Molecular structure, vibrational spectroscopic studies and analysis of 2-fluoro-5-methylbenzonitrile

Indian Journal of Pure & Applied Physics Vol. 47, July 2009, pp. 481-490 Molecular structure, vibrational spectroscopic studies and analysis of 2-fluoro-5-methylbenzonitrile N Sundaraganesan a*, G Elango a, S Sebastian a & P Subramani b a Department of Physics (Engg.), b Department of Chemistry Annamalai University, Annamalai Nagar 608 002 E-mail: sundaraganesan_n2003@yahoo.co.in Received 29 October 2008; revised 12 March 2009; accepted 30 April 2009 Quantum mechanical calculations of energies, geometries and vibrational wavenumbers of 2-fluoro-5-methylbenzonitrile (2F5MBN) have been carried out by using density functional theory (DFT/B3LYP) method with 6-311++G(d,p), cc-pvdz, Aug-cc-pvdz as basis sets. The optimized geometrical parameters obtained by DFT calculations are found to be in good agreement with experimental X-ray data. The best level of theory in order to reproduce the experimental wavenumbers is B3LYP method with the 6-311++G(d,p) basis set. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman spectra of 2F5MBN has also been reported. The entropy of the compound under study is also performed at / 6-311++G(d,p), cc-pvdz, Aug-cc-pvdz levels. The theoretical spectrogram for FT-IR and FT-Raman spectra of the molecule have been constructed. Keywords: Molecular structure, Vibrational spectroscopy, Quantum mechanical calculations, Benzonitrile, Density functional theory 1 Introduction Benzonitrile is a phenolic cyanide compound. It is a colourless liquid with a boiling point of 197 ºC and having a smell of bitter almonds. Many derivatives of benzonitrile are widely used in industry and medicinal fields. The main products of benzonitrile-benzoic acid is used in medicine as urinary antiseptic in the form of salt and in vapour form for disinfecting bronchial tubes. Benzonitrile derivatives are used in dye industry for making aniline blue and also use d for preserving food products 1. Because of its wide use and structural simplicity, a large number of studies on benzonitrile and its derivatives were reported 2-3. The Raman spectrum of benzonitrile in phenol solution was studied by Abramczyk et al 2. They analyzed the effect of hydrogen bond on vibrational relaxation of the proton accepting group C N of benzonitrile in phenol and in deuterated phenol-od using Raman spectroscopy. The Raman and IR spectra of mono substituted benzonitriles was thoroughly studied by Green et al 4. They have analyzed the spectral frequencies using the guidance of previous interpretations and complete vibrational assignments of the molecule were also reported. They have calculated thermo dynamical properties of benzonitrile in the ideal gas state. Ram et al. 5 reported the vibrational spectra of three isomeric hydroxyl benzonitriles in solid form. Infrared spectra were recorded in the region 350-4000 cm 1. Complete assignments of the observed frequencies were made assuming C s point group for all the three isomers. Joshi et al. 6 reported the infrared and electronic absorption spectra of 2,6-, 3,5-dichloro benzonitrile and 3-chloro-4-methyl benzonitrile. The various modes of vibrations were assigned and the effects of substitution were analyzed. Passinghan et al. 7 reported the Raman spectra of some aromatic nitro compounds. Correlations were found between frequencies of the nitro vibrations and the electron donating and withdrawing effect of substitutions of the phenol ring. The infrared absorption spectra of liquid m- and p-benzonitrile have been recorded by Johri et al 8. The spectra were recorded forming a thin film of compound between the KBr plates. They carried out the complete vibrational spectra on the basis of C s and C 2v point group symmetry. Mohan et al. 9 reported the laser Raman spectrum of 2-chloro-6-methyl benzonitrile. They carried out assignment of most of the prominent bands in the spectrum based on the assumption that the benzonitrile belongs to the C s point group. Kumar et al. 10 studied the vibrational spectral studies of some substituted benzonitrile assignments. Vibrational spectra of benzonitrile and its radical anion have been studied by an ab initio study by Dimitrova 11.

482 INDIAN J PURE & APPL PHYS, VOL 47, JULY 2009 In the present paper, the vibrational spectra of 2F5MBN have been studied and the various normal modes with greater wavenumber accuracy have been identified. Density functional theory (DFT/B3LYP) calculations have been performed to support the present wavenumber assignment. Density functional theory calculations are reported to provide excellent vibrational frequencies of organic compounds if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlation, for basis set deficiencies and for the anharmonicity. 2 Methodology 2.1 FT-IR and FT-Raman measurements The compound 2-fluoro-5-methylbenzonitrile (2F5MBN) was obtained from Sigma-Aldrich Chemical Company, USA with a stated purity of greater than 99% and it was used as such without further purification. The FT-Raman spectrum of 2F5MBN has been recorded using 1064 nm line of Nd: YAG laser as excitation wavelength in the region 50-3500 cm 1 on a Bruker model IFS 66 V spectrophotometer. The FT-IR spectrum of this compound was recoded in the region 400-4000 cm 1 on IFS 66 V spectrophotometer using KBr pellet technique. The spectra were recorded at room temperature, with scanning speed of 30 cm 1 min 1 and the spectral resolution of 2.0 cm 1. The observed experimental FT-IR and FT-Raman spectra of 2F5MBN compound are shown in Figs 1 and 2 and comparison of experimental and theoretical FT-IR is shown in Fig. 3. The Theoretical FT-Raman by 6-311++G(d,p) (best-level) for the 2F5MBN molecule is shown in Fig. 4. The spectral measurements were carried out at Central Electro Chemical Research Institute (CECRI), Karaikudi, Tamil Nadu. 2.2 Computational details DFT calculations were performed using GAUSSIAN 03 12 program package without any constraint on the geometry 13. Geometries of the model 2F5MBN, have been first optimized with full relaxation on the potential energy surfaces at 6-311++G(d,p) level and the resultant geometries have been used as inputs for further calculations at DFT level. Polarization functions have been added for the better treatment of the fluoro, and methyl groups. The optimized structural parameters were used in the vibrational frequency calculations at Fig. 1 FT-IR spectrum of 2-fluoro-5-methylbenzonitrile Fig. 2 FT-Raman spectrum of 2-fluoro-5-methylbenzonitrile DFT levels to characterize all stationary points as minima. We have utilized DFT/ 6-311++G(d,p)/cc-pvdz, Aug-cc-pvdz approach for the computation of molecular structure, vibrational frequencies and energies of optimized structures, in the present work. By the use of GAUSSVIEW program 14 with symmetry considerations along with available related molecules vibrational frequency assignments were made with a high degree of accuracy. 2.3 Prediction of Raman intensities The Raman activities (S i ) calculated with Gaussian 03 program converted to relative Raman intensities (I i ) using the following relationship derived from the intensity theory of Raman scattering 15,16. i 4 o i i f( v v ) S Ii = v[1 exp( hcv / kt)] i where ν o is the exciting frequency in cm 1, ν i the vibrational wave number of the i th normal mode, h, c and k fundamental constants, and f is a suitably chosen common normalization factor for all peak

SUNDARAGANESAN et al.: VIBRATIONAL SPECTRA OF 2-FLUORO-5-METHYLBENZONITRILE 483 Fig. 3 Comparison of experimental and theoretical FT-IR spectrum of 2-fluoro-5-methylbenzonitrile intensities. For simulation of calculated FT-Raman spectra have been plotted using pure Lorentizian band shape with a bandwidth of (FWHM) of 10 cm 1 as shown in Fig. 4. 3 Results and Discussion 3.1 Geometric structure Our optimized bond length and angles in 2F5MBN are given in Table 1, along with atom numbering scheme given in Fig. 5. All the geometries determined belong to the true minimum proven by real wavenumbers in the vibrational analysis. Experimental data of the isolated molecules in gas phase has not been reported. Thus, present calculated values were compared with the microwave data (benzonitrile) in the solid phase determined by X-ray as published by Casoda et al 17. Since bond length in Fig. 4 Theoretical FT-Raman spectgrum of 2-fluoro- 5-methylbenzonitrile the microwave data is usually smaller than in the gas phase, the DFT calculations may describe the bond lengths of 2F5MBN in the gas phase correctly. The benzene ring appears a little distorted with C1-C2 (C1-C6) and C4-C5 bond length next to the substitution place (~1.40Å) longer than the bonds C2-C3 (C5-C6) in the middle of the phenyl ring (~1.39Å). These distortions are explained in terms of the change in hybridization affected by the substituent at the carbon place to which it is attached. The increase of the C-C bond lengths adjacent to the C2-F10, C1-C15 and C5-C11 bonds in the substitution place is accompanied by slightly irregular hexagonal structure of the angles C2-C1-C6, C1-C2-C3 and C2-C3-C4, 118.4, 121.4 and 118.9, respectively, at the 6-311++G(d,p) level. A notable difference among the methods occurs in the computed C N, C2-F10, C5-C11 bond lengths as shown in Table 1. Thus, at the 6-311++G(d,p) level, the nitrile (C N) bond length in 1.155 Å which is very close to the experimental value of 1.152 Å when compared to other methods such as cc-pvdz at 1.164 Å and Aug-cc-pvdz at 1.163 Å. The other bond length and angles, computed by DFT method, show good agreement with microwave data 17. 3.2 Vibrational assignments 2-fluoro-5-methylbenzonitrile molecule have 16 atoms and belonging to the point group C s. The three cartesian displacements of the 16 atoms provide 48 internal modes, namely. Γ int er = 32A +16 A From the character table for the C s point group, since and Γ trans = 2A + A and Γ rot = A +2A, one get: Γ vib = Γ 48 Γtrans Γrot = 29A +13A normal modes of vibrations

484 INDIAN J PURE & APPL PHYS, VOL 47, JULY 2009 Table 1 Geometrical parameters optimized in 2-fluoro-5-methylbenzonitrile, bond length (A ), bond angle ( ) and dihedral angle ( o ) Parameters 6-311++G(d,p) BLYP/cc-pvdz Aug-cc-pvdz a Microwave data (benzonitrile) Bond length (A o ) C1-C2 1.396 1.401 1.399 1.399 C1-C6 1.406 1.409 1.410 1.398 C1-C15 1.428 1.434 1.433 1.430 C2-C3 1.385 1.390 1.389 1.387 C2-F10 1.345 1.343 1.351 C3-C4 1.391 1.395 1.395 1.391 C3-H7 1.083 1.091 1.089 C4-C5 1.403 1.407 1.407 1.391 C4-H8 1.085 1.093 1.091 C5-C6 1.392 1.397 1.396 1.387 C5-C11 1.510 1.510 1.511 C6-H9 1.084 1.091 1.089 C11-H12 1.091 1.099 1.097 C11-H13 1.094 1.103 1.100 C11-H14 1.094 1.103 1.100 C15-N16 1.155 1.164 1.163 1.152 Bond Angle (º) C2-C1-C6 118.41 118.63 118.39 120.0 C2-C1-C15 120.88 120.39 120.93 119.9 C6-C1-C15 120.71 120.98 120.68 119.9 C1-C2-C3 121.45 121.17 121.56 119.9 C2-C1-C6 119.25 119.19 119.19 C3-C2-F10 119.30 119.64 119.26 C2-C3-C4 118.98 119.11 118.85 120.2 C2-C3-H7 119.28 119.08 119.38 C4-C3-H7 121.75 121.82 121.77 C3-C4-C5 121.61 121.61 121.64 120.1 C3-C4-H8 118.93 118.96 118.88 C5-C4-H8 119.46 119.43 119.48 C4-C5-C6 118.11 118.08 118.13 120.2 C4-C5-C11 120.69 120.64 120.60 C6-C5-C11 121.20 121.29 121.27 C1-C6-C5 121.45 121.41 121.43 119.7 C1-C6-H9 118.31 118.35 118.31 C5-C6-H9 120.24 120.24 120.27 C5-C11-H12 111.36 111.52 111.33 C5-C11-H13 111.13 111.20 111.09 C5-C11-H14 111.12 111.20 111.10 H12-C11-H13 107.84 107.80 107.92 H12-C11-H14 107.84 107.81 107.92 H13-C11-H4 107.37 107.11 107.30 C1-C15-N16 181.26 181.03 181.35 Dihedral angle (º) C6-C1-C2-C3 0.01 0.00 0.00 C6-C1-C2-F10-179.99 180.00 180.00 C15-C1-C2-C3-179.99-180.00-180.00 C15-C1-C2-F10 0.01 0.00 0.00 C2-C1-C6-C5 0.00 0.00 0.00 C2-C1-C6-H9 180.00 180.00 180.00 C15-C1-C6-C5 179.99 180.00 180.00 C15-C1-C6-H9 0.00 0.00 0.00 C1-C2-C3-C4-0.01 0.00 0.00 C1-C2-C3-H7 179.99 180.00-180.00 F10-C2-C3-C4 179.99-180.00-180.00 F10-C2-C3-H7-0.01 0.00 0.00 Contd

SUNDARAGANESAN et al.: VIBRATIONAL SPECTRA OF 2-FLUORO-5-METHYLBENZONITRILE 485 Table 1 Geometrical parameters optimized in 2-fluoro-5-methylbenzonitrile, bond length (A ), bond angle ( ) and dihedral angle ( o ) Contd Parameters 6-311++G(d,p) BLYP/cc-pvdz Aug-cc-pvdz C2-C3-C4-C5 0.00 0.00 0.00 C2-C3-C4-H8-180.01-180.00-180.00 H7-C3-C4-C5 180.00-180.00-180.00 H7-C3-C4-H8-0.01 0.00 0.00 C3-C4-C5-C6 0.01 0.00 0.00 C3-C4-C5-C11 180.01-180.00 179.99 H8-C4-C5-C6 180.01 180.00 180.00 H8-C4-C5-C11 0.01-0.01-0.01 C4-C5-C6-C1-0.01 0.00 0.00 C4-C5-C6-H9-180.01-180.00-180.00 C11-C5-C6-C1-180.00-179.99 180.01 C11-C5-C6-H9-0.01 0.00 0.01 C4-C5-C11-H12 179.88 180.00-179.99 C4-C5-C11-H13-59.86-59.63-59.68 C4-C5-C11-H14 59.64 59.63 59.70 C6-C5-C11-H12-0.12 0.00 0.01 C6-C5-C11-H13 120.13 120.36 120.31 C6-C5-C11-H14-120.37-120.37-120.31 a Taken from Ref [17] a Microwave data (benzonitrile) Fig. 5 Numbering system adopted in this study for 2-fluoro-5- methylbenzonitrile All 42 fundamental vibrations are active in both IR and Raman. For an N-atomic molecule, 2N-3 of all vibration is in-plane and N-3 is out-of-plane 18. Thus, for 2-fluoro-5-methyl benzonitrile; 29 of all the 42 vibrations are in-plane and 13 are out-of-plane. Since the molecules belong to the C s group all vibrations being anti-symmetric through the mirror plane of symmetry σ h will belong to the species A and the others being symmetric through σ h will belong to the species A. Thus, all the vibrations of the A species will be in plane and those of the A speices will be out-of-plane. The harmonic-vibrational frequencies calculated for 2F5MBN at B3LYP level using 6-311++G(d,p), cc-pvdz and Aug-cc-pvdz basis sets have been given in Table 2. The observed FT-IR and FT-Raman frequencies for various modes of vibrations are presented in Table 2. Comparison of the frequencies calculated by 6-311++G(d,p) method shows good agreement with experimental value. The DFT hybrid BL3YP functional tends to overestimate the fundamental modes; therefore scaling factors have to be used for obtaining a considerable better agreement with experimental data. In present study, three different scaling factors 6-311++ G(d,p): 0.9668, cc-pvdz: 0.9700 and Aug-cc-pvdz: 0.9704 (Ref. No. 19) have been followed. C-H vibrations The C-H stretching modes usually appear with strong Raman intensity and are highly polarized. May be owing to this high polarization, Raman bands have not been observed in experimental spectra. The C-H stretching vibrations of benzene derivatives generally appear above 3000 cm 1. In the FT-IR spectrum of 2F5MBN the bands at 3075 and 3066 cm 1 are assigned to C-H stretching vibrations of aromatic ring. In the

486 INDIAN J PURE & APPL PHYS, VOL 47, JULY 2009 Table 2 Comparison of the observed (FT-IR and FT-Raman) and calculated vibrational frequencies of 2-fluoro-5-methylbenzonitrile Mode nos. Species Experimental (cm 1 ) Scaled wavenumbers (cm 1 ) FT-IR FT-Raman BLYP/ IR int BLYP/ 6-311++G(d,p) Km/mol cc-pvdz Aug-cc-pvdz Vibrational assignments 1 A 29 0.04 47 70 τ CH 3 2 A 135vw 122 0.53 125 123 τ C N 3 A 155m 134 2.96 133 132 β C N 4 A 175s 147 0.52 150 149 γ C-CH 3 +γ C N 5 A 280w 288 0.09 289 289 β C-CH 3 +β C-F 6 A 324 4.69 331 326 γ C-CH 3 7 A 360m 379 2.19 378 376 β C-CH 3 +β C N 8 A 412w 410m 389 0.36 392 388 16a γ C-C-C 9 A 444m 450m 433 5.39 433 429 β C-C-C 10 A 483s 485w 488 11.63 491 481 16b γ C-C-C 11 A 504vw 510m 493 0.49 494 491 6b β C-C-C 12 A 582 0.85 583 578 6a β C-C-C 13 A 579vw 565m 583 0.95 589 585 β C N 14 A 692w 690s 693 1.27 696 691 β C-C-C 15 A 708m 710s 720 0.17 734 716 γ C-C-C+t C N 16 A 766s 770m 758 22.94 764 757 β C-C-C+υ C-F 17 A 821vs 812 39.39 822 809 17b γ C-H 18 A 892m 895w 879 4.12 890 874 γ C-H+τ C N 19 A 917w 920vw 904 2.31 908 906 1 Ring breathing 20 A 956vw 955s 939 1.63 952 943 10a γ C-H 21 A 1008vw 989 4.17 983 988 Trigonal bending 22 A 1042w 1030 3.99 1023 1023 ρ CH 3 23 A 1116s 1098 22.48 1100 1094 β C H+υ C-F 24 A 1137m 1130w 1126 2.51 1123 1124 β C H 25 A 1225s 1203 35.78 1207 1199 β C H+υ C-F 26 A 1255s 1250vw 1228 44.36 1248 1235 υ C-C+β C H 27 A 1275w 1270s 1254 14.08 1252 1248 β C H 28 A 1316vw 1310w 1289 0.30 1315 1308 γ C-CH 3 29 A 1383w 1373 1.57 1359 1361 CH 3 sym. deform 30 A 1395m 1380 10.67 1383 1381 υ C-C+CH 3 deform 31 A 1425vw 1438 8.08 1413 1420 CH 3 asym. deform 32 A 1483m 1451 16.65 1432 1438 14 υ C-C 33 A 1500vs 1480 115.18 1492 1479 19a υ C-C 34 A 1608vs 1610s 1568 5.87 1586 1578 8b υ C-C (semi-circle stretch) 35 A 1627m 1595 9.44 1614 1605 8a υ C-C 1783w overtone/combination 1875vw overtone/combination 1933w overtone/combination 36 A 2252s 2261 33.74 2275 2260 υ C N 2513w overtone/combination 2853m overtone/combination 37 A 2925s 2929 22.75 2942 2943 υ sym CH 3 38 A 2954s 2950w 2978 12.40 3000 2999 υ asym CH 3 39 A 3015m 3008 12.43 3031 3031 υ asym CH 3 40 A 3066s 3065vw 3065 6.03 3085 3085 20a** arom. υ C-H 41 A 3075w 3080m 3084 1.63 3104 3104 20b** arom. υ C-H 42 A 3098 0.75 3119 3117 20a** arom. υ C-H w-weak; vw-very weak; s-strong; vs-very strong; m-medium; br,sh-broad, shoulder; υ-stretching; υ sym -symmetric stretching; υ asy - asymmetric stretching; β-in plane bending; γ-out-of-plane bending; ω-wagging; t-twisting; δ-scissoring; τ-torsion; *-Wilson s notation; IR int-ir intensities.

SUNDARAGANESAN et al.: VIBRATIONAL SPECTRA OF 2-FLUORO-5-METHYLBENZONITRILE 487 FT-Raman spectrum, the bands observed at 3080 and 3065 cm 1 are attributed to C-H stretching vibrations. The computed vibrations by 6-311++G(d,p) assigned to aromatic C-H stretch in the region 3098-3065 cm 1 (ref. 20) are in agreement with experimental assignment 3045-3085 cm 1 (ref. 21). The three in plane C-H bending vibrations appear in the range 1000-1300 cm 1 for the substituted benzenes and the three out of plane bending vibrations occur in the frequency range 750-1000 cm 1 (rer. 22). The C-H in plane bending vibrations assigned in the region 1106-1234 cm 1, even though found to be contaminated by C-F stretching vibration are in the range found earlier 23,24 while the experimental observations are at 1116-1275 cm 1 in FT-IR and 1130-1270 cm 1 in FT-Raman spectra respectively. Theoretically computed vibrational frequencies by 6-311++G(d,p) method at 1254-1098 cm 1 (mode nos.27-23) are assigned to C- H in plane bending vibrations. The calculated frequencies 939, 879-812 cm 1 (mode nos. 20, 18-17) by 6-311++G(d,p) for C-H out of plane bending vibration fall in the FT-IR values at 956-821 cm 1. C-F vibrations The vibrations belonging to the bond between the ring and halogen atoms are worth to discuss here since mixing of vibrations are possible due to the lowering of the molecular symmetry and the presence of heavy atoms on the periphery of the molecule 25. The assignments of C-F stretching and deformation vibrations have been made by comparison with similar molecules, p-bromophenol 26 and the halogen-substituted benzene derivatives 27. Mooney 28,29 assigned vibrations of C-X group (X =Cl, F, Br, I) in the frequency range of 1129-480 cm 1. In the FT-IR spectrum of 2F5MBN the strong bands at 766 and 770 cm 1 in FT-Raman spectrum are assigned to C-F stretching vibration. The theoretical calculations by 6-311++G(d,p)/cc-pvdz method at 758/764 cm 1 show good agreement with experimental as well as above previous data. The C-F in plane bending and out of plane bending vibrations are assigned to the Raman bands at 280 and 175 cm 1 respectively. This in agreement with computed values at 288/147 cm 1 (mode no. 5/4) by 6-311++G(d,p) method. C N vibrations The geometry of the cyano group (C N) is affected significantly by a new substituent of the phenyl ring. Hence the vibrational wavenumber on the cyano group remains almost unchanged from the benzonitro molecule. For the aromatic compound which bears a C N group attached to the ring, a band of very good intensity has been observed in the region 2240-2221 cm 1 (ref. 30) and it is being attributed to C N stretching vibrations. A strong IR band at 2252 cm 1 in 2F5MBN indicate the C N stretching vibration. The theoretically computed and assigned by Gauss view program package at 2261/2275 cm 1 by 6-311++G(d,p)/cc-pvdz at (mode no. 36 ) is assigned to C N stretching vibrations. Experimentally 31,27 in mono-substituted benzonitrile, the vibration appears in the 2220-2240 cm 1 range and in di-substituted at 2230 cm 1, in accordance with scaled values of 2238 cm 1 in 4-amino benzonitrile and 2248 cm 1 in benzonitrile and with an IR intensity that varies from medium weak to strong depending on the substituent. As in the benzonitrile molecule and its derivatives, this stretching mode appears with the strongest Raman intensity, in accordance with present results. The intensity is enhanced by the conjugation of the aromatic ring. In-plane and out-of-plane bending modes of C N group, by contrast appear with weak IR intensity and with null Raman activity and strongly coupled with C-C-C bending modes. The C-C-N in plane bending and out of plane bending vibrations are assigned to the Raman bands at 360 and 155 cm 1 respectively. The theoretically computed 6-311++G(d,p) method for the above said vibrations are at 379 and 134 cm 1 respectively. This is in agreement with the previous data 32,8 Methyl group vibrations The molecule 2-fluoro- 5-methylbenzonitrile, under consideration possesses one CH 3 group in fifth position of the ring. For the assignments of CH 3 group frequencies one can expect nine fundamentals can be associated to each CH 3 group, namely the symmetrical stretching in CH 3 (CH 3 sym. stretch) and asymmetrical stretching (CH 3 asym. stretch), in-plane stretching modes (i.e. in-plane hydrogen stretching mode); the symmetrical (CH 3 sym. deform), and asymmetrical (CH 3 asym. deform) deformation modes; the in-plane rocking (CH 3 ipr); out-of- plane rocking (CH 3 opr) and twisting (tch 3 ) bending modes. For the OCH 3 group compounds, the mode (stretch)ν appears in the range 2825 2870 cm 1, lower in magnitude compared to its value in CH 3 compounds (2860 2935 cm 1 ). The theoretically

488 INDIAN J PURE & APPL PHYS, VOL 47, JULY 2009 computed asymmetric and symmetric C-H stretching vibration in CH 3 is at 3008, 2978 and 2929 cm 1 by 6-311++G(d,p) method. The FT-IR bands at 3015, 2954 and 2925 cm 1 represent asymmetric and symmetric CH 3 stretching vibrations. The rocking vibration of (CH 3 ) has been identified at 1042cm 1 i.e. ρ r. The theoretically calculated value by B3LYP method using 6-311++G(d,p) basis set is at 1030 cm 1. The asymmetric and symmetric in-plane bending modes of CH 3 group are at 1373 and 1289 cm 1 (mode no. 29 and 28). These assignments find support from the work of Singh and Prasad 33 and are within the frequency intervals given by Varsanyi 27. The CH 3 torsional mode could be assigned at 29/47 cm 1 by BL3YP/6-311++G(d,p)/ cc-pvdz. It is to be noted here that this is a pure mode (mode no. 1). Carbon skeleton vibrations In general the bands around 1400 to 1650 cm 1 in benzene derivatives are assigned to skeletal stretching C-C bands. The bands observed at 1627, 1608, 1500 cm 1 in FT-IR and 1610 and 1395 cm 1 in FT-Raman spectrum are assigned to C-C stretching vibrations. The theoretically computed C-C stretching vibrations by BL3YP/6-311++G(d,p) method at 1595, 1568, 1480 and 1380 cm 1 ( mode nos. 35-33, 30) show excellent agreement with recorded spectral data. The C-C aromatic stretching known as semicircle stretching vibrations, predicted at 1568 cm 1 is also in excellent agreement with experimental observations of 1608 cm 1 in FT-IR and 1610 cm 1 in FT-Raman spectra. In the benzene, fundamental (992 cm 1 ) and (1010 cm 1 ) represent the ring breathing mode and carbonyl trigonal bending modes. Under the C s point group both the vibrations are very close, there is an appreciable interaction between these vibrations and consequently their energies will be modified. The ring breathing and trigonal bending modes of 2F5MBN are assigned at 917 and 1008 cm 1 in FT-IR spectrum respectively. The theoretically computed values at 904 and 989 cm 1 by 6-311++G(d,p) method exactly coincide with experimental observations. The theoretically calculated C-C-C out and in plane bending vibrational modes have been found to be consistent with experimental observations. 3.3 Vibrational contribution to NLO activity Many organic molecules, containing conjugated л electrons and characterized by large values of molecular first hyperpolarizabilities, were analyzed by means of vibrational spectroscopy 34,35. In most cases, even in the absence of inversion symmetry, the strongest band in the Raman spectrum is weak in the IR spectrum and vice versa. But the intramolecular charge transfer from the donor to accepter group through a single double bond conjugated path can induce large variations of both the molecular dipole moment and the molecular polarizability, making IR and Raman activity strong at the same time. The experimental spectroscopic behaviour described above is well accounted for by ab initio calculations in л conjugated systems that predict exceptionally large Raman and infrared intensities for the same normal modes 35. It is also observed that in 2F5MBN molecule the bands at 1608, 766, 708, 444, cm 1 in FT-IR spectrum have their counterparts in Raman at 1610, 770, 710, 450 cm 1, which shows that the relative intensities in IR and Raman spectra are comparable resulting from the electron cloud movement through л conjugated frame work from electron donor to electron acceptor groups. The analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state and is mainly described by one-electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied orbital (LUMO). The LUMO of л nature (i.e. benzene ring) is delocalized over the whole C-C bond. By contrast, the HOMO is located over fluoro and methyl group, consequently the HOMO LUMO transition implies an electron density transfer to nitrile and benzene ring of л conjugated system from fluoro and methyl group. Moreover, these three orbitals significantly overlap in the para position of the benzene ring for 2F5MBN. The atomic orbital compositions of the frontier molecular orbital are shown in Fig. 6. The HOMO-LUMO energy gaps of 2F5MBN, calculated at the 6-311++G(d,p) level, reveals that the energy gap reflect the chemical activity of the molecule (Table 3). LUMO as an electron acceptor represents the ability to obtain an electron, HOMO represents the ability to donate an electron HOMO energy = 0.27420a.u LUMO energy = 0.06833a.u HOMO LUMO energy gap = 0.20587a.u The calculated self-consistent field (SCF) energy of 2F5MBN is - 463.170 a.u. Moreover the lower in the HOMO and LUMO energy gap explains the eventual

SUNDARAGANESAN et al.: VIBRATIONAL SPECTRA OF 2-FLUORO-5-METHYLBENZONITRILE 489 charge transfer interactions taking place within the molecule. 3.4 Other molecular properties Several calculated thermodynamic parameters are presented in Table 4. Scale factors have been recommended 36 for an accurate prediction in determining the zero-point vibration energies (ZPVE), and the entropy, Svib(T). The variations in the ZPVE s seem to be insignificant. The total energies and the change in the total entropy of 2F5MBN at room temperature at different methods are also presented. Fig. 6 The atomic orbital compositions of the frontier molecular orbital for 2F5MBN Table 3 HOMO-LUMO energy value calculated by 6-311++G(d,p), cc-pvdz, Aug-cc-pvdz method Parameters 6-311++G(d,p) cc-pvdz Aug-cc-pvdz HOMO (a.u) -0.27420-0.26477-0.27244 LUMO(a.u) -0.06833-0.05649-0.06718 HOMO-LUMO (a.u) -0.20587-0.20828-0.33962 Table 4 Theoretically computed energies (a.u.), zero-point vibrational energies (kcal mol 1 ), rotational constants (GHz), entropies (cal mol 1 K 1 ) and dipole moment (D) for 2-fluoro-5- methylbenzonitrile Parameters 6-311++G(d,p) cc-pvdz Aug-cc-pvdz Total energy -463.170-463.069-463.096 Zero-point energy 73.965 74.110 73.859 Rotational constants 2.249 2.249 2.234 1.066 1.056 1.059 0.726 0.722 0.721 Entropy Total 93.825 92.809 92.282 Translational 40.614 40.614 40.614 Rotational 29.602 29.616 29.621 Vibrational 23.61 22.579 22.047 Dipole moment 6.023 5.443 5.946 4 Conclusions Density functional theory calculation have been carried out on the structure and vibrational spectra of 2F5MBN. The equilibrium geometry computed by DFT level for both the bond lengths and bond angles are performed better. The vibrational frequencies analysis by 6-311++G(d,p) method agree satisfactorily with experimental results. On the basis of agreement between the calculated and experimental results, assignments of all the fundamental vibrational modes of 2F5MBN were examined and proposed. Therefore, the assignments made at higher level of theory with higher basis set with only reasonable deviations from the experimental values, seems to be correct. HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. This study demonstrates that scaled DFT/B3LYP calculations are powerful approach for understanding the vibrational spectra of medium sized organic compounds. References 1 Bahl B S & Arun Bhal, Advanced Organic Chemistry, 4 th ed (S Chand and Company Ltd), 1995, p 1117. 2 Abramczyk H & Reimschussel W, Proceedings of the IV International Conference of Raman Spectroscopy, Tokyo, 1984, p 174. 3 Green J H S, Spectrochim Acta, 17 (1961) 607. 4 Green J H S & Harrison D J, Spectrochim Acta, 32A (1976) 1279. 5 Ram S, Gupta B K & Tadav J S, Indian J Pure Appl Phys, 19 (1981) 1110. 6 Joshi A, Suryanarayana Rao K & Shaahidar M A, Curr Sci, 57 (1988) 477. 7 Passingam C, Hendra P J & Hodge C, Spectrochim Acta, 47A (1991) 1235. 8 Johri G K, Prabkash V & Srivastava C L, Indian J Pure Appl Phys 14 (1976) 418. 9 Mohan S, Murugan R & Srinivasan S, Proc Nat Acad Sci India, 62A (1992) 12.

490 INDIAN J PURE & APPL PHYS, VOL 47, JULY 2009 10 Kumar A P & Rao G R, Spectrochimica Acta, 53 A (1997) 2049. 11 Dimitrova Y, J Mol Struc, (Theochem), 391 (1997) 241. 12 Gaussion 03 program, (Gaussian Inc., Wallingford CT) 2004. 13 Schegel H B, J Comput Chem, 3 (1982) 214. 14 Frisch, Nielson A B, Holder A J, GAUSSVIEW User Manual, (Gaussian Inc Pittsburgh, PA), 2000. 15 Keresztury G, Holly S, Varga J, Besenyei G, Wang A Y & Durig J R, Spectrochim Acta, A49 (1993) 2007. 16 Snehalatha M, Ravikumar C, Joe I H, Sekar N, Jayakumar V S, Spectrochimica Acta, 72 A (2009) 654. 17 Casado J, Nygaard L & Sorenson G O, J Mol Struct, 8 (1971) 211. 18 Wilson E B, Decius J C & Cross P C, Molecular vibrations (Dover Publ Inc New York) 1980. 19 Scott A P & Radom L, J Phys Chem, 100 (1996) 16502. 20 Rastogi K, Palafox M A, Tanwar R P & Mittal L, Spectrochim. Acta, 58A (2002) 1989. 21 Silverstein M, Basseler G C & Morill C, Spectrometic Identification of Orgonic Compounds, (Wiley, New York), 1981. 22 Dwivedi C P D & Sharma S N, Indian J Pure App Phys, 11 (1973) 447. 23 Beck D, J Chem Phys, 98 (1993) 5648. 24 Lee C, Yang W & Parr R G, Phys Rev, 37B (1998) 785. 25 Bakiler M, Masler I V & Akyiiz S, J Mol Struct, 475 (1999) 83. 26 Zlerkiewiez W, Michalska D & Zeegers-Huyskens T, J Phys Chem, 104A (2000) 11685. 27 Varsanyi G, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, Vol 1-2, (Adam Hilger, New York), 1974. 28 Mooney E F, Spectrochim Acta, 20 (1964) 1021. 29 Mooney E F, Spectrochim Acta, 19 (1963) 877. 30 George W O & Mcintyre P S, Infrared spectrosc, (John Wiley & Sons) 1987. 31 Palafox M A, Rastogi V K & Mittal L, Int J Quant Chem, 94(2003)189. 32 Aquel R & Verma P K, Indian J Pure Appl Phys, 20 (1982) 672. 33 Singh RN & Prasad S C, Spectrochim Acta, A 34 (1974) 39. 34 Ataly Y, Avci D & BaŞoğlu A, Struct Chem, 19 (2008) 239. 35 Vijayakumar T, Joe H, Nair C P R & Jayakumar V S, Chemical Physics, 343 (2008) 83. 36 Palafox M A, Int J Quant Chem, 77 (2000) 661. Supplementary materials Appendix A Table 1 and 4 with the geometrical set of parameters and thermodynamical properties respectively for 2F5MBN is available from the authors, on request.