Vibrational spectroscopic studies and ab initio calculations of 5-methyl-2-(p-methylaminophenyl)benzoxazole

Article abstract of DOI:10.1016/j.saa.2007.04.034

FT-IR spectra of 5-methyl-2-(p-methylaminophenyl)benzoxazole was recorded and analysed. The vibrational frequencies of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values.

Full text of DOI:10.1016/j.saa.2007.04.034

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 782–788
Vibrational spectroscopic studies and ab initio calculations of
5-methyl-2-(p-methylaminophenyl)benzoxazole
a
a
b
c,∗
K.R. Ambujakshan , V.S. Madhavan , Hema Tresa Varghese , C. Yohannan Panicker ,
d
d
d
Ozlem Temiz-Arpaci , Betul Tekiner-Gulbas , Ilkay Yildiz
a
Department of Physics, MES Ponnani College, Ponnani South, Malappuram, Kerala, India
Department of Physics, Fatima Mata National College, Kollam 691001, Kerala, India
Department of Physics, TKM College of Arts and Science, Kollam 691005, Kerala, India
Ankara University, Faculty of Pharmacy, Department of Pharmaceutical Chemistry,
b
c
d
Tandogan 06 100, Ankara, Turkey
Received 12 January 2007; accepted 23 April 2007
Abstract
FT-IR spectra of 5-methyl-2-(p-methylaminophenyl)benzoxazole was recorded and analysed. The vibrational frequencies of the compound have
been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values.
©
2007 Elsevier B.V. All rights reserved.
Keywords: FT-IR; HF ab initio calculation; Benzoxazole
1
. Introduction
2. Experimental
Benzoxazole derivatives are the structural isoesters of natu-
For the synthesis of the benzoxazole derivative, using aque-
ous mineral acids as the condensation reagent did not provide
successful results, because of the oxazole ring which was eas-
ily hydrolysed under these conditions [12]. Reactions which
occurred in pyridine or xylene also has low yields. Finally,
polyphosphoricacidwaschosenasthereagentforcyclodehydra-
tion in the synthesis of the title compound as it has good solvent
power and contains anhydride group which combine with the
water formed in the reaction centre in order to prevent effective
acidity. The stability of the oxazole ring increases under these
conditions and high temperature could be used [12]. The title
compound was prepared by heating 2-hydroxy-5-methylaniline
withtheappropriatecarboxylicacidinpolyphosphoricacid[12].
A mixture of 2-hydroxy-5-methylaniline (0.01 mol) and appro-
priate carboxylic acid (0.02 mol) was heated in polyphosphonic
acid (12 g) with stirring for 2.5 h. At the end of the reaction
period, the residue was poured into ice-water and neutralized
with excess of 10% NaOH solution. After extracted with ben-
zene, the benzene solution was dried over anhydrous sodium
sulphate and evaporated under reduced pressure. The residue
was boiled with 200 mg charcoal in ethanol and filtered. The
rally occurring nucleotides such as adenine and guanine, which
allows them to interact easily with the biopolymers of living
systems and different kinds of biological activity have been
obtained [1–5]. Besides, due to their kind of action, it has been
reported that they have shown low toxicity in warm-blooded
animals [4,6]. Synthesis and microbiological activity of 5-
substituted-2-(p-substitutedphenyl)benzoxazole derivatives are
reported by Yalcin et al. [7–10]. Anto et al. [11] reported the
vibrational spectroscopic and ab initio studies of 5-methyl-
2
-(p-fluorophenyl)benzoxazole. Ab initio quantum mechanical
method is at present widely used for simulating IR spectrum.
Such simulations are indispensable tools to perform normal
coordinate analysis so that modern vibrational spectroscopy is
unimaginable without involving them. In the present study the
FT-IR and theoretical calculations of the wavenumbers of the
title compound are reported.
∗
Corresponding author.
E-mail address: cyphyp@rediffmail.com (C.Y. Panicker).
◦
filtrate was left to crystallize by addition of water. MP 179.1 C,
1
386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.04.034

K.R. Ambujakshan et al. / Spectrochimica Acta Part A 69 (2008) 782–788
783
Table 1
Optimized geometrical parameters of 5-methyl-2-(p-methylaminophenyl)
benzoxazole, atom labeling is according to Fig. 2
´
◦
◦
Bond lengths ( A˚ )
Bond angles ( )
Dihedral angles ( )
C1–C2
C1–C6
C1–H7
C2–C3
C2–N8
C3–C4
C3–O15
C4–C5
C4–H9
1.3879
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(2,3,15)
A(4,3,15)
A(3,4,5)
A(3,4,9)
A(5,4,9)
A(4,5,6)
A(4,5,10)
A(6,5,10)
A(1,6,5)
118.3
120.4
121.3
120.1
131.6
108.3
123.5
107.2
129.4
115.9
122.1
122.0
122.4
118.8
118.8
119.9
120.7
119.4
104.6
111.2
111.2
111.2
107.8
107.5
107.8
104.8
121.2
119.8
119.0
120.4
119.2
120.5
118.3
122.3
119.4
120.9
119.4
119.8
120.8
119.4
119.8
118.4
119.9
121.7
114.2
122.4
114.7
115.2
127.5
117.3
108.4
110.7
113.1
107.6
108.5
108.4
D(6,1,2,3)
D(6,1,2,8)
D(7,1,2,3)
D(7,1,2,8)
D(2,1,6,5)
D(2,1,6,11)
D(7,1,6,5)
D(7,1,6,11)
D(1,2,3,4)
D(1,2,3,15)
D(8,2,3,4)
−0.0
−180.0
−180.0
0.0
1.3860
1.0746
1.3784
1.3892
1.3768
1.3590
1.3839
1.0739
1.4034
1.0761
1.5128
1.2753
1.0860
1.0836
1.086
1.3492
1.3799
1.3899
1.0738
1.3978
1.0728
1.4007
1.3782
1.3752
1.0762
1.3939
1.0737
1.4610
0.9946
.14435
1.0819
1.0835
1.0887
0.0
−180.0
180.0
0.0
0.0
C5–C6
−180.0
−180.0
−0.0
−180.0
0.0
−0.0
180.0
180.0
−0.0
−0.0
180.0
0.0
C5–H10
C6–C11
N8–C27
C11–H12
C11–H13
C11–H14
O15–C27
C16–C17
C16–C21
C16–H22
C17–C18
C17–H23
C18–C19
C18–N24
C19–C20
C19–H25
C20–C21
C20–H26
C21–C27
N24–H28
N24–C29
C29–H30
C29–H31
C29–H32
D(8,2,3,15)
D(1,2,8,27)
D(3,2,8,27)
D(2,3,4,5)
Fig. 1. FT-IR spectrum of 5-methyl-2-(p-methylaminophenyl)benzoxazole.
D(2,3,4,9)
A(1,6,11)
A(5,6,11)
A(2,8,27)
D(15,3,4,5)
D(15,3,4,9)
D(2,3,15,27)
D(4,3,15,27)
D(3,4,5,6)
D(3,4,5,10)
D(9,4,5,6)
D(9,4,5,10)
D(4,5,6,1)
D(4,5,6,11)
D(10,5,6,1)
D(10,5,6,11)
D(1,6,11,12)
D(1,6,11,13)
D(1,6,11,14)
D(5,6,11,12)
D(5,6,11,13)
D(5,6,11,14)
D(2,8,27,15)
D(2,8,27,21)
D(3,15,27,8)
D(3,15,27,21)
D(21,16,17,18)
◦
yield 52%, reaction temperature 165–170 C, UV λmax. 214,
3
7
1
26, 354 (shoulder), H NMR δ ppm 2.68 (s, 3H), 3.49 (s, 3H),
.59–8.89 (m, 7H).
A(6,11,12)
A(6,11,13)
A(6,11,14)
A(12,11,13)
A(12,11,14)
A(13,11,14)
A(3,15,27)
A(17,16,21)
A(17,16,22)
A(21,16,22)
A(16,17,18)
A(16,17,23)
A(18,17,23)
A(17,18,19)
A(17,18,24)
A(19,18,24)
A(18,19,20)
A(18,19,25)
A(20,19,25)
A(19,20,21)
A(19,20,26)
A(21,20,26)
A(16,21,20)
A(16,21,27)
A(20,21,27)
A(18,24,28)
A(18,24,29)
A(28,24,29)
A(8,27,15)
A(8,27,21)
A(15,27,21)
A(24,29,30)
A(24,29,31)
A(24,29,32)
A(30,29,31)
A(30,29,32)
A(31,29,32)
180.0
−180.0
0.0
The FT-IR spectrum (Fig. 1) was recorded using a Perkin-
Elmer FT-IR 1760x spectrometer. The spectral resolution was
−1
4
cm . Standard KBr technique with 1 mg sample per 300 mg
−0.0
180.0
−190.0
−0.0
−120.1
−0.1
120.1
59.8
180.0
−59.9
−0.0
−180.0
0.0
KBr was used.
3
. Computational details
Calculations of the title compound were carried out with
Gaussian03 program [13] using the HF/6-31G* basis set to
predict the molecular structure and vibrational wavenumbers.
Molecular geometry was fully optimized by Berny’s optimiza-
tion algorithm using redundant internal coordinates. Harmonic
vibrational wavenumbers were calculated using the analytics
second derivatives to confirm the convergence to minima on
the potential surface. The wavenumber values computed at
the Hartree-Fock level contain known systematic errors due to
the negligence of electron correlation [14]. We therefore, have
used the scaling factor value of 0.8929 for HF/6-31G* basis
set. Parameters corresponding to optimized geometry of the
title compound (Fig. 2) are given in Table 1. The absence of
imaginary wavenumbers on the calculated vibrational spectrum
confirms that the structure deduced corresponds to minimum
180.0
0.3
D(21,16,17,23) −179.9
D(22,16,17,18) −179.9
D(22,16,17,23)
D(17,16,21,20)
D(17,16,21,27)
−0.1
−0.1
179.8
D(22,16,21,20) −179.9
D(22,16,21,27)
D(16,17,18,19)
D(16,17,18,24)
D(23,17,18,19)
D(23.17,18,24)
D(17,18,19,20)
D(17,18,19,25)
0.0
−0.3
178.3
179.9
−1.5
0.1
−
1
energy. In total there are 90 vibrations from 3448 to 30 cm
.
179.7
D(24,18,19,20) −178.5
D(24,18,19,25)
D(17,18,24,28)
D(17,18,24,29)
D(19,18,24,28)
1.1
161.6
15.7
−19.8
D(19,18,24,29) −165.8
D(18,19,20,21)
0.0
D(18,19,20,26) −180.0
D(25,19,20,21) −179.5
Fig. 2. Optimized geometry of 5-methyl-2-(p-methylaminophenyl)benzo-
xazole.
D(25,19,20,26)
D(19,20,21,16)
0.5
−0.0

7
84
K.R. Ambujakshan et al. / Spectrochimica Acta Part A 69 (2008) 782–788
in the regions 1140 ± 15, 1055 ± 25 and 985 ± 65 cm− are
due to two methyl-rocking vibrations and a N–C stretching
vibration strongly coupled among themselves [17]. The bands
in the above regions are assigned to the N–C stretch as well
1
Table 1 (Continued )
´
◦
◦
Bond lengths ( A˚ )
Bond angles ( )
Dihedral angles ( )
D(19,20,21,27)
D(26,20,21,16)
D(26,20,21,27)
D(16,21,27,8)
D(16,21,27,15)
D(20,21,27,8)
D(20,21,27,15)
D(18,24,29,30)
D(18,24,29,31)
D(18,24,29,32)
D(28,24,29,30)
D(28,24,29,31)
D(28,24,29,32)
−180.0
180.0
0.1
−0.0
180.0
180.0
−0.1
175.0
−67.2
54.6
−1
as methyl rock [25]. The absorption near 1135, 1070 cm
in the spectra of N-methyl sulfonamide is attributed to ρCH3
and υN–C stretching vibrations [26,27]. For the title compound
ρMe are calculated at 1129, 1114 cm and only one band is
observed in the IR spectrum at 1131 cm . The band calcu-
−
1
−1
−
1
lated at 1036 cm is assigned as υC24–N29 stretching mode.
In the spectra of N-methyl-substituted benzeneamines the NH
−1
29.2
wag is observed in the range of 635 ± 35 cm [17]. The band
−
1
146.9
−91.2
observedat642 cm intheIRspectrumandthecalculatedvalue
−1
at 639 cm is assigned as wagging NH mode. In N-methyl-
substituted aliphatic amines and sulfonamides the δC–N–C is
−
1
found in the region 360 ± 50 cm [17]. In N-methyl benzene
The assignments of the calculated wavenumbers are aided by the
animation option of MOLEKEL program, which gives a visual
presentation of the vibrational modes [15,16].
amines this vibration is coupled to the substituent sensitive Ph–N
deformation. The methyl torsion is often assigned in the region
−1
2
30 ± 30 cm and the NHMe torsion may be expected in the
−
1
neighbourhood of 100 cm [28,29].
4
. Results and discussion
In aromatic compounds the asymmetric stretching vibrations
−1
of CH3 are expected in the range of 2905–3000 cm and sym-
−1
The observed IR bands with their relative intensities
metric CH3 vibrations in the range o 2860–2870 cm [17,30].
The first of these results from the antisymmetric stretching
υasCH3 mode in which two C–H bonds of the methyl group are
extending while the third one is contracting. The second arises
fromsymmetricalstretchingυsCH3 inwhichallthreeoftheC–H
bonds extend and contract in-phase. The asymmetric stretching
and calculated wavenumbers and assignments are given in
Table 2.
In aromatic compounds the υNH appears moderately to
−
1
strongly in the region 3400 ± 40 cm [17]. The HF calcula-
−
1
tions give a value of 3448 cm for this υNH mode and a weak
−
1
−1
−1
band is observed in the IR spectrum at 3428 cm . The anti-
symmetric stretching vibrations of the methyl group of NHMe
absorb in the regions 2965 ± 25 and 2930 ± 25 cm [17]. The
band intensity varies from weak in N-methyl-substituted sulfon-
amides to moderate in N-methyl-substituted amines. The methyl
symmetric stretch absorbs, sharply and clearly separated from
modes of the methyl group are calculated to be 2930, 2905 cm
and the symmetric mode at 2860 cm . The band at 2904 cm
−
1
−
1
in the IR spectrum is assigned as υasCH3. Two bending can
occur within a methyl group. The first of these, the symmet-
rical bending vibration, involves the in-phase bending of the
C–H bonds. The second, the antisymmetrical bending vibration,
involves out-of-phase bending of the C–H bonds. The asymmet-
−
1
the antisymmetric counter parts, in the region 2855 ± 70 cm
,
−1
with a moderate to strong intensity in amines [17]. These low
wavenumbers, attributed to a reduced force constant of the CH
bond under the influence of the free electron pair of the nitro-
gen atom, is typical for N–Me compounds [18–23] but also for
methyl esters. The asymmetrical stretching modes of the methyl
ric deformations are expected in the range of 1400–1485 cm
[17]. The calculated values of δasCH3 modes are at 1470 and
−
1
1458 cm . Experimentally no bands are observed. In many
molecules the symmetric deformation δsCH3 appears with an
intensity varying from medium to strong and expected in the
−
1
−1
group of NHMe are calculated to be 2948, 2911 cm and the
symmetric mode at 2843 cm . The weak bands observed at
2
υasCH3 and υsCH3, respectively for NHMe group for the title
compound. In N-methylamines the δNH [17] should be expected
in the region 1530 ± 50 cm , but the very weak intensity of
the band hinders the assignment. Both methyl antisymmetric
deformations absorb weakly to moderately and often coincide
range of 1380 ± 25 cm [17]. The HF calculation gives δsCH3
−
1
−1
at 1400 cm . Aromatic molecules display a methyl rock in the
neighbourhoodof1045 cm [17]. Thesecondrockintheregion
−
1
−1
940, 2915 and 2820 cm in the IR spectrum are assigned as
−
1
970 ± 70 cm [17] is more difficult to find among the C–H
out-of-plane deformations. For the title compound, these modes
−
1
−1
ρCH3 are calculated at 1047 and 988 cm . The bands at 1052,
988 cm in the IR spectrum are assigned as ρCH3 modes.
−
1
The C N stretching skeletal bands are observed in the range
−
1
−1
and is expected in the regions 1470 ± 15 and 1460 ± 15 cm
of 1672–1566 cm [12,31–33]. Saxena et al. [31] reported a
−
1
−1
[
17]. The region 1410 ± 35 cm
deformation is typical for N-methyl-substituted amines and
thio)amides alike [24]. The intensity of the symmetric defor-
mation slightly exceeds that of the antisymmetric modes. The
of the methyl symmetric
value 1608 cm for polybenzodithiazole and Klots and Col-
lier [34] reported a value of 1517 cm for benzoxazole as
−
1
(
υC N stretching mode. Yang et al. [35] reported a band at
−
1
1626 cm in the IR spectrum as υC N for the oxazole ring. For
the title compound, the HF calculation give the υC27 N8 mode
−
1
calculated values for δasCH3 are at 1479, 1462 cm . Exper-
imentally one band is observed at 1479 cm . The band at
1
−
1
−1
−1
at 1638 cm . The authors have reported a value of 1661 cm
−1
−1
−1
−1
431 cm
in the IR spectrum and at 1442 cm
(HF) are
(Raman), 1671 cm (IR) and 1643 cm (HF) as υC N mode
assigned to δsCH3. The three weak to moderate absorptions
[11].

K.R. Ambujakshan et al. / Spectrochimica Acta Part A 69 (2008) 782–788
785
Table 2
Calculated vibrational wavenumbers, measured infrared band positions and assignments for 5-methyl-2-(p-methylaminophenyl)benzoxazole
υ(calculated) (cm 1)
−
υ(IR) (cm
−1
)
IR intensity (KM/mole)
Raman activity (A**4/amu)
Assignments
3
3
3
3
3
3
2
2
2
2
2
2
2
2
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
1
1
1
1
1
448
043
037
034
028
023
996
995
948
930
911
905
860
843
638
616
611
597
577
520
495
483
479
470
462
458
442
423
402
400
323
298
273
273
264
201
196
184
168
129
121
114
097
096
066
047
036
3428 w
3066 w
34.6
9.3
7.0
131.8
102.7
58.1
129.8
46.7
68.5
110.1
102.9
144.3
65.7
79.6
116.3
190.8
121.4
430.6
1286.7
102.0
1038.3
679.1
84.2
6.7
υNH
υCH II
υCH II
υCH I
υCH II
υCH I
3031 w
2940 w
10.3
11.5
10.1
15.9
26.5
39.7
24.7
52.6
33.4
50.1
79.8
86.5
362.9
1.5
υCH I
υCH II
υasMe II
υasMe I
υasMe II
υasMe I
υsMe I
υsMe II
2915 w
2904 w
2820 w
1608 vs
υC
N
υPh II
υPh I
υPh II
υPh II
υPh I
υPh I
δNH
165.4
130.8
429.6
0.9
1571 m
1517 vs
1500 vs
1485 sh
1479 s
5.6
3.2
73.4
41.5
9.3
57.9
35.4
29.3
22.6
10.1
147.4
70.0
28.1
153.6
116.5
74.3
1.9
405.1
12.6
14.3
20.9
177.9
4.6
14.3
11.1
13.1
6.7
48.7
1.3
12.0
1.9
2.4
1.5
5.0
0.1
37.9
102.4
1.4
0.3
3.3
1.4
0.3
24.9
9.2
5.4
δasMe II
δasMe I
δasMe II
δasMe I
δsMe II
υPh I, II
δCH II
δsMe I
υC2–N8
υPh II
υC18–N24
δCH I, II
υasC–O–C
δCH I
δCH II
δCH I
δCH II
ρMe II
δCH I
ρMe II
δCH II
δCH I
υsC–O–C
ρMe I
υC24–N29
γCH II
ρMe I
γCH II
γCH I
γCH I
δNCO
δPh(X) I
γCH I
γCH II
γCH II
γCH I
δPh I
Ring breathing II
γPh I, II
4.2
1431 s
1419 sh
11.3
31.6
91.8
6.1
68.0
128.9
167.7
22.8
76.1
1.6
39.0
15.5
191.2
3.0
31.2
3.7
7.7
3.1
10.9
3.2
23.1
0.1
14.5
5.6
0.1
1.9
3.6
1331 s
1291 s
1279 m
1261 s
1200 m
1176 vs
1131 m
1119 m
1100 sh
1076 m
1052 m
9
9
9
9
9
9
9
8
8
8
8
8
7
7
7
7
7
90
88
85
76
54
23
18
93
38
20
15
13
82
52
44
41
01
988 w
942 w
927 m
13.5
9.4
57.7
3.1
56.9
25.3
4.8
18.5
13.6
4.1
864 w
824 s
792 s
752 m
γPh I, II
Ring breathing I
γPh II
738 m
702 m
8.9
0.2
3.6

7
86
K.R. Ambujakshan et al. / Spectrochimica Acta Part A 69 (2008) 782–788
Table 2 (Continued )
υ(calculated) (cm 1)
−
υ(IR) (cm
−1
)
IR intensity (KM/mole)
Raman activity (A**4/amu)
Assignments
6
6
5
5
5
5
4
4
4
4
3
3
3
3
2
2
2
2
1
1
1
39
21
94
88
43
21
93
53
37
11
93
77
47
16
80
58
28
14
93
75
27
642 w
619 sh
597 m
4.5
0.6
3.3
2.8
7.6
0.1
10.5
0.6
0.9
2.3
7.5
0.4
0.1
9.1
9.2
11.9
4.2
1.1
1.8
0.7
2.7
4.0
1.2
1.8
0.5
0.1
0.5
0.5
0.2
ωNH
δPh II
γPh(X) I
δPh(X) I
δPh I, II
γPh II
δPh I, II
δPh(X) I, II
γPh I
3.0
536 w
509 m
15.0
10.9
37.1
24.2
3.8
0.1
γPh II
41.5
53.7
97.3
8.8
12.5
1.8
0.8
0.4
1.1
1.7
1.7
1.8
1.0
0.2
1.8
0.3
γPh(X) I
δPh(X) I, δC–N–C II
δPh I, τMe I
γPh I, II
γPh(X) I
γPh(X) I, τMe II
τMe I
τMe II
τPh I, II
τMe II
τNHMe
τMe II
τPh I, II
τPh I, II
τMe I, II
τMe I, II
8
7
4
3
3
4
1
1
7
0
υ: stretching; δ: in-plane deformation; γ: out-of-plane deformation; ρ: rocking; τ: torsional; Ph: phenyl; I: Ph attached to methyl; II: Ph attached to NHMe; v: very;
s: strong; m: medium; w: weak; sh: shoulder; X: substituent sensitive; Subscripts: as: asymmetric; s: symmetric.
Since the identification of allthe normalmodesof vibrationof
large molecules is not trivial, we tried to simplified 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 multi-
ple homo- and heteroaromatic rings [36–40]. In the following
discussion, the phenyl rings attached with the methyl group and
NHMe group 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 inter-
actions 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 sig-
nificant amount of splitting is observed. Such observations have
already been reported [36–38,41].
symmetric C–O–C stretching vibration appears as a weak band
−
1
in the IR spectrum at 1076 cm [12,32]. The ab initio calcu-
−
1
lation give 1264 and 1066 cm as asymmetric and symmetric
C–O–C modes, respectively. The C–O–C stretching is reported
−
1
at 1250 and 1073 cm for 2-mercaptobenzoxazole [42] and at
−
1
−1
−1
1061 cm (Raman), 1055 cm (IR), 1060 cm (HF) and at
1269 cm (Raman), 1263 cm (IR) and 1261 cm (HF) for
−
1
−1
−1
5-methyl-2-(p-fluorophenyl)benzoxazole [11]. Fukukawa and
−1
Ueda [44] reported the symmetric C–O–C stretch at 1045 cm
for benzoxazole.
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
and is typically exhibited as a multiplicity of weak to moder-
ate bands, compared with the aliphatic C–H stretch [45]. Klots
and Collier [34] reported the bands at 3085, 3074, 3065 and
Primary aromatic amines with nitrogen directly on the ring
−
1
−1
absorb strongly at 1330–1260 cm due to stretching of the
phenyl carbon–nitrogen bond [30]. For 2-mercaptobenzoxazole
3045 cm as υC–H modes for benzoxazole. In the present
case, ab initio calculations give υC–H modes in the range of
−
1
−1
−1
−1
this mode [42] is reported at 1340 cm (Raman) and 1325 cm
ab initio calculations). Sandhyarani et al. [43] reported υCN
3043–2995 cm . The bands observed at 3066 and 3031 cm
(
in the IR spectrum are assigned as υC–H modes of the benzene
ring.
−
1
at ∼1318 cm for 2-mercaptobenzothiazole. For benzoxazole
−
1
υC2–N8, the authors have reported [11] a value at 1332 cm
For the para-disubstituted benzene, the υPh modes are
−
1
−1
−1
(
IR), 1315 cm (Raman) and 1315 cm (HF). For the title
compound υC2–N8 mode is observed at 1331 cm in the IR
spectrum and the calculated value is 1323 cm . The υC18–N24
in the IR spectrum and at
273 cm theoretically.
As expected the asymmetric C–O–C stretching vibration pro-
expected in the range of 1280–1620 cm and for the trisub-
−
1
stituted benzene ring these modes are seen in the region
−
1
−1
1250–1610 cm [17]. For para-substituted benzenes the δCH
−
1
−1
band is observed at 1279 cm
modes are seen in the range of 995–1315 cm
[17] and
−1
1
for trisubstituted benzenes these modes are in the range of
−
1
1050–1280 cm [17]. For the title compound, the HF calcu-
lation give 1402, 1273, 1201, 1196, 1184, 1168, 1121, 1097,
−
1
duce strong band at 1261 cm in the IR spectrum [12,32]. The

K.R. Ambujakshan et al. / Spectrochimica Acta Part A 69 (2008) 782–788
787
−
1
1
096 cm 1 as δCH modes. These bands are observed at 1200,
for benzoxazole, the bond lengths, N8–C27, C27–O , O –C3,
15 15
−
1
176, 1119, 1100 cm in the IR spectrum.
The C–H out-of-plane deformations are observed between
000 and 700 cm [17]. Generally the C–H out-of-plane defor-
mations with the highest wavenumbers have a weaker intensity
C3–C4, C2–C1, C2–C3 and N8–C2 are 1.291, 1.372, 1.374,
˚
1.390, 1.4, 1.403and1.401 Arespectivelyandthecorresponding
−
1
1
valuesforthetitlecompoundare1.2753, 1.3492, 1.3590, 1.3768,
˚
1.3879, 1.3784 and 1.3892 A. These changes in bond lengths
than those absorbing at lower wavenumbers. These γCH modes
for the title compound can be attributed to the conjugation of
the phenyl ring. Purkayastha and Chattopadhyay [50] reported
−
1
are observed at 942, 864, 824 cm in the IR spectrum. The ab
initio calculations give these modes at 990, 985, 976, 954, 893,
˚
N8–C27, N8–C2 bond lengths as 1.3270, 1.400 A for benzoth-
−
1
˚
iazole and 1.3503, 1.407 A for benzimidazole compounds. For
8
38, 820, 815 cm
For benzoxazole, Klots and Collier [34] reported the bands at
.
˚
the title compound we have obtained 1.2753 A as N8–C27 and
−
1
˚
1.3892 A as N8–C2 bond lengths. The carbon–carbon bond
9
32, 847 and 746 cm as out-of-plane deformations of the ben-
˚
zene ring. According to Collier and Klots [46] the non-planar
vibrations are observed at 970, 932, 864, 847, 764, 746, 620,
lengths in the phenyl ring I lies between 1.3768 and 1.4034 A
˚
while for phenyl ring II, the range is 1.3752–1.4007 A. The
−
1
˚
5
8
74, 417, 254, 218 cm for benzoxazole and at 973, 939, 856,
CH bond lengths lies between 1.0739 and 1.0761 A for Ph I
−
1
˚
24, 757, 729, 584, 489, 419. 207, 192 cm for benzothia-
zole. The planar modes below 1000 cm for benzoxazole are
reported to be at 920, 870, 778, 622, 538, 413 cm and for
benzothiazole at 873, 801, 712, 666, 531, 504, 530 cm [46].
We have also observed the planar and non-planar modes for
the title compound in this region. Two very strong CH out-of-
and 1.0728–1.0762 A for Ph II. Here for the title compound,
−
1
benzene is a regular hexagon with bond lengths somewhere in
−
1
˚
between the normal values for a single (1.54 A) and a double
−
1
˚
(1.33 A) bond [51]. The HF calculation gives shortening of angle
◦
C19–C18–N24 by 0.6 and increase of angle C17–C18–N24 by
◦
◦
2.3 from 120 at C18 position and this asymmetry of exocyclic
angle reveals the repulsion between NHMe group and the phenyl
ring.
−
1
plane deformation bands, occurring at 840 ± 50 cm is typical
for 1,4-disubstituted benzenes [17]. For the title compound, a
−
1
very strong γCH is observed at 824 cm . Again, according to
literature [17,30] a lower γCH absorbs in the neighbourhood
Acknowledgement
−
1
8
20 ± 45 cm , but is much weaker or infrared inactive. The
−
1
ab initio calculations gives a γCH at 820 cm and no band is
Hema Tresa Varghese would like to thank the University
Grants Commission, India for a minor research grant.
experimentally observed for this mode. In the case of trisubsti-
−
1
tuted benzenes, two γCH bands are observed at 890 ± 50 cm
−
1
and 815 ± 45 cm in the IR spectrum. Theoretically the values
References
−
1
observed are 954 and 893 cm
.
The benzoxazole ring stretching vibrations exist in the range
[
[
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1
1
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7
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−
1
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15
˚
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˚
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15
1
˚
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˚
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