VIBRATIONAL SPECTROSCOPIC STUDIES OF IMIDAZOLE

Armenian Journal of Physics, 2015, vol. 8, issue 1, pp. 51-55 VIBRATIONAL SPECTROSCOPIC STUDIES OF IMIDAZOLE R. RAMASAMY Department of Physics, Natio...
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Armenian Journal of Physics, 2015, vol. 8, issue 1, pp. 51-55

VIBRATIONAL SPECTROSCOPIC STUDIES OF IMIDAZOLE R. RAMASAMY Department of Physics, National College (Autonomous), Trichy – 620 001, Tamilnadu, India. e-mail: [email protected] Received 19 August 2014

Abstract – Normal coordinate calculations of imidazole have been carried out using Wilson’s FG matrix mechanism. On the basis of General Valence Force Field (GVFF) for both in-plane and out-ofplane vibrations. The potential energy constants obtained in this study are refined using numerical methods. Keywords: Imidazole, FTIR, Normal coordinate analysis, potential energy distribution.

1. Introduction Imidazole is an organic compound with the formula C3 H4 N2. This aromatic heterocyclic is classified as an alkaloid. Imidazole refers to the parent compound where as imidazoles are a class of heterocycles with similar ring structure but varying substituents. This ring system is present in important biological building blocks such as histidine and the related hormone histamine. Imidazole can serve as a base and as a weak acid. Many drugs contain an imidazole ring, such as anti fungal drugs and nitroimidazole. 2. Experimental details Pure chemical of imidazole is obtained from Lancaster chemical company (England) and used as such without any further purification. The FTIR spectrum of imidazole was recorded in the region 4000-400 cm-1 using KBr pellet. The Bruker IFS 66V model FTIR spectrometer was used for the spectral measurements. The globar and mercury arc sources, KBr beam splitters are used while recording FTIR spectra of the title compound. 2.1. Structure and Symmetry The molecular structure of imidazole is shown in Fig. 1. From the structural point of view the molecule is assumed to have Cs point group symmetry. The 21 fundamental modes of vibrations arising for this molecule are distributed into 15 A' and 6 A" species. The A' and A" species represent the in-plane and out-of-plane vibrations. 4 C

3 N

H 9

8 H

2 C

5 C 1 N

H6

7 H

Fig. 1. Molecular structure of Imidazole.

 

RAMASAMY || Armenian Journal of Physics, 2015, vol. 8, issue 1

2.2. Normal coordinate analysis The evaluation of potential energy constants are made on the basis of GVFF by applying Wilson’s FG Matrix mechanism [1]. The structural parameters were taken from the Sutton’s table [2]. The vibrational secular determinants have been solved using the computer programmes with the SIMPLEX optimization procedure [3]. The initial set of force constants and the vibrational frequencies required for the calculations were taken from the literature [4]. All the force constants have been refined via a non-linear square fit analysis between the calculated and observed frequencies. The refinement converged smoothly in three cycles. 2.2.1. Symmetry coordinates Detailed description of vibrational modes can be given by means of normal coordinate analysis. For this purpose, the full set of 31 standard internal valence coordinates (containing 10 redundancies) were defined as given in Table 1. From these, a non-redundant set of local internal coordinates were constructed (Table 2) much like the natural internal coordinates recommended by IUPAC [5,6]. Theoretically calculated force fields were transformed to the latter set of vibrational co-ordinates and used subsequent calculations. Table 1. Definition of internal coordinates of imidazole. No(i) Symbol Type Stretching 1-4 ri C-N 5-7 Ri C-H C-C 8 qi N-H 9 Qi In-plane-bending 10-13 αi N-C-H 14-15 αi C-C-H C-N-H 16-17 βi Ring 18-22 φi Out-plane-bending 23-25 ωi C-H 26 σi N-H Torsion 27-31

 

ti

τRing

Definition C2-N3, C2-N1, C4-N3, C5-N1, C2-H8, C4-H9, C5-H6 C4-C5 N1-H7 N1-C2-H8, N3-C2-H8, N1-C5-H6, N3-C4-H9 C5-C4-H9, C5-C4-H6 C2-N1-H7, C5-N1-H7 N1-C2-N3, C2-N3-C4, N3-C4-C5, C4-C5-N1, C5-N1-C2 H8-C2-N1-N3, H9-C4-N3-N1, H6-C5-C4-N1 H7-N1- C5-C2 C2-N1-C5-C4, N1-C5-C4-N3, C5-C4-N3-C2, C4-N3-C2-N1, N3-C2-N1-C5

 

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Vibrational Spectroscopic Studies of Imidazole || Armenian Journal of Physics, 2015, vol. 8, issue 1

Table 2. Definition of local symmetry coordinates of imidazole. No(i)

Type

Definition

1-4

CN

r1, r2, r3, r4

5-7

CH

R5, R6, R7

8

CC

q8

9

NH

Q9

10-12

bCH

(α10 – α11)/√2, (α12 – α13)/√2, (α14 – α15)/√2,

13

bNH

(β16 – β17)/√2

14

Rbend1

φ18 + (φ19 + φ22) + b(φ20 – φ21)

15

Rbend 2

(a – b) (φ19 – φ22) + (1 – a) (φ20 + φ21)

16-18

ωCH

ω23, ω24, ω25

19

σNH

σ26

20

R torsion 1

τ29 + b(τ27 + τ31) + a(τ28 + τ30)

21

R torsion 2

(a – b) (τ30 – τ28) + (1 – a) (τ31 – τ27)

a = cos 144º and b = cos 72º

2.2.2. Vibrational band assignments

Transmittance (%)

The FT-IR spectrum of the title compound is shown in Fig. 2.

Wavenumber (cm-1)

Fig. 2. FTIR spectrum of Imidazole. The observed frequencies of the title compound together with relative intensities, probable assignments, calculated frequencies and PEDS are presented in Table 3.

 

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RAMASAMY || Armenian Journal of Physics, 2015, vol. 8, issue 1

Table 3. Vibrational frequencies and assignments of imidazole. S. No.

Species

Observed frequency (cm-1) FT-IR

Calculated frequency (cm-1)

1

A’

3376

3365

N-H stretching(94)

2

A’

3126

3116

C-H stretching(99)

3

A’

3040

3028

C-H stretching(98)

4

A’

2922

2911

C-H stretching(96)

5

A’

1593

1583

C-C stretching(73)

6

A’

1486

1475

C-N stretching(76)

7

A’

1440

1430

C-N stretching(74)

8

A’

1367

1355

C-N stretching(72)

9

A’

1325

1314

C-N stretching(76)

10

A’

1251

1241

C-H in-plane-bending(71)

11

A’

1223

1212

N-H in-plane-bending(69)

12

A’

1142

1131

C-H in-plane-bending(70)

13

A’

1107

1116

C-H in-plane-bending(72)

14

A”

1057

1067

C-H out-of-plane-bending(66)

15

A”

929

937

C-H out-of-plane-bending(65)

16

A’

891

899

Ring deformation in-plane-bending(52)

17

A”

833

842

N-H out-of-plane-bending(60)

18

A’

817

823

Ring deformation in-plane-bending(53)

19

A”

750

745

C-H out-of-plane-bending(66)

20

A”

655

661

Ring deformation out-of-plane-bending(58)

21

A”

615

622

Ring deformation out-of-plane-bending(59)

Assignment (% PED)

C-H vibrations The molecular structure shows the presence of C-H stretching vibrations in the region 3000-3100 cm-1 which is the characteristic region for the ready identification of C-H stretching vibrations [7,8]. In this region, the bands are not affected appreciably by the nature of the substituents. Hence, in the present investigation, the C-H vibrations have been found at 3126, 3040 and 2922 cm-1 in the FTIR spectrum. C-C vibrations The bands between 1400 and 1650 cm-1 in benzene derivatives are due to C-C stretching vibrations [9]. Therefore the C-C stretching vibrations of the title compound are observed at 1593 cm-1 in the FTIR spectrum.

 

 

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Vibrational Spectroscopic Studies of Imidazole || Armenian Journal of Physics, 2015, vol. 8, issue 1

N-H Vibrations In all the heterocyclic compounds, the N-H stretching vibrations [10] occur in the region 3500-3000 cm-1. The FTIR band appeared at 3376 cm-1 of the title compound has been assigned to N-H stretching modes of vibrations. C-N vibrations The identification of C-N stretching frequency is a very difficult task, since the mixing of bands is possible in this region. Hence, the FTIR bands at 1486, 1440, 1367 and 1325 cm-1 in imidazole have been designated to C-N stretching modes of vibrations. These assignments are made in accordance with the assignments proposed by Krishnakumar et al. [11]. 3. Conclusion Based on the normal coordinates analysis a complete vibrational analysis was performed for imidazole. A systematic set of symmetry coordinates have been constructed. The closer agreement obtained between the calculated and observed frequencies and the PED calculations are also supporting the assignments made for various functional groups present in the molecule. REFERENCES 1. E.B. Wilson, Phys. Rev., 45, 706 (1934). 2. L.E. Sutton, The inter atomic bond distance and bond analysis in molecules and ions, Chemical Society, London, 1958. 3. J.A. Nelder, R. Mead, Comput. J., 7, 308 (1965). 4. V. Suryanarayan, Pavankumar, G. Ramanarao, Spectrochim., Acta A, 48, 1481 (1992). 5. IUPAC commission on molecular structure on spectroscopy, Pure & Appl Chem,. 50, 1707 (1978). 6. P. Pulay, G. Fogarasi, F. Prog, J.E. Boggs, J. Am. Chem. Soc., 101, 2550 (1979). 7. H.G. Silver, J.L. Wood, Trans. Faraday Soc., 60, 5 (1964). 8. R. Ramasamy, Journal of Applied Spectroscopy, 80, 506 (2013). 9. V. Krishnakumar, K. Parasuraman, A. Natarajan, Indian J. Pure & Appl. Phys., 35, 1 (1997). 10. V. Krishnakumar, R. Ramasamy, Spectrochim. Acta A, 69, 8 (2008). 11. V. Krishnakumar, R. Ramasamy, Indian J. Pure & Appl. Phys., 40, 252 (2002).

 

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