CHAPTER-IV. FT-IR and FT-Raman investigation on m-xylol using ab-initio HF and DFT calculations

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1 4.1. Introduction CHAPTER-IV FT-IR and FT-Raman investigation on m-xylol using ab-initio HF and DFT calculations m-xylol is a material for thermally stable aramid fibers or alkyd resins [1]. In recent year, there is a demand for m-xylol, as a material for thermally stable fibers such as poly (mphenylene isophthalamide) fibers which can be produced through step conversion from m-xylol to isophthaloyl. m-xylol is used as varnish solvent in varnish and wood stains industries, dyes, organic synthesis, insecticides and aviation fuel [2-3]. Potential health impacts associated with m-xylol include cardiovascular or blood toxicity, developmental toxicity, gastrointestinal or liver toxicity, immune toxicity, neurotoxicity, respiratory toxicity, and skin sensitivity. The introduction of one or more substituents in the benzene ring leads to the variation of charge distribution in the molecule and consequently this greatly affects the structural, electronic and vibrational parameters [4]. Arjunan et al [5] have investigated structural and harmonic vibrational properties of 2- nitro-, 4-nitro- and 5-nitro-m-xylene. The experimental vibrational frequency was compared with that obtained theoretically by ab initio HF and DFT B3LYP gradient calculations employing the standard 4-31G(d,p) basis set for the optimized geometries of the compounds. The complete vibrational assignment, analysis and correlation of the fundamental modes of the compounds were carried out using the experimental FTIR and FT-Raman data and ab initio HF and DFT quantum chemical studies. The geometrical parameters and the wave numbers of normal modes of vibration obtained from the HF and DFT methods are in good agreement with the 89

2 experimental values. The potential energy distribution of the fundamental modes was calculated with ab initio force fields utilizing Wilson s FG matrix method. The influence of bulky methyl groups on the nitro group fundamental modes and on the ring skeletal vibrations are investigated. Stuart et al [6] have studied the xylene swelling of polycarbonate using FT-Raman spectroscopy. The Fourier transform (FT) Raman spectrum of polycarbonate (PC) was reported. The effect of Xylene on the structural properties of PC has been investigated. Changes to the FT- Raman spectrum of PC in the presence of xylene are believed to be due to an increase in the amount of the cis-trans conformation produced in the polymer. Xylene causes plasticization of PC while inducing ordering in the polymer. The FT-Raman spectra of PC before and after exposure to xylene, a solvent which causes significant changes to the physical properties of the polymer, were investigated. A stretching mode of the O-C (O)-O group and a phenyl ring vibration were found to change in intensity after PC was exposed to xylene. These changes to the FT-Raman spectrum of PC in the presence of xylene were believed to be due to an increase in the amount of the cis-trans conformation of the polymer. The study has demonstrated the effectiveness of FT Raman spectroscopy as a technique for the investigation of polymer-solvent interactions. The results obtained support those findings made as a result of a FTIR spectroscopy study, emphasizing the usefulness of combining these complimentary techniques. The structural characteristics and vibrational spectroscopic analysis of m-xylol by the quantum mechanical ab initio HF and DFT methods have not been studied so far. Thus, considering the industrial and medical importance of m-xylol, an extensive experimental and theoretical ab initio-hf and DFT studies have been undertaken in the present work to obtain a complete, reliable and accurate vibrational assignments and structural characteristics of the compound. 90

3 4.2. Computational details In this work, ab-initio HF and DFT (B3LYP) calculated with 6-31+G (d, p), G (d, p) and G (d, p) basis sets are carried out. All these calculations were performed using GAUSSIAN 03W program package on Pentium IV processor in personal computer. The optimized structure and structural parameters are obtained using GAUSSVIEW program. The calculation of molecular structural parameters, vibrational frequencies and energies of m-xylol by DFT method with the hybrid exchange-correlation function B3LYP (Becke s three-parameter (B3) exchange in conjunction with the Lee Yang Parr s (LYP) correlation function) is very effective for vibrational studies and is also important in systems containing extensive electron conjugation and/or electron lone pairs [7-14]. The calculated frequencies are scaled down by suitable factors in comparison with the experimental frequencies. The scaling factors are 0.915, 0.920, and in the case of HF/6-31G (d, p) basis set, in agreement with the literature [15-16]. In the case of B3LYP, the scaling factors are 0.960, 0.968, 0.985, and 1.01 for 6-31+G (d, p) basis set, for B3LYP with G (d, p) set the scaling factors are 0.985, 0.955, 0.958, and 1.02 and for G (d, p) basis set the scaling factors are 0.962, 0.959, 0.943, 0.985, and 1.03, in agreement with the literature [17-18] Results and Discussion Molecular Geometry The optimized molecular structure of the m-xylol, obtained from GAUSSVIEW is shown in Figure 4.1. The molecule consists of couple of methyl groups connected with benzene ring. The zero point vibrational energy of the molecule is , 97.2, 97.2 and 96.8 Kcal/mol as predicted by HF/6-31+G (d, p), B3LYP/6-31+G (d, p), B3LYP/6-31++G (d, p), and 91

4 B3LYP/ G (d, p) respectively. The Figure 4.2 and 4.3 give the comparative IR and Raman spectra of experimental and theoretical frequencies for different methods. The structural parameters; bond lengths, bond angles and dihedral angles calculated using different basis sets are presented in Table 4.1. In comparison with the experimental values, it is observed that most of the calculated bond length values are slightly larger than the experimental values. This may be due to the fact that the calculations are performed for the isolated molecules (gaseous phase) while the experimental spectra are recorded in solid phase. This is in accordance with the earlier work [19]. The comparative graphs, for different basis sets, of bond lengths, bond angles and dihedral angles are presented in the Figures 4.6, 4.7 and 4.8 respectively, which show that all these parameters are slightly larger in HF when compared to that in other methods. The introduction of a couple of methyl group in the benzene ring of m-xylol causes no appreciable change in the bond distances. In this case, the order of the bond lengths lie as C1-C6 = C5-C6 < C1-C2 = C2-C3 = C3-C4 = C4-C5. The calculated bond length of C2-C3 at the point of CH 3 substitution is greater by Å than the experimental value (1.390 Å) [19-20]. From the above observation, it is clear that the left half of the benzene ring is found to be sheared slightly than the right half due to the impact of CH 3 groups. The distortion in the benzene ring is found to be evident from the order of bond angle C1- C2-C3 = C3-C4-C5 <C1-C6-C5 < C2-C1-C6 = C4-C5-C6 < C2-C3-C4. The evaluated bond angle of C2-C1-C6 and C4-C5-C6 are , 0.96 less than the experimental value [20-21]. 92

5 Vibrational assignments The molecule m-xylol, as it possesses one axis of rotational symmetry and one plane of reflection symmetry belongs to C 2v point group symmetry. It consists of 18 atoms; hence the number of normal modes of vibrations is 48. Of the 48 normal modes of vibrations, 16 modes of vibrations are stretching, 15 modes of vibrations are in- plane bending and 17 modes of vibrations are out-of-plane bending. Thus the 48 normal modes of vibrations of m-xylol are distributed among the symmetry species as Γ Vib = 16A 1 + 7A B B 2 A 1 and B 2 irreducible representations correspond to stretching, ring deformation and in plane bending vibrations while A 2 and B 1 correspond to ring, torsion and out of plane bending vibrations. However the intensity of certain peaks is so weak, they are found missing in the spectra, which are verified from the intensity values calculated theoretically. The calculated and experimental frequency values for different methods and basis sets and the corresponding assignments are presented in the Table 8.2. All the observed bands are assigned to different possible modes of vibrations based on the earlier works on structurally similar molecules, the characteristic frequencies of the functional groups, GAUSSVIEW program and the calculated IR intensity and Raman activity etc. The discussion on individual assignments and probable reasons are provided under separate titles in the following paragraphs Computed IR intensity and Raman activity analysis The computed IR intensities and Raman activities of the m-xylol for different modes of vibrations with corresponding frequencies, calculated by different methods and basis sets are given in the Table 4.3. The IR intensity values predicted by HF methods are found to be larger 93

6 when compared to hybrid methods, whereas the Raman activity values predicted by hybrid methods are found to be larger when compared to HF. The comparison of IR intensity and Raman activity among different methods and basis sets are graphically shown in Figures 6.4 and 4.5 respectively Computed vibrational frequency analysis The comparative graph between calculated vibrational frequencies by HF and DFT methods at HF/6-31+G (d, p), B3LYP/6-31+G (d, p), B3LYP/6-31++G (d, p) and B3LYP/ G (d, p) basis sets for the molecule are given in the Figure 4.9. From the Figure, it is found that the calculated (unscaled) frequencies of B3LYP with G (d, p) basis set is closer to the experimental frequencies than HF method with 6-31+G (d, p) basis set. This observation is supported by the earlier work [22]. The standard deviation (SD) calculation made between experimental and computed frequencies (HF/DFT) for the m-xylol is presented in the Table 4.4 and the SD comparative graph of computed frequencies for four sets is presented in the Figure According to the SD, the computed frequency deviation decrease in going from HF/6-31+G (d, p) to B3LYP/6-31+G (d, p) to B3LYP/6-31++G (d, p) to B3LYP/ G (d, p). The deviation ratio between HF/6-31+G (d, p) to B3LYP/6-31+G (d, p) is 2.43, B3LYP/6-31++G (d, p) and B3LYP/ G (d, p) is It also proved that, the calculated frequencies by B3LYP / G (d, p) basis set is closer to the experimental frequencies than HF method. 94

7 4.3.5 C-H vibration: The homo aromatic structure of m-xylol shows the presence of C-H stretching vibration in the region cm -1 [23-24]. There are four stretching vibrations identified for C-H stretching at 3060, 3040, 3020 and 3000 cm -1. The bands due to the ring C-H in-plane bending are usually observed in the region cm -1. In the present molecule, these vibrations are observed at 1360, 1320, 1300 and 1270 cm -1 which are in good agreement with the reported values [25]. The C-H out-of-plane bending vibrations are usually observed between 750 and 1000 cm -1 [26]. In the present molecule, these vibrations are observed at 990, 960, 900 and 860 cm -1. The values for C-H vibrations calculated by B3LYP/6-31++G (d, p) and B3LYP/ G (d, p) methods show a good agreement with recorded spectrum. Though all the C-H stretching and out of plane bending vibrations are observed within the expected range, except one band all the in plane bending vibrational modes are found to be elevated abruptly from the expected region. This may be due to the favoring of CH 3 in the meta positions of the ring. It is also observed that most of the bands are found in the IR spectrum C-C vibrations: The ring C=C and C-C stretching vibrations, known as semicircle stretching usually occurs in the region cm -1 [27-28].The C-C stretching vibrations are observed with weak and strong intensity at 1460, 1450 and 1430 cm -1 in Raman spectrum and C=C stretching vibrations are observed with strong and medium intensity at 1580, 1560 and 1470 cm -1 in IR spectrum. This observation indicates that the C=C vibrations are slightly lowered than the expected range. The CCC in plane and out of plane bending vibrations is observed in IR spectrum at 520, 500 and 460 cm -1 and 400, 420 and 210 cm -1 respectively. Except for the last out of plane bending 95

8 vibration, all these assignments are in good agreement with the literature [24, 29]. One band is suppressed by the CH 3 groups Methyl group vibrations: The assignments of methyl group vibration make a significant contribution to m-xylol. The presence of C-H vibrations ensures the place of methyl group in benzene ring. The asymmetric C-H vibration for methyl group usually occurs in the region between cm -1 [27, 30-31] and the symmetric C-H vibrations for methyl group are usually occurs in the region of cm -1. In m-xylol, methyl groups are attached to third and fifth carbon atoms of the benzene ring. The methyl C-H stretching vibrations are observed with very strong intensity at 3000, 2995, 2940, 2930, 2920 and 2910 cm -1 in IR spectrum only. The C-H in-plane bending vibrations of methyl group are found at 1150, 1130, 1080 and 1030 cm -1 in IR spectrum and 1230 cm -1 in Raman spectrum. The C-H out of plane bending vibrations is observed at 890, 790, 760, 750, 710 and 680 cm -1. These assignments are in agreement with the reported literature values [10, 32]. Except last two bands of C-H out of plane bending vibrations, all the vibrations are found within the expected region. Most of the vibrational bands are observed in IR spectrum. The values for C-H vibrations calculated by B3LYP/ G (d, p) method nearly coincide with IR observed values. The IR band for C-C stretching vibration for CH 3 is identified at 1420 and 1390 cm -1. This assignment is supported by the literature values [33-34]. The C-C in plane bending vibration is observed at 270 cm -1 in Raman spectrum and CH 3 twisting vibrations are observed at 210 and 200 cm -1. Though the assignments agree well with reported values and they also support the existence of two methyl groups in title molecule all the C-H stretching vibrations associated with the methyl group lay in the asymmetric region. 96

9 4.4 Conclusion A complete vibrational investigation on m-xylol is performed by HF and DFT (B3LYP) levels of theory. The observed and simulated spectra have shown a good frequency fit. The difference between theoretical and experimental wave numbers within 10 cm 1 is confirmed by the qualitative agreement between the calculated and observed frequencies. The global minimum energy between the different methods shows the difference in optimizations between the same and the different sets. Various quantum chemical calculations help us to identify the structural and symmetry properties of the titled molecule. From the vibrational investigation, the following observations are made; 1. The introduction of one or more substituents in the benzene ring leads to the variation of charge distribution in the molecule and consequently this greatly affects the structural, electronic and vibrational parameters. 2. The introduction of a couple of methyl group in the benzene ring of m-xylol causes no appreciable change in the bond distances. 3. The distortion in the benzene ring is found to be evident from the order of bond angle C1- C2-C3 = C3-C4-C5 <C1-C6-C5 < C2-C1-C6 = C4-C5-C6 < C2-C3-C4. The evaluated bond angle of C2-C1-C6 and C4-C5-C6 are , 0.96 less than the experimental value Though all the C-H stretching and out of plane bending vibrations are observed within the expected range, except one band all the in plane bending vibrational modes are found to be elevated abruptly from the expected region. 5. The observation indicates that the C=C vibrations are slightly lowered than the expected range. 97

10 6. Except for the last out of plane bending vibration, all these assignments are in good agreement with the literature and one band is suppressed by the methyl group. 7. Except for the last out of plane bending vibration, all these assignments are in good agreement with the literature. 8. Though the assignments agree well with reported values and they also support the existence of two methyl groups in title molecule all the C-H stretching vibrations associated with the methyl group lay in the asymmetric region. 98

11 FIGURES

19 Table 4.1: Optimized geometrical parameters for m-xylol computed at HF/6-31+G (d, p), B3LYP/6-31+G (d, p), B3LYP/6-31++G (d, p) and G (d, p) basis sets Geometrical Parameters HF/6-31+G (d, p) B3LYP/ 6-31+/6-31++G (d, p) Methods B3LYP/ G (d, p) Experimental Value Bond length (Å) C 1 -C C 1 -C C 1 -C C 2 -C C 2 -C C 3 -C C 3 -C C 4 -C C 4 -C C 5 -C C 5 -H C 6 -H C 11 -H C 11 -H C 11 -H C 15 -H C 15 -H C 15 - H Bond angle ( ) C 2 -C 1 -C C 2 -C 1 -H C 6 -C 1 -H C 1 -C 2 -C C 1 -C 2 -C C 3 -C 2 -C C 2 -C 3 -C

20 C 2 -C 3 -H C 4 -C 3 -H C 3 -C 4 -C C 3 -C 4 -C C 5 -C 4 -C C 4 -C 5 -C C 4 -C 5 -H C 6 -C 5 -H C 1 -C 6 -C C 1 -C 6 -H C 5 -C 6 -H C 4 -C 11 -H C 4 -C 11 -H C 4 -C 11 -H H 12 -C 11 -H H 12 -C 11 - H H 13 -C 11 -H C 2 -C 15 -H C 2 -C 15 -H C 2 -C 15 -H H 16 -C 15 -H H 16 -C 15 -H H 17 -C 15 -H Dihedral angle ( ) C 6 -C 1 -C 2 -C C 6 -C 1 -C 2 -C H 7 -C 1 -C 2 -C H 7 -C 1 -C 2 -C C 2 -C 1 -C 6 -C C 2 -C 1 -C 6 -H H 7 -C 1 -C 6 -C H 7 -C 1 -C 6 -C C 1 -C 2 -C 3 -C C 1 -C 2 -C 3 -H

21 C 15 -C 2 -C 3 -C C 15 -C 2 -C 3 -H C 1 -C 2 -C 15 -H C 1 -C 2 -C 15 -H C 1 -C 2 -C 15 -H C 3 -C 2 -C 15 -H C 3 -C 2 -C 15 -H C 3 -C 2 -C 15 -H C 2 -C 3 -C 4 -C C 2 -C 3 -C 4 -C H 8 -C 3 -C 4 -C H 8 -C 3 -C 4 -C C 3 -C 4 -C 5 - C C 3 -C 4 -C 5 -H C 11 -C 4 -C 5 -C C 11 -C 4 -C 5 -H C 3 -C 4 -C 11 -H C 3 -C 4 -C 11 -H C 3 -C 4 -C 11 -H C 5 -C 4 -C 11 -H C 5 -C 4 -C 11 -H C 5 -C 4 -C 11 -H C 4 -C 5 -C 6 -C C 4 -C 5 -C 6 -H H 9 -C 5 -C 6 -C H 9 -C 5 -C 6 -H

22 Sl. No. Table 4.2: Observed and HF /6-31+G (d, p), B3LYP/6-31+G (d, p), B3LYP/6-31++G (d, p) and B3LYP/ G (d, p) level calculated vibrational frequencies of m-xylol Symmetry species C 2V Observed fundamentals (cm 1 ) Calculated Frequencies (cm -1 ) Vibrational assignments HF/6-31+G b3lyp/6- b3lyp/6-31+g b3lyp/6-31++g FTIR FT (d, p) 311++G(d, p) (d, p) (d, p) Raman Unscaled Scaled Unscaled Scaled Unscaled Scaled Unscaled Scaled 1 A (s) (C-H) υ 2 A (vs) (C-H) υ 3 A (vs) 3030 (m) (C-H) υ 4 B (vs) 3020 (m) (C-H) υ 5 B (vs) CH 3 (C-H) υ 6 B (s) CH 3 (C-H) υ 7 B (vs) CH 3 (C-H) υ 8 B (vs) CH 3 (C-H) υ 9 B (vs) CH 3 (C-H) υ 10 B (vs) 2910 (vs) CH 3 (C-H) υ 11 A (m) 1580 (m) (C=C) υ 12 A (m) (C=C) υ 13 A (vs) 1470 (w) (C=C) υ 14 A (vs) (C-C) υ 15 A (s) (C-C) υ 16 A (w) (C-C) υ 17 A (w) (C-C) υ 18 A (m) 1390 (s) (C-C) υ 19 B (s) (C-H) δ 20 B (w) (C-H) δ 21 A (w) (C-H) δ 22 A (w) 1270 (m) (C-H) δ 23 B (w) (C-H)δ

23 24 B (m) 1150 (m) CH 3 (C-H) δ 25 B (m) 1130 (w) CH 3 (C-H) δ 26 B (m) 1080 (m) CH 3 (C-H) δ 27 B (m) 1030 (m) CH 3 (C-H) δ 28 A (w) 1000 (vs) CH 3 (C-H) δ 29 B (w) (C-H) γ 30 B (w) (C-H) γ 31 A (m) (C-H) γ 32 A (m) 890 (w) (C-H) γ 33 B (w) CH 3 (C-H) γ 34 B (w) CH 3 (C-H) γ 35 B (vs) CH 3 (C-H) γ 36 B (vs) 750 (w) CH 3 (C-H) γ 37 B (w) 710 (vs) CH 3 (C-H) γ 38 B (m) CH 3 (C-H) γ 39 A (m) 520 (s) (CCC) δ 40 A (vs) (CCC) δ 41 A (m) 460 (w) (CCC) γ 42 A (vs) 420 (w) (CCC) γ 43 B (vs) 400 (w) (C-C) δ 44 A (vs) 270 (m) (CCC) γ 45 A (m) (CH 3 ) τ 46 A (s) (CH 3 ) τ VS Very Strong; S Strong; m- Medium; w weak; as- Asymmetric; s symmetric; υ stretching; δ - In plane bending; γ out plane bending; α-deformation; τ Twisting:

24 Table 4.3: Comparative values of IR intensity and Raman Activity between HF/6-31+G (d, p), B3LYP/6-31+G (d, p), B3LYP/6-31++G (d, p) and B3LYP/ G (d, p) of m-xylol Sl. No. Symmetry species C s Observed frequencies (cm -1 ) Calculated Frequencies (cm -1 ) HF/6-31+G (d, p) b3lyp/6-31+g(d, p) b3lyp/6-31++g(d, p) b3lyp/ g(d, p) FTIR FT IR Raman IR Raman IR Raman IR Raman Raman Intensity Activity Intensity Activity Intensity Activity Intensity Activity 1 A (s) A (vs) A (vs) 3030 (m) B (vs) 3020 (m) B (vs) B (s) B (vs) B (vs) B (vs) B (vs) 2910 (vs) A (m) 1580 (m) A (m) A (vs) 1470 (w) A (vs) A (s) A (w) A (w) A (m) 1390 (s) B (s) B (w) A (w) A (w) 1270 (m) B (w)

25 24 B (m) 1150 (m) B (m) 1130 (w) B (m) 1080 (m) B (m) 1030 (m) A (w) 1000 (vs) A (w) A (w) A (m) A (m) 890 (w) B (w) B (w) B (vs) B (vs) 750 (w) B (w) 710 (vs) B (m) A (m) 520 (s) A (vs) A (m) 460 (w) A (vs) 420 (w) B (vs) 400 (w) A (vs) 270 (m) A (m) A (s)

26 Table 4.4: Standard Deviation of frequencies by HF/DFT (B3LYP) at 6-31+G (d, p), G (d, p) and G (d, p) basis sets S.No. Basic set levels Total Standard Deviation Average values Deviation ratio Experimental HF/6-31+(d, p) B3LYP/6-31+(d, p) B3LYP/6-31++(d, p) B3LYP/ (d, p)

CHAPTER INTRODUCTION

CHAPTER INTRODUCTION CHAPTER 3 A SCALED QUANTUM MECHANICAL APPROACH OF VIBRATIONAL ANALYSIS OF O-TOLUNITRILE BASED ON FTIR AND FT RAMAN SPECTRA, AB INITIO, HARTREE FOCK AND DFT METHODS 3.1. INTRODUCTION o-tolunitrile or ortho