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

Article abstract of DOI:10.1016/j.saa.2006.08.026

FT-Raman and FT-IR spectra of 5-methyl-2-(p-fluorophenyl)benzoxazole were 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.2006.08.026

Spectrochimica Acta Part A 67 (2007) 744–749
Vibrational spectroscopic studies and ab initio calculations of
5-methyl-2-(p-fluorophenyl)benzoxazole
P.L. Antoa, C. Yohannan Panickerb,∗, Hema Tresa Varghesec, Daizy Philipd,
Ozlem Temiz-Arpacie, Betul Tekiner-Gulbase, Ilkay Yildize
a Department of Physics, St. Thomas College, Trichur 680 001, Kerala, India
b Department of Physics, TKM College of Arts and Science, Kollam 691 005, Kerala, India
c Department of Physics, Fatima Mata National College, Kollam 691 001, Kerala, India
d Department of Physics, Mar Ivanios College, Nalanchira, Thiruvananthapuram 695 015, Kerala, India
e Ankara University, Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Tandogan 06 100, Ankara, Turkey
Received 14 March 2006; accepted 2 August 2006
Abstract
FT-Raman and FT-IR spectra of 5-methyl-2-(p-fluorophenyl)benzoxazole were 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.
© 2006 Elsevier B.V. All rights reserved.
Keywords: FT-Raman; FT-IR; HF ab initio calculation; Benzoxazole
1. Introduction
successful results, because of the oxazole ring which was eas-
ily hydrolysed under these conditions [11]. 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 [11]. The title
compound was prepared by heating 2-hydroxy-5-methyl ani-
line with the appropriate carboxylic acid in polyphosphoric acid
[11] and the structure of the title compound were supported by
elemental analysis and spectral data by Yalcin et al. [11]. The
FT-IR spectrum (Fig. 1) was recorded using a Perkin-Elmer FT-
Benzoxazole derivatives are the structural isoesters of natu-
rally occuring nucleotides such as adenine and guanine, which
allows them to interact easily with the biopolymers of living sys-
temsanddifferentkindsofbiologicalactivityhavebeenobtained
[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]. Ab initio quantum mechanical method is
at present widely used for simulating IR spectrum. Such sim-
ulations are indispensable tools to perform normal coordinate
analysis so that modern vibrational spectroscopy is unimagin-
able without involving them. In the present study the FT-IR,
FT-Raman and theoretical calculations of the wavenumbers of
the title compound are reported.
IR 1760× spectrometer. The spectral resolution was 4 cm−1
.
Standard KBr technique with 1 mg sample per 300 mg KBr was
used. The FT-Raman spectrum (Fig. 2) was obtained on a Bruker
IFS 66 V NIR-FT instrument equipped with a FRA106 Raman
module. A Nd/YAG laser at 1064 nm with an output on 300 mW
was used as the exciting source. The detector was a Ge-diode
cooled to liquid nitrogen temperature. One thousand scans were
accumulated with a total registration time of about 30 min. The
spectral resolution after apodization was 6 cm−1. A correction
according to the fourth power scattering factor was performed,
but no correction to instrumental response was done.
2. Experimental
For the synthesis of the benzoxazole derivative, using aque-
ous mineral acids as the condensation reagent did not provide
∗
Corresponding author.
E-mail address: cyphyp@rediffmail.com (C.Y. Panicker).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.08.026

P.L. Anto et al. / Spectrochimica Acta Part A 67 (2007) 744–749
745
imaginary wavenumbers on the calculated vibrational spectrum
confirms that the structure deduced corresponds to minimum
energy. In total there are 75 vibrations from 3050 to 38 cm−1
.
The assignments of the calculated wavenumbers are aided by the
animation option of MOLEKEL program, which gives a visual
presentation of the vibrational modes [14,15].
4. Results and discussion
The observed Raman and IR bands with their relative inten-
sities and calculated wavenumbers and assignments are given
in Table 2 . In aromatic compounds the asymmetric stretching
vibrations of CH3 are expected in the range 2905–3000 cm−1
and symmetric CH3 vibrations in the range 2860–2870 cm−1
[16,17]. The first of these results from the asymmetric stretch-
ing νaCH3 mode in which two C–H bonds of the methyl group
are extending while the third one is contracting. The second
arises from symmetrical stretching νsCH3 in which all three of
the C–H bonds extend and contract in phase. The asymmetric
stretching modes of the methyl group are calculated to be 2931,
2907 cm−1 and the symmetric mode at 2861 cm−1. νaCH3 vibra-
tions are observed at 2921 (IR), 2928 cm−1 (Raman) and νsCH3
at ∼2875 cm−1 in both spectra. Two bending can occur within a
methyl group. The first of these, the symmetrical bending vibra-
tion, involves the in-phase bending of the C–H bonds. The sec-
ond, the asymmetrical bending vibration, involves out-of-phase
bending of the C–H bonds. The asymmetric deformations are
expected in the range 1400–1485 cm−1 [16]. The calculated val-
ues of δaCH3 modes are at 1469 and 1457 cm−1. Experimentally
no bands are observed. In many molecules the symmetric defor-
mation δsCH3 appears with an intensity varying from medium
to strong and expected in the range 1380 25 cm−1 [16]. The
band at 1391 cm−1 in the Raman spectrum and at 1400 cm−1
(calculated) are assigned to δsCH3. Aromatic molecules display
a methyl rock in the neighbourhood of 1045 cm−1 [16]. The
second rock in the region 970 70 cm−1 [16] is more difficult
to find among the C–H out-of-plane deformations. For the title
compound, these modes are calculated at 1048 and 989 cm−1
and only one band is observed at 1053 cm−1 in the IR spectrum.
The torsional modes of CH3 are seen in the range 38–481 cm−1
[16].
Fig. 1. FT-IR spectrum of 5-methyl-2-(p-fluorophenyl)benzoxazole.
Fig. 2. FT-Raman spectrum of 5-methyl-2-(p-fluorophenyl)benzoxazole.
3. Computational details
Calculations of the title compound were carried out with
Gaussian03 program [12] 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 analytic
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 [13]. 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. 3) are given in Table 1 . The absence of
Fluorine atoms directly attached to an aromatic ring give rise
to bands in the region 1270–1100 cm−1 [17]. Many of these
compounds including the simpler ones with one fluorine only
on the ring absorb near 1230 cm−1 [17]. Saxena et al. [18]
reported a value 1157 cm−1 for νC–F stretching mode for poly-
benzodithiazole. For the title compound the stretching mode
νC–F is observed in the IR spectrum at 1223 cm−1 and the cal-
culated value is 1244 cm−1
.
The C N stretching skeletal bands are observed in the range
1672–1566 cm−1 [11,18–20]. Saxena et al. [18] reported a value
1608 cm−1 for polybenzodithiazole and Klots and Collier [21]
reported a value 1517 cm−1 for benzoxazole as νC N stretch-
ing mode. The bands at 1661 cm−1 in the Raman spectrum,
1671 cm−1 in the IR spectrum and 1643 cm−1 given by calcu-
lation are assigned as νC N.
Fig. 3. Optimized geometry of 5-methyl-2-(p-fluorophenyl)benzoxazole.

746
P.L. Anto et al. / Spectrochimica Acta Part A 67 (2007) 744–749
Table 1
Optimized geometrical parameters of 5-methyl-2-(p-fluorophenyl)benzoxazole, atom labeling is according to Fig. 3
Bond angle (◦)
Dihedral angle (◦)
˚
Bond length (A)
C1–C2
C1–C6
C1–H7
C2–C3
C2–N8
C3–C4
C3–O9
C4–C5
C4–H10
C5–C6
C5–H11
C6–C12
N8–C27
O9–H26
O9–C27
C12–H13
C12–H14
1.3884
1.385
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(2,3,9)
118.2
120.4
121.4
120.2
131.5
108.2
123.4
107.2
129.4
115.8
122.1
122.1
122.5
118.8
118.7
119.8
120.8
119.4
104.5
170.6
104.6
84.8
111.2
111.2
111.2
107.5
107.8
107.8
120.4
120.4
119.2
118.6
121.7
119.7
122.4
118.8
118.9
118.6
119.8
121.7
120.4
119.7
119.9
119.6
119.3
121.1
96.8
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(2,1,6,12)
D(7,1,6,5)
D(7,1,6,12)
D(1,2,3,4)
D(1,2,3,9)
D(8,2,3,4)
D(8,2,3,9)
D(1,2,8,27)
D(3,2,8,27)
D(2,3,4,5)
D(2,3,4,10)
D(9,3,4,5)
D(9,3,4,10)
D(2,3,9,26)
D(2,3,9,27)
D(4,3,9,26)
D(4,3,9,27)
D(3,4,5,6)
D(3,4,5,11)
D(10,4,5,6)
D(10,4,5,11)
D(4,5,6,1)
D(4,5,6,12)
D(11,5,6,1)
D(11,5,6,12)
D(1,6,12,13)
D(1,6,12,14)
D(1,6,12,15)
D(5,6,12,13)
D(5,6,12,14)
D(5,6,12,15)
D(2,8,27,9)
D(2,8,27,21)
D(3,9,26,20)
D(27,9,26,20)
D(3,9,27,8)
D(3,9,27,21)
D(26,9,27,8)
D(26,9,27,21)
D(21,16,17,18)
D(21,16,17,23)
D(22,16,17,18)
D(22,16,17,23)
D(17,16,21,20)
D(17,16,21,27)
D(22,16,21,20)
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)
D(24,18,19,20)
D(24,18,19,25)
D(18,19,20,21)
D(18,19,20,26)
D(25,19,20,21)
D(25,19,20,26)
−0.001
−179.99
179.99
0.004
1.0744
1.3776
1.3903
1.3777
1.3594
1.3828
1.0737
1.4047
1.076
1.5126
1.2727
2.4689
1.347
1.0859
1.0858
1.0835
1.3802
1.3931
1.0731
1.3808
1.0734
1.3778
1.3267
1.3836
1.0734
1.3902
1.073
0.008
−179.94
−179.98
0.065
−0.005
−179.99
179.98
−0.003
179.99
0.002
A(4,3,9)
A(3,4,5)
A(3,4,10)
A(5,4,10)
A(4,5,6)
A(4,5,11)
A(6,5,11)
A(1,6,5)
0.003
−179.99
179.99
−0.002
179.77
0.002
A(1,6,12)
A(5,6,12)
A(2,8,27)
A(3,9,26)
A(3,9,27)
A(26,9,27)
A(6,12,13)
A(6,12,14)
A(6,12,15)
A(13,12,14)
A(13,12,15)
A(14,12,15)
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(9,26,20)
A(8,27,9)
A(8,27,21)
A(9,27,21)
C
12–H15
C16–C17
16–C21
C16–H22
17–C18
C17–H23
18–C19
C
−0.21
C
−179.98
0.005
C
−179.99
179.99
−0.002
−0.011
179.94
179.98
−0.06
C18–F24
C19–C20
C
19–H25
C20–C21
20–H26
C21–C27
C
1.4667
119.34
−120.95
−0.79
−60.61
59.10
179.25
−0.0004
−178.00
−179.82
−0.04
−0.00
178.00
−179.96
0.03
−0.004
178.00
180.00
−0.0005
0.00
180.00
−180.00
−0.001
0.001
180.00
−180.00
−0.004
0.001
180.00
−179.99
0.01
115.4
127.2
117.4
−0.001
−179.99
180.00
0.007

P.L. Anto et al. / Spectrochimica Acta Part A 67 (2007) 744–749
747
Table 1 (Continued )
Bond angle (◦)
Dihedral angle (◦)
˚
Bond length (A)
D(19,20,21,16)
D(19,20,21,27)
D(26,20,21,16)
D(26,20,21,27)
D(19,20,26,9)
D(21,20,26,9)
D(16,21,27,8)
D(16,21,27,9)
D(20,21,27,8)
D(20,21,27,9)
−0.002
−180.00
179.99
−0.006
−179.98
0.02
−0.03
179.96
179.96
−0.04
Since the identification of all the normal modes of vibrations
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 mul-
tiple homo-and heteroaromatic rings [22–26]. In the following
discussion, the phenyl rings attached with methyl group and flu-
orine are designated as rings I and II, respectively. The modes
in the two phenyl rings will differ in wavenumber and the mag-
nitude 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 con-
sidered as quasi-degenerate and for the other modes a significant
amount of splitting is observed. Such observations have already
been reported [22–24,27].
The benzoxazole ring stretching vibrations exist in the range
1504–1309 cm−1 in both spectra [18,28]. Bands observed at
1504, 1433, 1418 cm−1 in the Raman spectrum [18] and 1499,
1474, 1412 and 1309 cm−1 in the IR spectrum are assigned as
the ring stretching vibrations. Klots and Collier [21] reported
the bands at 1615, 1604, 1475 and 1451 cm−1 as fundamental
ring vibrations of the benzoxazole ring.
As expected the asymmetric C–O–C vibration produce strong
band at 1263 and 1269 cm−1 in the IR and Raman spectrum,
respectively [11,19]. The symmetric C–O–C stretching vibra-
tion appear as a weak band at 1055 cm−1 in the IR spectrum
and at 1061 cm−1 in the Raman spectrum [11,19]. The ab ini-
tio calculations give 1261 and 1060 cm−1 as asymmetric and
symmetric C–O–C modes, respectively. The C–O–C stretching
is reported at 1250 and 1073 cm−1 for 2-mercaptobenzoxazole
[29].
Primary aromatic amines with nitrogen directly on the ring
absorb strongly at 1330–1260 cm−1 due to stretching of the
phenyl carbon–nitrogen bond [17]. For 2-mercaptobenzoxazole
this mode [29] is reported at 1340 cm−1 (Raman) and 1325 cm−1
(ab initio calculations). Sandhyarani et al. [30] reported νCN at
∼1318 cm−1 for 2-mercaptobenzothiazole. We have observed
this mode at 1332 cm−1 in IR spectrum, 1315 cm−1 in Raman
spectrum and 1315 cm−1 theoretically.
and Collier [21] reported the bands at 3085, 3074, 3065 and
3045 cm−1 as νC–H modes for benzoxazole. In the present
case, ab initio calculations give νC–H modes in the range
3001–3050 cm−1. The bands observed at 3023, 3048 cm−1 in
the IR spectrum and at 3036, 3075 cm−1 in the Raman spectrum
are assigned as these modes
For the paradisubstituted light–heavy, fluorine attached
phenyl ring the νPh modes are expected in the range
1280–1620 cm−1 and for the trisubstituted benzene ring these
modes are seen in the region 1250–1610 cm−1 [16]. For para
substitued benzenes, the δ CH modes are seen in the range
995–1315 cm−1 [16] and for trisubstitued benzenes these modes
are in the range 1050–1280 cm−1 [16]. Bands observed at 1009,
1096, 1125, 1153 1201, 1291, 1309 cm−1 in the IR spectrum
and at 1154, 1208 cm−1 in the Raman spectrum are assigned as
δCH modes.
The C–H out-of-plane deformations are observed between
1000 and 700 cm−1 [16]. Generally the C–H out-of-plane defor-
mations with the highest wavenumbers have a weaker intensity
than those absorbing at lower wavenumbers. These γCH modes
are observed at 847, 882 cm−1 in the IR spectrum. The ab initio
calculations give these modes at 817, 834, 856, 895, 958, 973
and 993 cm−1. For benzoxazole, Klots and Collier [21] reported
the bands at 932, 847 and 746 cm−1 as out-of-plane deforma-
tions of the benzene ring. According to Collier and Klots [32] the
non-planar vibrations are observed at 970, 932, 864, 847, 764,
746, 620, 574, 417, 254, 218 cm−1 for benzoxazole and at 973,
939, 856, 824, 757, 729, 584, 489, 419, 207, 192 cm−1 for ben-
zothiazole. The planar modes below 1000 cm−1 for benzoxazole
are reported to be at 920, 870, 778, 622, 538, 413 cm−1 and for
benzothiazole at 873, 801, 712, 666, 531, 504, 350 cm−1 [32].
We have also observed the planar and non-planar modes for the
title compound in this region. Two very strong CH out-of-plane
deformation bands, occurring at 840 50 cm−1 is typical for
1,4-disubstituted benzene [16]. For the title compound, a very
strong γCH band is observed at 847 cm−1. Again, according to
literature [16,17] a lower γCH absorbs in the neighbourhood
820 45 cm−1, but is much weaker or infrared inactive. The
ab initio calculation gives a γCH at 834 cm−1 and no band is
experimentally observed for this mode. In the case of trisubsti-
tuted benzenes, two γCH bands are observed at 890 50 and
815 45 cm−1 [16]. We have observed bands at 882 cm−1 in
the IR spectrum and at 816 cm−1 in the Raman spectrum. The-
The existence of one or more aromatic rings in a structure
is normally readily determined from the C–H and C C–C ring
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 [31]. Klot
oretically the values obtained are 895 and 817 cm−1
.

748
P.L. Anto et al. / Spectrochimica Acta Part A 67 (2007) 744–749
Table 2 (Continued )
Table 2
Calculated vibrational wavenumbers, measured infrared and Raman band posi-
tions and assignments for 5-methyl-2-(p-fluorophenyl)benzoxazole
νcalculated (cm−1
)
νIR (cm−1
)
νRaman (cm−1
)
Assignments
388
347
324
260
253
235
186
120
81
γPh I
νcalculated (cm−1
)
νIR (cm−1
)
νRaman (cm−1
)
Assignments
343 w
277 w
δPh I, τCH3
γPh I, II, τCH3
γPh(X) I
τPh II, τCH3
τCH3
τPh I, II, τCH3
τPh I, II, τCH3
τPh I, II, τCH3
τPh I, II, τCH3
τPh I, II, τCH3
τCH3
3050
3049
3036
3035
3034
3025
3001
2931
2907
2861
1643
1623
1611
1605
1585
1511
1482
1469
1457
1423
1404
1400
1315
1286
1273
1261
1244
1199
1189
1161
1143
1122
1097
1066
1060
1048
994
3075 m
ν(CH)II
ν(CH)II
ν(CH)I
ν(CH)II
ν(CH)II
ν(CH)I
ν(CH)I
νaCH3
3048 w
3036 vw
3023 w
2921 w
46
40
38
2928 w
νaCH3
νsCH3
2873 w
1671 wbr
1617 m
2875 vw
1661 vw
1624 vvs
νC
N
ν: 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 fluorine; v:
very; s: strong; m: medium; w: weak; sh: shoulder; br: broad. Subscripts—a:
asymmetric; s: symmetric.
νPh II
νPh I
νPh I, II
νPh II
νPh I
1603 s
1566 m
1499 vvs
1474 s
1572 s
1504 w
νPh I
The substitution of fluorine in the phenyl ring shortens the
C–C bond lengths C18–C19 and C18–C17 of the benzene ring.
Fluorine is highly electronegative and tries to obtain additional
electron density. It attempts to draw it from the neighbour-
ing atoms, which move closer together in order to share the
remaining electrons more easily as a result. Due to this the
bond angle A(17,18,19) is found to be 122.361◦ in the present
calculation which is 120◦ for normal benzene. Similarly, the
bond lengths C18–C19 and C18–C17 are 1.3778 and 1.3808 A,
respectively, which is 1.3864 A for benzene. For the title com-
pound the bond lengths C3–O9, C2–N8 are found to be 1.3594
and 1.3903 A while for 2-mercaptobenzoxazole [29] these are,
respectively, 1.3436 and 1.3739 A. The bond lengths N8–C27,
O9–C27, C2–C3 are found to decrease to 1.2727, 1.3470 and
1.3776 A from the values 1.3001, 1.3804 and 1.3927 A obtained
for 2-mercaptobenzoxazole [29]. These changes in bond lengths
for the title compound can be attributed to the conjugation of the
phenyl ring.
δaCH3
δaCH3
νPh I
νPh I, II
δsCH3
νC–N
νPh II
νCH I
νaC–O–C
νC–F
δCH I
δCH I
δCH II
δCH II
δCH I
1433 m
1418 m
1391 vw
1315 w
1412 s
1332 m
1309 w
1291 m
1263 s
1223 vs
1201 s
1269 s
1208 w
1154 w
˚
˚
1153 s
˚
˚
1125 m
1096 m
δCH I
˚
˚
δCH II
νsC–O–C
ρCH3
δCH II
γCH II
ρCH3
1055 vs
1053 w
1009 m
1061 vw
993
989
973
958
926
919
895
856
834
819
817
795
752
742
γCH II
γCH I
δNCO
δPh(X)I
γCH I
γCH II
γCH II
δPh I, II
γCH I
δPh I
Acknowledgements
950 w
927 m
948 wsh
936 m
C. Yohannan Panicker would like to thank the University
Grants Commission, India for awarding a Teacher fellowship
and Hema Tresa Varghese thanks the University Grants Com-
mission, India for a minor research grant.
882 w
847 vs
823 m
816 w
References
809 vs
757 m
734 s
768 w
740 w
γPh I
γPh I, II
δPh I
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