CHAPTER - 5

FTIR INND RfiMLfiN STUDIES ON IEIlErYZIMIlDiZOLlE

5.1 INTRODUCTION

Benzimidazoles are known for commercial and

biological importance as phermaceuticals, veterinary

anthelminitics and fungicides. The vibrational spectra of

potentially pharmacologically active thiazoles and

benzothiazoles have been reported ( 1-5 ) . Among them many

products have emerged as antibiotics such as sulfathiazole

and a host of related compounds.

Chouteau et a1 ( 5 ) have discussed the infrared

spectrum of liquid thiazole above 650 cm-I as well as the

infrared spectra of its halogenated derivatives. They have

proposed the vibrat iona 1 assignrnon t of tho furldarnon to1 rr~urlos

of the unsubstituted ring together with an approximate

description of the molecular motions in terms of group

vibrations. For almost all the fundamentals the joint

evidence of the Raman polarization data and the infrared

vapour envelopes offers s definite and a unique assignment.

A value of 1620 cm-I for ll~iozolo lurldu~~~ulilol is oxcuodir~yly

high if one considers that the highest ring mode in

thiophene and iaothiazole at 1507 and 1489 cln-I

respectively. Other bands at 759 and 612 Cm-l are assigned

as A' fundamentals and the spectrum has strongest Raman

1 ines . The infrared apectrr in the region 2000-650 cm-'

of 73 thiazoles of known structure and of 5 partly known

structure are discussed in literature (4). The resolution

especially in lower frequency range, wee poor and no bands

below 770 cm-' were recorded. The band in the 1625 - 1535

cm-' region was observed in certain thiazolea only end is

usually weak. Nearly all thiazoles investigated showed one

or two medium to strong bands in the region 1535-1475 cm-l.

Similarly most of them have at least one medium to strong

bend in the range 1 4 4 5 - 1 3 8 5 cm-l, The only compounds to

show bands between 700-650 cm-I were thos'e containing a

monosubstituted benzene ring, some of methyl derivatives and

those containing an NH2 ~ r o u p in the form of primary amine,

amide, thioamide or hydrazide. There appears to be no

correlation between the number and position of the bands in

the 930-650 cm-I region and the pattern of the substitution,

is the same as benzene and pyridine series.

In a study of the infrared spectra of fifteen 2-

substituted thiazolines ( 3 ) the intensity of the bonds were

very weak in the region 1640-1550 cm-I. It was pointed out

that the heteroatomic compounds containing five mornberod

rings generally show three bands in the 1600-1350 cm-'

region near 1590, 1490 and 1400 cm-I due to ring stretching

modes. It was also reported in the literature that the

bands near 1610,1500 and 1380 cm-I are as approximate

frequencies for substituted thiazoles. The thiazole nucleus

contain the conjucated - C = C - N - C - system. With

increaeing C O ~ ~ U C U t ion, obsorvod fror~uunclos 110 lui~gc~r

correspond to the vibration of an individual bond, and when

the bonds are no longer alike, the assignment of the

vibrations to the oscillation of individual structures or

even to certain combinations of such oscillations becomes

impossible.

The ultraviolet spectra of thiazole and

benzothiazole were determined and assignments were made for

main absorption bands (6). It was observed that the

ultraviolet spectrum of thiazole has no resemblance to that

of benzothiazole.

Rao et a1 (7) reported the infrared spectra of 5

membered N- and N - S heterocyclic derivatives such as

triazoles, thiazoles, thiadiazoles. They have also

attempted empirical assignments for benzothiazole

derivatives. The CH out-of-plane deformation bands of

benzene rings fused to 5-membered heterocyclic rings were

found to be in the region 760 - 740 cm-I which is

characteristic of four adjacent benzenoid hydrogen atoms.

A number of instances were given by Bassignana et

a1 ( 8 ) in which the shift in the infrared frequency of tho

out-of-plane CH deformation of a 5 membered heterocyclic

ring can be correlated with the electronegativity of the

heteroatom. The successive introduction of functional

groups R, electron donors and acceptors into position 5 or 6

in the aromatic ring produces out-of-plane vibrational

dlaplacomotlls of tlio ell oclJocu~il LL I LIIU biubt l~ l lulucl C. A

decrease in the frequency of the nearly CH groups correspond

to a decrease of the electronegativity of the hetero atom.

This is true of benzo and 2-methyl benzo derivatives.

Changes occur when 2nd hetero atom is introduced in a p

position to that of the first hetero atoms (oxazole,

thiazolel. The vibrations are then influenced by the 2-

heteroatoms. In keeping the same ( N ) atom in 3- position in

the heterocyclic nucleus and in changing llio hotoroato~~~ 111

position 1, it is not possible to trace a straight line on a

plot of frequency verses electronegativity series. In

effect a broken line occurs to which corresponds the minimum

value of the out-of-plane CH deformation when the 2nd hetero

atom is N. An increase in CH frequency deformation with a

decrease in CH frequency deformation with a decrease in

electronegativity is established.

The infrared absorption spectra of 0.1 to 0.5

X I O - ~ M solutions of homo and heterocyclic 5 membered ring

compounds and their CsHs analogs were measured in CC14

solution with a . 5 to 5 . U cm line path ( 9 ) . The integrated

intensities (C-H stretching) of 5 membered heterocyclic

compounds are smaller ,than those of the correspondlng

homocyclic compounds and occured in the following order for

CqHq X . X 0 S . NH > 0. ICH of tho C01Iti onulogs, CBI16 x and

C12H8 x occurred in the order NH > S > 0.

The decrease in ICH for heterocyclic compounds was

accompanied by a hypsochromic shift in the center of gravity

of 3000 - 3100 cm-I absorbance. The introduction of

additional N atom in the ring augmented the effect both in

the ICH and the shift in the absorbance.

Recently the infrared and Raman spectra of

benzimidazole have been reported by Suwaiyan et a1 (10).

However, they have neither reported the normal coordinate

analysis nor the potential energy distribution associ~ted

with each vibrational mode. Hence the present study has

been undertaken to record and study the Fourier Transform

Infrared (FTIR) and laser Raman spectra of benzimidazole and

to assign the normal mode of vibrations on the basis of

normal coordinate calculations.

The pure benzimidazole was obtained from

Ms.BUrgOYne (Bombay) and was used as such. Tho FTIR

Spectrum of benzimidazo1.e in a KBr disk was recorded on a

Nicolet 20 DXB spectrometer in the region of 4 0 0 - 4 0 0 0 cm-I

at C.L.R.I. (Madras1 . The laser Raman spectrum was also

recorded in the region of 2 0 0 - 4 0 0 0 cm-I on a Cary model 82

grating spectrophotometer operating at 4 8 8 nm with 4W power.

The spectrum was recorded with a scanning speed of 3 0 cm-I

min-I with the spectral width 2 . 0 cm-l. The frequencies for

all sharp bands are accurate to ?cm-l. The structural

formula of the t i t l e compound lbnd tlie measured spoctpa e r o

shown in figures 1-3 respectively. The values of bond

length and bond angles are assumed from Sutton's table (11).

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 calculations were carried out using

Wilson's FG-Matrix method with the computer program written

by Mink and Mink(l2). Thirty nine coordinates were used to

obtain tho G-mntrlx. Iritnr~iril r:~~t~r+cIl~iirtcin fc~r. l l i c ~ 1 1 1 1 I -or-

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

FIG-!. STRUCTURE OF BENZIMIDAZOLE

of benzene. Thirty-nine symmetry coordinates were used to

calculate the corresponding potential energy distribution

which is given in table.1.

5.3 RESULTS AND DISCUSSION

The observed frequencies along with their relative

intensities and probable assignments are presented in

table.1 along with the calculated frequencies.

Benzimidazole molecule belongs to Cs symmetry and the 39

fundamental vibrations are active in both infrared and

Raman. The 39 fundamental vibrational frequencies are

distribute as

r vib=

27 a' (in-plane) + 12 a" (out of plane)

The results of the normal coordinate calculation

of the compound is also given in the same table. The

general agreement between the calculated and observed

frequencies for both in plane and out-of-plane modes are

good.

5 . 3 . 1 CARBON VIBRATIONS

The ( C n C ) vibrations possess more intensity if the

double bond is in conjucation with the ring. The actual

Positions are determined not so much by the nature of the

substituents but by the form of the substitution around the

ring (13). The two doubly degenerate e 2 8

modes

corresponding to ( C=C stretch in^ in benzene are assigned

to the bands at 1621, 1588, 1478, 1459 and 1247 cm-I in

benzimidazole.

The C-C ring breathing mode and C-C-C trigonal

bending are assigned to the bands at 835 and 1U07 cm-l. The

above C O ~ C ~ U S ~ O ~ S agree favourably well with Murray and

Galloway (14) and Golse and Thoi (15). 'The in-plane corbon

bending vibrations are obtained from the non-degenerate

blu (1010 cm-l) and degenerale e (6U6 cm-l) modes 01 2 8

benzene. The degenerate frequency under Cs symmetry hafi

been observed at 628 cm-I in benzimidazole.

The carbon out-or-plane bending vibrationfi ora

related to the non-degenerate b (703 cm-l) and degenerate 28

2u (404 cm-I) modes of benzene. ?he former is found Lo be

constant in substituted benzenes (16) and in this work it is

observed at 676 cm-l. The degenerate eZU (404 cm-l)

vibration splits into two non-totally symmetric components

and the bands observed at 478 and 577 cm-I in benzimidazole

are assigned t o this vibration.

5.3.2 C-H VIBRATIONS

The frequency of the C - l l strelcl~lng vibration8 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 3016 cm-I and

they are in good agreement with Augus et a1 (17) and Bailey

et a1 (18).

Studies on the spectra of benzene shows that there

appear to be two degenerate e (1178 cm-l) and elu 2 g

(1037 cm-l) and two non-degenerate bZU (1152 cm-'1 and a 2 8

(1340 cm-l) vibrations involving C-I1 in-plane bending

vibrations involving the hydrogen atom. The frequencies

1273, 1202, 1157, 1135 and 1114 cm-I in benzinlidazole are

assigned to C-H in-plane bending vibrations which belong to

a' species. These assignments are in agreement with values

given in the literature (1Y,20).

The C-H out-of-plane deformations result from b a g

(985 Cm-'), eZU (970 cm-l), e (850 cm-l) and aZU 1 g

(671 cm-') modes of benzene and they are expected to occur

in the region of 600 - 1000 cm-' (21.22). 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 (22,231. Hence the bands at 958, 933, 885, 769

and 749 cm" have been assigned to give C-H out-of-plane

bending vibration.

5.3.3 N-H STRETCHING

Tsuboi (24) reported the N-H stretching

frequency at 3481 cm-I insnlline. In lirte with 111s

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 deformations are assigned to bands at 1545

cm-I and 628 cm-I which agrees well with Venkateswaran and

Pandya (25) and Evans(26).

5.3.4 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 frequencies from other

vibrations. Pinchas et a1 (27) assigned the C-N stretching

band at 1368 cm-' in benzamide. Kahovec and Kohlrausch

(28) identified the stretching frequency of the C=N bond in

Salicylic aldoxime at 1617 cm-l. Referring to the above

workers, the bands at 1689 cm-' and 1365 cm-I are assigned

to C=N and C-N stretching, respectively.

The remainder of the observed frequencies in

table.1 may be occourrtod lor 111 lowotl coal~l~~nl l t l i ~ n nntl

overtones of the fundamentals which gives additional

support for their choice.

5 . 4 POTENTIAL BNRRGY DlSTHlLIUTlON (I'UU)

To check whether the choson sot of vibrotionol

frequencies contribute the maximum to the potential energy

associated with normal coordinates of the molecule, the

potential energy distribution has been calculated using the

relation

The close agreement be tween the observed and

calculated frequencies conlirrr~s Il~o vi11 I l l 1 t y ol 1l1o ~)rosor~l

assignment.

Observed and calculated frequencies and potential

energy distribution for benzimidzole

Species Freq. Observed Freq. Cnlcul- No. PTIH Italnen o lull Ase 1 HIIIIICIII I / IIL<D'+

f reqgyncy cm-I cm-I c rn

a ' 1 3460 M 3457 (N-H) stretching (98)

a' 2 3124 W 3121 (C-H) stretching 194)

a' 3 3104 W 3100 IC-H) stretching (931

a' 4 3068 M 3067 ( C - H ) stretching (97)

a' 5 3044 M 3042 (C-H) stretchlng (991

a' 7 1689 M 1691 (C=N] stretching (85) V I 1 (11)

a' 8 1621 M 1619 (CEC) stretching ( 9 1 )

1580 (CmC) stretching ( 8 0 ) v 1 5 (I ( ; )

(N-H) in plane bending ( 7 2 ) , V Z u ( 1 8 )

[ C = C ) stretching ( 8 9 )

( C = C ) stretching ( U s ) ",, ( 1 2 )

( C = C ) stretching ( 9 1 )

(C-N) stretching ( 8 0 )

( C - N ) stretching ( 9 0 )

(C-N) stretching ( 9 2 )

( C - H ) in plane bending ( 8 4 )

( C - C ) stretchlnp, ( U U ) V Z l ( 1 0 )

( C - H ) in plane bending ( 7 5 )

( C - t i ) in plane bending ( 8 7 )

( C - H ) i n plane bending ( 6 9 1 , v 3 4 ( 2 1 )

(C-C-C) lrigonal bending ( 7 7 1 , ~ ~ ~ ( 1 1 1

( C - H ) out o f plane bending ( 7 8 1

( C - H ) out of plane bending ( 6 9 ) , ~ ~ ~ ( 2 2 )

( C - t l ] out of plane bending ( 8 4 )

( C - C ) ring breathing mode ( 6 9 1 , v 2 5 ( 2 8 )

an 3 1 7 6 9 S 7 7 0 S 7 7 4 ( C - H ) out of plnno boncllng ( U S )

7 3 9 ( C - H ) out of plane bonding ( 7 1 1 , V J 4 ( 1 t i )

a I' 3 3 6 7 6 VW 6 6 7 ( C - C - C ) out of plane bonding ( 7 0 ) , V J 7 ( 2 1 )

a' 2 5 6 3 1 M 6 2 8 ( C - C - C ) in plane bending (81).vl5

a " 3 4 6 2 8 W 6 2 6 W 6 2 6 ( N - H ) out of plane bending ( 8 0 1 , v 3 0 ( 1 2 )

a " 3 5 5 7 7 W 5 7 0 ( C - C - C ) out o f plane bending ( 8 2 )

a' 2 6 5 4 5 VW 5 3 0 ( C - C - C ) in plane bending (651, v 2 1 ( 2 3 )

a " 3 6 4 7 8 W 4 7 2 (C-C-C) out of plane bending ( 7 5 )

a ' 2 7 4 2 1 S 4 1 7 M 4 1 1 (C-C-C) in plane bending ( 7 1 1 , v17(14)

2 7 2 w 2 6 4 (C-C-C) out of plono bending ( 7 0 ) , V Z Z ( 1 9 )

2 4 4 M 2 3 1 ( C - C - C ) out of plane bendin8 ( 6 5 ) , VZ! , (15)

2 2 8 W 2 1 4 ( C - C - C ) out o f plane bonding ( 6 1 ) , V 3 7 ( 3 0 )

VS - Very strong; S - Strong: M - Medium W - Weak; VW - Very weak; PED values less than 10% are not reported here,

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