Specrrochimica Acta. Vol. 47A. No. 8, pp. 1111-1115, 1991 Printed in Great Britain

0584-x539/91 s3.00+0.00 @ 1991 Pergamon Press plc

IWR and Raman studies on bemimidamle

S. MOHAN* and N. SUNDARAGANESAN

Raman School of Physics, Pondicherry University, JIPMER Campus, Pondicherry-605 006, India

and

J. MINK

Institute of Isotopes, Hungarian Academy of Sciences, H-1525 Budapest, Hungary

(Received 10 October 1990; infi~l form 3 April 1991; accepted 15 April 1991)

Ahatract-The FZlR and laser Raman spectra of benzimidazole have been recorded. The observed frequen- cies were assigned to various modes of vibrations on the basis of normal coordinate calculations, assuming C, Point group symmetry. The Potential energy distribution associated with normal modes is also reported here. The assignment of fundamental vibrations agrees well with the calculated frequencies.

THE present study is a part of our ongoing discussions on the vibrational spectra of the derivatives of benzene [l, 21. The vibrational spectra of potentially pharmacologically active thiazoles and benzothiazoles have been reported in the literature [3-51. Recently the IR and Raman spectra of benzimidazole have been reported by SUWAIYAN et al. [6]. However, they have neither reported the normal coordinate analysis nor the potential energy distribution associated with each vibrational mode. Hence the present study has been undertaken to record and study the FIIR and laser Raman spectra of benzimida- zole and to assign the normal mode of vibrations calculations.

EXPERIMENTAL

on the basis of normal coordinate

The pure benzimidazole was obtained from Burgoyne (Bombay) and used as such. The FTIR spectrum of benzimidazole in a KBr disc was recorded on a Nicolet 2ODXEl spectrometer in the region of 400-4OtXcm-’ at C.L.R.I. (Madras). The laser Raman spectrum was also recorded in the region of 200_4C@O cm-’ on a Cary model 82 grating spectrophotometer operating at 488 nm with 4 W power. The spectrum was recorded with a scanning speed of 30 cm-’ min-’ with the spectral width 2.0 cm-‘. The frequencies for all sharp bands are accurate to f 1 cm-‘.

The values of bond length and bond angles are assumed from SU-ITON’S table [7]. The normal coordinate calculations were performed to support the assignment of the fundamental vibrational frequencies and obtained the potential energy distribution for the normal modes. These calcula- tions were carried out using Wilson’s FG-matrix method with the computer program written by MINK and MINK [8]. Thirty-nine coordinates were used to obtain the G-matrix. Internal coordi- nates for the out-of-plane torsional vibrations are defined as recommended by IUPAC. The general quadratic valence force field is adopted for both in-plane and out-of-plane vibrations. The initial set of force constants were taken from similar derivatives of benzene. Thirty-nine symmetry coordinates were used to calculate the corresponding potential energy distribution which is given in Table 1.

RESULTS AND DISCUSSION

The observed frequencies along with their relative intensities are presented in Table 1. Since the benzimidazole molecule belongs to C, symmetry, all the 39 fundamental

* Author to whom correspondence should be addressed.

1111

1112 S. MOHAN et al.

Table 1. Observed and calculated frequencies and potential energy distribution for benzimidaaole

Frequency Observed frequency Calculated No. FTIR Raman frequency

Species V (cm-‘) (cm-‘) (cm-‘) AssignmentlPED %

a’ 1

a’ 2 a’ 3 a’ 4 a’ 5 a’ 6

a’ 7 a’ 8 a’ 9 a’ 10

a’ 11 a’ 12 a’ 13 a’ 14 a’ 15 a’ 16 a’ 17 a’ 18 a’ 19 a’ 20 a’ 21 a’ 22 a’ 23 a” 28 a” 29 a” 30 a’ 24 a” 31 a” 32 d 33 a’ 25 a” 34 d’ 35 a’ 26 a” 36 a’ 27 a” 37 a” 38 a“ 39

3460m 3124 w 3104 w 3068 m 3044m 3016 vm 2985 m 2952 m 2900vw 2852 m 2811 w 1930 w 1895 w 1772 1689 m 1621 m 1588 m 1545 VW 1567 m 14% VW 1478 s 1486 w 1459 s 1410 vs 1408 w 1365 m 1348 w 1302 s 1305 m 1273 s 1270 s 1247 vs 1256 m 1202 m 1157 w 1135 m 1114 vw 1004m 1007 s 958 s 933 w 885 m 890m 835 vw 769 s 770 s 749 vs 676 VW 631 m 628 w 626 w 577 w 545 VW 478 w 421 s 417 m

272 w 244 m 228 w

3457 3121 3100 3067 3042 3015

1691 1619 1580 1553

1471 1449 1405 1358 1352 1308 1265 1241 1185

1130 1104 1012 951 906 881 827 774 739 667 628 626 570 530 472 411 264 231 214

(N-H) stretching (98) (C-H) stretching (94) (C-H) stretching (93) (C-H) stretching (97) (C-H) stretching (99) (C-H) stretching (99) (1698 + 1302) (1588 + 1365) (1621+ 1273) (1478 + 1365) (2 x 1410) (3044 - 1114) (1410+478) (1545 + 228) (C=N) stretching (85), vu (11) (C=C) stretching (91) (C=C) stretching (80), vu (16) (N-H) in-plane bending (72), vs,, (18) (2 x 749) (C=C) stretching (89) (C=C) stretching (85) v19 (12) (C=C) stretching (91) (C-N) stretching (80) (C-N) stretching (90) (C-N) stretching (92) (C-H) in-plane bending (84) (C-C) stretching (88), v,, (10) (C-H) in-plane bending (81) (C-H) in-plane bending (75) (C-H) in-plane bending (87) (C-H) in-plane bending (69) vM (21) (C-C-C) trigonal bending (77) vz (11) (C-H) out-of-plane bending (78) (C-H) out-of-plane bending (69), vx (22) (C-H) out-of-plane bending (84) (C-C) ring breathing mode (69), vz (28) (C-H) out-of-plane bending (85) (C-H) out-of-plane bending (71), v, (16) (C-C-C) out-of-plane bending (70), v,, (21) (C-C-C) in-plane bending (81), vIs (N-H) out-of-plane bending (80), v, (12) (C-C-C) out-of-plane bending (82) (CC-C) in-plane bending (65), vzI (23) (C-C-C) out-of-plane bending (75) (C-C-C) in-plane bending (71). v,, (14) (C-C-C) out-of-plane bending (70) vx (19) (C-C-C) out-of-plane bending (65). vB (15) (C-C-C) out-of-plane bending (61) vs, (30)

vs, Very strong; s, strong; m, medium; w, weak; vw, very weak. PED values less than 10% are not reported here.

vibrations are active in both IR and Raman. Of these, 27 are in-plane vibrations and 12 are out-of-plane vibrations.

Carbon vibrations

The (C=C) vibrations are more interesting if the double bond is in conjugation with the ring. The actual positions are determined not so much by the nature of substituents

FTIR and Raman studies on benzimidazole 1113

but by the form of the substitution around the ring [9]. The two doubly degenerate ezg modes corresponding to (C=C) stretching in benzene are assigned to the bands at 1621, 1588,1478,1459 and 1247 cm-’ in benzimidazole.

The C-C ring breathing mode and C-C-C trigonal bending are assigned to the bands at 835 and 1007 cm- ‘. The above conclusions agree favourably well with MURRAY and GALLOWAY [lo] and GOLSE and THOI [ll]. The in-plane carbon bending vibrations are obtained from the non-degenerate bI, (1010 cm-‘) and degenerate eze (606 cm-‘) modes of benzene. The degenerate frequency under C, symmetry has been observed at 628 cm-’ in benzimidazole.

The carbon out-of-plane bending vibrations are related to the non-degenerate b, (703 cm-‘) and degenerate ez,, (404 cm-‘) modes of benzene. The former is found to be constant in substituted benzenes [12] and in this work it is observed at 676 cm-‘. The degenerate eti (404 cm-‘) vibration splits into two non-totally symmetric components and the bands observed at 478 and 577 cm-’ in benzimidazole are assigned to this vibration.

C-H Vibrations

The frequency of the C-H stretching vibrations of the methyl and methylene groups in the side chain do not differ very much from those found in the spectra of aliphatic compounds. They are not appreciably affected by the nature of the substituents. In the present case they are observed at 3124, 3104, 3068,3044 and 3016cm-’ and they are in good agreement with Aucus et al. [13] and BAILEY et al. [ 141.

Studies on the spectra of benzene shows that there appear to be two degenerate eze (1178cm-‘) and e,, (1037 cm-‘) and two non-degenerate bzu (1152cm-‘) and a% (1340 cm-‘) vibrations involving C-H in-plane bending vibrations involving the hydro- gen atom. The frequencies 1273, 1202, 1157, 1135 and 1114cm-’ in benzimidazole are assigned to C-H in-plane bending vibrations which belong to a’ species. These assign- ments are in agreement with values given in the literature [15,16].

The C-H out-of-plane deformations result from b, (985 cm-‘), ezu (970 cm-‘), elg (850 cm-‘) and a2,, (671 cm-‘) modes of benzene and they are expected to occur in the

I I I 1 I , I

4000 3200 2400 1600 600 400 200

Fig. 1. Infrared and Raman spectra of benzimidazole.

1114 S. MOHAN et al.

region of 600-1000 cm-’ [17,18]. The changes in the frequencies of these deformations from their values in benzene are almost determined exclusively by the relative position of the substituents and are almost independent of their nature [18,19]. Hence the bands at 958, 933, 885, 769 and 749 cm-’ have been assigned to give C-H out-of-plane bending vibration.

N-H Stretching

TSUBOI [20] reported the N-H stretching frequency at 3481 cm-’ in aniline. In line with his observation, (N-H) stretching is assigned to the band at 3460 cm-’ in the present work. The N-H in-plane bending and N-H out-of-plane bending are assigned to the bands at 1545 cm-’ and 628 cm-’ which agrees well with VENKATESWARAN and PANDYA [21] and EVANS [22].

C=N, C-N Vibrations

The identification of the C-N stretching frequency in the side chains is a rather difficult task since there are problems in identifying these frequncies from other vibrations. PINCHAS ef al. [23] assigned the C-N stretching band at 1368 cm-’ in benzamide. KAHOVEC and KOHLRAUSCH [24] identified the stretching frequency of the C=N bond in salicylic aldoxime at 1617 cm-‘. Refering to the above workers, the bands at 1689 cm-i and 1365 cm-’ are assigned to C=N and C-N stretching, respectively.

The remainder of the observed frequencies in Table 1 may be accounted for from allowed combinations and overtones of the fundamentals which gives additional support for their choice.

Potential energy distribution

To check whether the chosen set of vibrational frequencies contribute the maximum to the potential energy associated with normal coordinates of the molecule, the potential energy distribution has been calcualted using the relation

The close agreement between the observed and calculated frequencies confirms the validity of the present assignment.

REFERENCES

[l] S. Mohan and A. R. Prabakaran, J. Raman Spectrosc. 28,263 (1989). [2] A. R. Prabakaran and S. Mohan, Indian I. Phys. 63B, 468 (1989). [3] A. Taurins, J. G. E. Fenynes and R. Norman-Jones, Can. I. Chem. 35,423 (1957). [4] B. Ellis and P. J. F. Griffiths, Spectrochim. Acta 21, 1881 (1965). [5] C. N. R. Rao and R. Venkataraghavan, Can. I. Chem. 42,43 (1964). [6] A. Suwaiyan, R. Zwarich and N. Baig, J. Raman Specrrosc. 21,243 (1990). [7] L. E. Sutton, The Interatomic Bond Distances and Bond Angles in Molecules and Ions. The Chemical

Society, London (1958). [8] J. Mink and L. M. Mink, Computer Progam System for Vibrational Analysis of Molecules. Erlangen

(1983). [9] L. J. Bellamy, The Infrared Spectra of Complex Molecules. John Wiley, New York (1959).

[lo] M. J. Murray and W. S. Gallaway, 1. Am. Chem. Sot. 70, 3867 (1948). [ll] R. Golse and L. V. Thoi, Comp. Rend. AC. Sci. Paris 230, 210 (1950). [12] J. H. S. Green, Spectrochim. Acta. 18, 39 (1%2). [13] W. R. Augus, C. K. Ingold and A. H. Leckie, J. Chem. Sot. 925 (1936). [14] C. R. Bailey, R. R. Gordon and J. B. Hale, J. Chem. Sot. 299 (1946). [15] E. F. Mooney, Specrrochim. Acta. 20, 1343 (1964). [16] G. Joshi and N. L. Singh, Spectrochim. Acta. IDA, 1341 (1%7). [17] H. W. Thompson and R. B. Temple, J. Chem. Sot. 1432 (1948). [18] G. Varsanyi, Acta Chim. Hung. 50, 225 (1966).

FTIR and Raman studies on benzimidazole

D. H. Whiffen and H. W. Thompson, J. Chem. Sot. 268 (1945). M. Tsuboi, Spectrochim. AC& 16, 505 (1960). C. S. Venkateswaran and N. S. Pandya, Proc. Id AC. Sci. A15,390 (1942). J. C. Evans, Spectrochim. Acto 16,428 (1960). S. Pinchas, D. Samuel and M. Weiss-Broday, J. Chem. Sot. 1688 (l%l). L. Kahovec and K. W. F. Kohlrausch, Mom&. Chem. 74, 333 (1941).

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