the aromatic ring are
−
1
−1
observed at 1172, 1066 cm for ring I and 1120 cm for ring II.
The presence of these modes suggests that the benzene rings are
oriented tilted to the silver surface [67,68]. Also the benzene ring
−
1
−1
vibrations observed at 1575, 1432 cm (ring II), 1554, 1319 cm
(ring I) in the SERS spectrum supports this fact. According to the
surface selection rules [77,78] the presence of in-plane vibrational
−1
and SERS spectra is not more than 5 cm , the molecular plane is
expected to be perpendicular to the silver surface [69].
−
1
−1
modes at 1120, 462, 360 cm (ring II), 1172, 1066 cm (ring I) and

1
38
C.Y. Panicker et al. / Spectrochimica Acta Part A 74 (2009) 132–139
−
1
of the out-of-plane vibrational modes at 939, 588, 509 cm (ring
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−1
II), 996, 877, 836, 780, 736, 703, 416, 158 cm (ring I) in the SERS
spectrum of the title compound suggest that there is a certain angle
between the rings and the surface of the silver particle. The substi-
tute sensitive in-plane and out-of-plane bending modes are also
detected at the same time, suggesting a tilted orientation of the
molecule [79,80].
The charge transfer mechanism of SERS can be explained by the
resonant Raman mechanism in which charge transfer excitation
from the metal to the adsorbed molecule or vice versa occurs at
the energy of the incident laser wavenumber [81,82]. The frontier
orbital theory plays a significant role in the understanding of the
charge transfer mechanism of SERS [83,84]. Two types of charge
transfer mechanisms are predicted. One is molecule to metal and
the other is metal to molecule. Molecule to metal charge transfer
excitation occurs when an electron is transferred from the high-
est occupied molecular orbital (HOMO) of the adsorbate to the
Fermi level of the metal. Conversely, transfer of an electron from the
Fermi level of the metal to the lowest unoccupied molecular orbital
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6
. Conclusion
The FT-IR,
FT-Raman
and
SERS
spectra
of
2-
phenoxymethylbenzothiazole were studied. The molecular
geometry and wavenumbers were calculated using the
Hartree–Fock method with 6-31G* basis set. The observed
wavenumbers were found to be in agreement with the calculated
values. Optimized geometrical parameters of the title compound
are in agreement with the reported values. It may be inferred that
for 2-phenoxymethylbenzothiazole adsorbed on silver colloid, the
molecule is adsorbed through the oxygen atom with the molecular
plane tilted on the metal surface. Analysis of the phenyl ring
modes shows that the C–C stretching mode is equally active as
strong bands in both IR and Raman spectra, and is responsible
for hyperpolarizability enhancement leading to nonlinear optical
activity.
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