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Vibrational spectroscopic investigation (FT-IR and FT-Raman) on 1,2-dibromobenzene by HF and hybrid (LSDA and B3LYP) calculations
Spectrochimica Acta Part A 86 (2012) 449–455
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectroscopic investigation (FT-IR and FT-Raman) on 1,2-dibromobenzene by HF and hybrid (LSDA and B3LYP) calculations G. Shakila a,∗ , S. Periandy b , S. Ramalingam c a b c
Department of Physics, Bharathidasan Government College for Women, Puducherry, India Department of Physics, Tagore Arts College, Puducherry, India Department of Physics, A.V.C. College, Mayiladuthurai, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 29 June 2011 Received in revised form 17 October 2011 Accepted 29 October 2011 Keywords: FT-IR FT-Raman HF DFT Vibrational pattern 1,2-dibromobenzene
a b s t r a c t The FT-IR and FT-Raman spectra of the compound 1,2-dibromobenzene have been recorded in the region 4000–100 cm−1 . The vibrational analysis has been made using HF and DFT (B3LYP and LSDA) level of theory by employing 6-31 +G (d, p) and 6-311 ++G (d, p) basis sets. Optimized geometrical parameters have been calculated, interpreted and compared with the reported experimental values of some halogensubstituted benzene. The experimental geometrical parameters show satisfactory agreement with the theoretical prediction of HF and DFT. The geometrical structure of the compound is fractured by the substitutions of couple of Br in the ring. From the vibrational assignments it is observed that, the vibrational pattern of the fundamental modes is realigned slightly with respect to the substitutions. The simulated FT-IR and FT-Raman spectra of the compound for different methods are compared with the experimental spectra. The impact of Br in the vibrational assignments of the molecule is also investigated. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The aromatic benzene derivative, bromobenzene is organic halogen compounds, formed by replacing hydrogen atoms in benzene by atoms of bromine. It is a pale yellow liquid with a pleasant, sweet smell and they are practically insoluble in water and denser than water [1]. The water solubility is decreasing if more brominated. Benzene is mostly commonly used as an additive to other chemicals and is used in the manufacturing processes of drugs, detergents, pesticides and explosives [2]. The commercial bromobenzenes are used as heavy liquid solvents, motor oil additives and as an intermediate to manufacture organic chemicals including pharmaceuticals, pesticides and flameretardants for polymeric materials [3]. It is used to make styrene, which is used to make plastics and polymers, and in the manufacturing process of nylon. Investigations on the structure of benzene and its derivative have been a subject of great interest because of their peculiar physical properties and industrial uses [4,5]. Suzuki and Ito [6] have studied the polarized Raman spectra of p-dichlorobenzene and p-dibromobenzene single crystals. Schultz et al. [7] has studied the molecular structure and ring distortions of p-dibromobenzene as determined by electron diffraction.
∗ Corresponding author. Tel.: +91 9444811389, fax: +91 9444811389. E-mail address: [email protected] (G. Shakila).
Literature survey reveals that no HF and DFT (LSDA/B3LYP) calculations with different basis sets of 1,2-dibromobenzene have been reported so far. Therefore, the purpose of the present study is to make a comprehensive vibrational analysis using both experimentally observed IR and Raman wave numbers and theoretically calculated vibrational spectra. The ab initio (HF) and DFT calculations are performed to obtain the ground state optimized geometries and the vibrational wave numbers of the different normal modes as well as to predict the corresponding intensities for the different modes of the molecule. These methods predict relatively accurate molecular structure and vibrational spectra with moderate computational effort. In particular, for polyatomic molecules the DFT methods lead to the prediction of more accurate molecular structure and vibrational frequencies than the conventional ab initio Hartree-Fock calculations. In DFT methods, the LSDA and Becke’s three parameter exact exchange-functional (B3) [8] combined with gradient-corrected correlational functional of Lee, Yang and Parr (LYP) [9,10] are the best predicting results for molecular geometry and vibrational wave numbers for moderately larger molecule [11–13]. 2. Experimental details The spectroscopic grade 1,2-dibromobenzene was purchased from Sigma Aldrich chemicals, U.S.A. and used as such for recording spectra without further purification. The FT-IR spectrum of the 1-Br-4-CB was recorded in Bruker IFS 66V spectrometer in
1386-1425/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.10.066
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Table 1 Zero point vibrational energies in J/Mol and kcal/Mol based on HF/6-31 +G, B3LYP/631 +G, 6-311 ++G and LSDA/6-311 ++G basis sets. Type
J/Mol
kcal/Mol
HF/6-31G B3LYP/6-31G B3LYP/6-311G LSDA/6-311G
231661.0 217058.0 209448.7 205590.8
55.36831 51.87810 50.05944 49.13737
the range of 4000–100 cm−1 . The spectral resolution is ±2 cm−1 . The FT-Raman spectrum of same compound was also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 m with 200 Mw power. Both the spectra were recorded in the range of 4000–100 cm−1 with scanning speed of 30 cm−1 min−1 of spectral width 2 cm−1 . The frequencies of all sharp bands are accurate to ±1 cm−1 . 3. Computational methods The molecular geometry optimization, energy and vibrational frequency calculations are carried out with Gaussian 03W software package [14,15] using the HF, B3LYP and LSDA functional combined with standard 6-31G and 6-311G basis sets. The optimized structural parameters have been evaluated for the calculations of vibrational frequencies at different level of theories and a variety of basis sets by assuming Cs point group symmetry. As a result, the calculated frequencies reduced mass, force constant infrared intensity, Raman activity, and depolarization ratios are obtained. All the computations have been done by adding polarization function (p) and diffuse function (d) on heavy atoms and on hydrogen atoms, in addition to triple split valence basis set; 6-311G (d, p), for better treatment of polar bonds. In order to fit the theoretical wave numbers to the experimental wave numbers, overall scaling factors have been introduced. The calculated frequencies are scaled by 0.9113, 0.9209, 0.9413, 0.9644 and 0.9924 for HF/6-31 +G (d, p) [16,17]. The B3LYP with 6-31 +G (d, p) set is scaled with 0.9604, 0.9754, 0.997, 1.0248and 1.0781 and B3LYP/6-311 ++G (d, p) basis set is scaled with 0.9628, 1.0388 1.0620, 1.08, 1.1583 and 1.203 and LSDA/6-311 ++G basis set is scaled with 0.9837, 0.9988, 1.0367, 1.0575, 1.0819, 1.102, 1.1180 and 1.221 [18]. After scaling by suitable scale factors, the deviation from the experiments is less than ±10 cm−1 with a few exceptions. The assignments of the calculated normal modes have been made and Gauss view program [15] has been considered to get visual animation and also for the verification of the normal modes assignment.
Fig. 1. Molecular structure of 1,2-dibromobenzene.
lengths of B3LYP with those of HF, as a whole the formers are bigger than later and the LSDA/B3LYP calculated values correlates well compared with the experimental data. Although there are differences, calculated geometrical parameters are the bases for the calculating other parameters, such as vibrational frequencies and thermodynamics properties. The benzene ring appears little distorted and angles slightly out of perfect hexagonal structure. It is due to the substitutions of the bromine atom in the place of H atoms. According to the calculated values for LSDA and B3LYP with 6-311 ++G (d, p) set, the order of the optimized bond lengths of the six C–C bonds of the ring lie as C3–C4 C5–C6 < C4–C5 < C1–C6 C2–C3 < C1–C2. From the order, it is clear that the C–C bond length is extended exactly in the substitution place. The bond length of C–Br 1.9095 A˚ calculated by B3LYP/6-311 ++G (d, p) is 0.0425 A˚ higher than the reported experimental value 1.867 A˚ [19,20]. Like bond
4. Results and discussion 4.1. Molecular geometry The zero point vibrational energy obtained by the HF and DFT structure optimization based on B3LYP/6-31G, 6-311G and LSDA/6311G basis sets of 1-2 DBB is presented in Table 1.The structure of the title compound belonging to Cs point group symmetry is shown in Fig. 1. The title compound consists of two Bromine atoms connected to a benzene ring. The comparative optimized geometrical parameters namely bond lengths, bond angles and dihedral angles at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6311 ++G (d, p) levels are given in Table 2. The comparative graphs of bond lengths, bond angles and dihedral angles of 1-2 DBB for four sets are presented in Figs. 2–4, respectively. From the theoretical values, it is found that all of the optimized bond lengths are slightly larger than the experimental values especially for B3LYP and LSDA methods. Comparing bond angles and
Fig. 2. Bond length differences between theoretical approach [HF and DFT].
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Table 2 Optimized geometrical parameters for 1,2-dibromobenzene computed at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6-311 ++G (d, p) basis sets. Methods Geometrical parameters
HF/6-31 +G (d, p)
˚ Bond length (A) C1 –C2 C1 –C6 C1 –Br7 C2 –C3 C2 –Br8 C3 –C4 C3 –H9 C4 –C5 C4 –H10 C5 –C6 C5 –H11 C6 –H12
B3LYP/6-31 +G (d, p)
B3LYP/6-311 ++G (d, p)
LSDA/6-311 ++G (d, p)
1.3857 1.3883 1.883 1.3883 1.883 1.3821 1.0748 1.3863 1.0748 1.3821 1.0748 1.0734
1.3991 1.3979 1.8949 1.3979 1.8949 1.3938 1.0844 1.3961 1.0855 1.3938 1.0855 1.0844
1.3982 1.3955 1.9095 1.3955 1.9095 1.3911 1.0824 1.3929 1.0836 1.3911 1.0836 1.0824
1.3917 1.3873 1.874 1.3873 1.874 1.3873 1.0927 1.3862 1.093 1.3837 1.093 1.0927
Bond angle (◦ ) C2 –C1 –C6 C2 –C1 –Br7 C6 –C1 –Br7 C1 –C2 –C3 C1 –C2 –Br8 C3 –C2 –Br8 C2 –C3 –C4 C2 –C3 –H9 C4 –C3 –H9 C3 –C4 –C5 C3 –C4 –H10 C5 –C4 –H10 C4 –C5 –C6 C4 –C5 –H11 C6 –C5 –H11 C1 –C6 –C5 C1 –C6 –H12 C5 –C6 –H12
119.6843 122.4515 117.8642 119.6843 122.4515 117.8642 120.5022 119.2624 120.2354 119.8135 119.6396 120.5469 119.8135 120.5469 119.6396 120.5022 119.2624 120.2354
119.6876 122.0092 118.3033 119.6876 122.0092 118.3033 120.4603 119.1845 120.3552 119.8522 119.5743 120.5735 119.8522 120.5735 119.5743 120.4603 119.1845 120.3552
119.7501 122.3275 117.9225 119.7501 122.3275 117.9225 120.2695 119.2278 120.5027 119.9805 119.5221 120.4975 119.9805 120.4975 119.5221 120.2695 119.2278 120.5027
119.8001 121.5044 118.6955 119.8001 121.5044 118.6955 120.1831 118.9347 120.8822 120.0168 119.5176 120.4656 120.0168 120.4656 119.5176 120.1831 118.9347 120.8822
Dihedral angle (◦ ) C6 –C1 –C2 –C3 C6 –C1 –C2 –Br8 Br7 –C1 –C2 –C3 Br7 –C1 –C2 –Br8 C2 –C1 –C6 –C5 C2 –C1 –C6 –H12 Br7 –C1 –C6 –C5 Br7 –C1 –C6 –H12 C1 –C2 –C3 –C4 C1 –C2 –C3 –H9 Br8 –C2 –C3 –C4 Br8 –C2 –C3 –H9 C2 –C3 –C4 –C5 C2 –C3 –C4 –H10 H9 –C3 –C4 –C5 H9 –C3 –C4 –H10 C3 –C4 –C5 –C6 C3 –C4 –C5 –H11 H10 –C4 –C5 –C6 H10 –C4 –C5 –H11 C4 –C5 –C6 –C1 C4 –C5 –C6 –H12 H11 –C5 –C6 –C1 H11 –C5 –C6 –H12
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
length, according to the calculated values of B3LYP/LSDA/6-311 ++G (d, p), the order of the optimized bond angle are lie as C2–C1–C6 C1–C2–C3 < C3–C4–C5 C4–C5–C6 < C2–C3–C4 C1–C6 –C5. 4.2. Vibrational assignments The title molecule consists of 12 atoms, which undergoes 30 normal modes of vibrations. Of the 30 normal modes of vibrations, 21 modes are in-plane and remaining 9 are out-of-plane. The bands that are in the plane of the molecule are represented as A and out of
Experimental values 1.384 1.396 1.867 1.395 1.867 1.384 1.090 1.387 1.090 1.383 1.090 1.090 121.8 124.9 114.4 119.2 121.8 – 119.7 – – 120.1 – – 121.4 – – 117.7 – – – – – – – – – – – – – – – – – – – – – – – – – –
plane as A . Thus the 30 normal modes of vibrations are distributed as vib = 21 A + 9 A In agreement with CS symmetry all the 30 fundamental vibrations are active in both Raman scattering and IR absorption. The detailed vibrational assignment of the experimental wave numbers is based on comparison with theoretically scaled wave numbers by HF and B3LYP/LSDA methods. The scaled wave numbers following LSDA/6-311G method are found closest to experimental data than the results obtained using other methods.
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Table 3 Assignments of fundamental vibrations of 1,2-dibromobenzene computed at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6-31 ++G (d, p) basis sets. S. No.
Symmetry species Cs
Observed fundamentals (cm−1 )
Calculated frequencies (cm−1 )
FT-IR
HF/6-31 +G(d, p)
FT-Raman
Unscaled 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
– 3080m – 3060w 1700vw 1600w – 1572s 1450vs 1350w – 1160w – 1100vs 1080w – 1020s 1010vs 950m 850w 750vs 700s 610w
3120w – 3070m – – – 1580w – – – 1280vw – 1150w – 1080w 1030s – – – – – – – 520w 390s 380w 300w 250w 220w 130m
3391 3387 3373 3358 1773 1755 1624 1584 1463 1393 1315 1234 1225 1200 1136 1127 1108 1096 977 858 771 703 636 572 410 402 278 275 149 141
B3LYP/6-31 +G (d,p)
Scaled 3122 3086 3073 3060 1709 1599 1575 1571 1451 1343 1275 1161 1153 1093 1095 1027 1020 1009 942 851 743 697 613 521 395 378 299 250 219 129
Vibrational assignments
Unscaled 3223 3219 3207 3194 1622 1613 1491 1461 1409 1327 1286 1188 1149 1130 1058 1013 1009 986 874 774 719 654 593 537 383 374 259 256 138 130
Scaled 3119 3091 3080 3067 1662 1608 1580 1575 1443 1359 1282 1158 1145 1102 1084 1038 1029 1010 942 835 736 705 607 515 392 383 299 249 219 129
B3LYP/6-311 ++G(d, p) Unscaled 3205 3201 3189 3175 1605 1602 1477 1455 1301 1277 1186 1143 1115 1052 1018 988 956 866 760 710 689 653 502 439 375 359 252 211 128 113
Scaled 3108 3081 3070 3056 1704 1599 1580 1571 1444 1356 1280 1154 1148 1092 1081 1026 1015 1003 950 852 744 705 603 526 389 381 302 253 218 130
LSDA/6-31 ++G(d,p) Unscaled 3135 3129 3122 3111 1598 1596 1442 1417 1378 1225 1138 1116 1112 1045 997 953 916 826 727 705 672 641 490 424 386 358 252 205 127 112
Scaled 3119 3077 3071 3060 1689 1594 1589 1561 1457 1350 1278 1156 1152 1105 1078 1031 1024 1008 945 860 751 706 598 517 389 378 307 250 219 136
(C–H) (C–H) (C–H) (C–H) (C C) (C C) (C C) (C–C) (C–C) (C–C) (C–H) ␦ (C–H) ␦ (C–H) ␦ (C–H) ␦ (C–H) ␥ (C–H) ␥ (C–H) ␥ (C–H) ␥ (CCC) ␦ (CCC) ␦ (CCC) ␦ (C–Br) (C–Br) (CCC) ␥ (CCC) ␥ (CCC) ␥ (C–Br) ␦ (C–Br) ␦ (C–Br) ␥ (C–Br) ␥
VS – very strong; S – strong; m – medium; w – weak; as – asymmetric; s – symmetric; – stretching; ␦ – In-plane bending; ␥ – out-plane bending.
The calculated frequencies are usually higher than the corresponding experimental quantities, due to the combination of electron correlation effects and basis set deficiencies. After applying, the overall uniform scaling factor theoretical calculations reproduce the experimental data well in agreement. The observed and scaled theoretical frequencies using HF/6-31G and DFT method (B3LYP/LSDA) with 6-31G (d, p) and 6-311G (d, p) basis sets are listed in Table 3. The comparative experimental and calculated (HF/B3LYP) IR and Raman spectra of 1-2 DBB are given in Figs. 5 and 6, respectively.
Fig. 3. Bond angle differences between theoretical [HF and DFT] approaches.
4.2.1. Computed IR intensity and Raman activity analysis Computed vibrational spectral IR intensities and Raman activities of the 1-2 DBB for corresponding wave numbers by HF and DFT methods with B3LYP/LSDA at 6-311G ++(d, p) basis sets have been collected in Table 4. The title molecule is a non-polar molecule with CS point group. Comparison of IR intensity and Raman activity calculated by HF and DFT with B3LYP/LSDA at 6-31 +G (d, P) and 6-311G ++(d, p) methods with experimental values show the variation of IR intensities and Raman activities. The IR intensity values predicted by HF methods are found to be larger when compared
Fig. 4. Dihedral angle differences between theoretical [HF and DFT] approaches.
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Table 4 Comparative values of IR intensity and Raman activity between HF/6-31 +G (d, p), B3LYP/6-31 +G (d, p), B3LYP/6-311 ++G (d, p) and LSDA/6-311 ++G (d, p) of 1,2dibromobenzene. S. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Observed frequency
3120 3080 3070 3060 1700 1600 1580 1572 1450 1350 1280 1160 1150 1100 1080 1030 1020 1010 950 850 750 700 610 520 390 380 300 250 220 130
Calculated with HF/6-31G (d, p)
Calculated with B3LYP/6-31G (d, p)
Calculated with B3LYP/6-311G (d, p)
Calculated with LSDA/6-311G (d, p)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
6.3798 3.2177 8.7778 1.6888 10.1928 4.4998 53.8106 23.1789 0.0000 6.5696 0.3102 2.6026 15.6389 3.8835 0.0000 9.0801 2.0908 42.3649 0.0000 77.7322 14.2803 10.6193 0.0000 11.5513 3.1297 0.0080 1.2943 0.4583 0.0000 0.0298
232.1062 25.2160 109.0976 48.5400 30.9332 10.5016 3.5747 0.0170 0.0979 0.0564 2.3524 0.0096 10.1546 18.9813 0.4185 44.6663 0.2207 4.8043 0.3363 0.8631 0.1078 8.7333 0.0107 0.0967 17.0610 2.4043 1.2346 2.3574 2.4450 0.8753
4.6495 2.7077 7.8334 1.5507 12.3129 4.8115 50.3028 19.6923 0.0000 0.3465 5.9495 0.0209 1.3283 24.0613 11.7462 0.0000 42.5158 1.7490 0.0000 59.2364 14.5742 10.2231 0.0000 11.4732 3.4783 0.0017 1.2403 0.5785 0.0000 0.0726
275.8349 32.3917 128.6354 52.8351 28.3299 11.6239 2.4148 0.0021 0.0205 11.7678 0.0142 6.5198 0.2769 16.4839 47.4668 0.3778 3.1646 0.1303 0.0984 0.5718 1.0814 7.0696 0.0153 0.1694 14.4991 1.8225 1.4993 1.8222 2.9302 0.9753
4.0340 1.7724 6.6471 1.4574 11.8282 4.0740 50.4415 20.6861 0.1881 5.2381 0.0004 0.8995 23.5199 13.4031 43.9301 0.0000 1.3877 0.0000 66.3556 20.6338 0.0000 10.6326 0.0000 4.9487 3.9428 0.0029 0.6802 2.0427 0.0627 0.0000
271.9115 31.5187 124.7663 49.5878 24.7710 11.1154 2.2211 0.0239 11.9279 0.0048 6.2674 0.2586 12.3473 48.7047 3.5102 0.1913 0.0542 0.0049 0.0045 0.6218 0.1180 6.2209 0.2203 0.0125 14.5876 2.5737 1.8856 1.0324 1.2778 2.6101
0.9979 0.0240 1.3193 0.6707 13.886 6.7268 62.2552 24.6051 2.0658 3.1309 1.4582 0.0784 27.1955 8.4835 38.4494 0.0000 1.1351 0.0000 70.8925 17.1974 0.0000 10.2303 0.0000 5.4950 3.1056 0.0346 0.4569 2.5398 0.0829 0.0000
327.3245 41.5121 101.1985 39.5847 26.1101 9.8646 3.5142 0.0000 11.4381 0.0002 4.2208 0.6443 37.5469 29.6461 2.1610 0.1364 0.1612 0.1332 0.0422 1.6911 0.7551 5.6702 0.4858 0.0204 12.9113 2.3456 1.3396 1.1521 1.1680 2.8892
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 Figs. 7 and 8, respectively. The similar effect was also observed in the earlier work [21].
4.2.2. Computed vibrational frequency analysis The comparative graph of calculated vibrational frequencies by HF and DFT methods at HF/6-31 +G (d, p), B3LYP/6-31 +G (d, p), B3LYP/6-311 +G (d, p) and LSDA/6-311 ++G (d, p) basis sets for the 1-2 DDB are given in Fig. 9. From the figure, it is found that the calculated (unscaled) frequencies by LSDA with 6-311 ++G (d, p) basis sets are closer to the experimental frequencies than the
Fig. 5. Experimental [A], calculated [B], [C], [D] and [E] FT-IR spectra of 1,2dibromobenzene.
Fig. 6. Experimental [A], calculated [B], [C], [D] and [E] FT-Raman spectra of 1,2dibromobenzene.
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other three methods. This observation is in line with our earlier work [22]. According to the SD, the computed frequency deviation decreases in going from LSDA/6-311 ++G (d, p) to HF/6-31 +G (d, p) to B3LYP/6-311 ++G (d, p) to B3LYP/6-31 +G (d, p). The deviation ratio between HF/6-31 +G (d, p) and B3LYP/6-31 +G (d, p) is 1.72, HF/6-31 +G (d, p) and B3LYP/6-311 +G (d, p) is 1.04 and HF/6-31 +G (d, p) and LSDA/6-311 +G (d, p) is 0.99.
Fig. 7. Comparative graph of IR intensities by HF and DFT (LSDA/B3LYP).
Fig. 8. Comparative graph of Raman activities by HF and DFT [B3LYP and LDSA).
Fig. 9. Comparative graph of experimental and calculated frequencies HF and DFT [LSDA and B3LYP].
4.2.3. C–H vibrations The aromatic compounds particularly benzene derivative commonly exhibit multiple bands in the region 3100–3000 cm−1 [23,24] due to aromatic C–H stretching vibrations. In the present study, the C–H vibrations are observed at 3120 and 3070 cm−1 in Raman spectrum and at 3080 and 3060 cm−1 in IR. The calculated wave numbers HF and B3LYP/LSDA methods for C–H aromatic stretching lie within the range 3122–3060 cm−1 which are in good agreement with the literature. Some of the assigned bands are moved to the top of the expected range which is due to the presence of heavy halogen atoms in the benzene ring. The bands due to C–H in-plane and out-of plane bending vibrations are normally observed as a number of bands in the region 1300–1000 cm−1 and 950–800 cm−1 [25–32], respectively. In this compound, four in-plane bending vibrations are observed at 1280, 1160, 1150 and 1110 cm−1 . The theoretically scaled vibrations by HF and B3LYP/LSDA methods also show good agreement with experimentally recorded data. The vibrations are identified at 1080, 1030, 1020 and 1010 cm−1 are assigned to C–H out-of plane bending. Most of the vibrations are observed in the IR spectrum except one. All C–H in-plane bending vibrations are line with the literature whereas all C–H out-of-plane bending vibrations are lie above the expected region. This is mainly due to the substitutions of heavy halogens atoms. 4.2.4. C–C vibrations Generally the CC stretching vibrations in aromatic compounds are seen in the region of 1430–1650 cm−1 . The six ring carbon atoms undergo coupled vibrations which are known as skeletal vibrations give a maximum of three bands in the region 1700–1580 cm−1 due to C C vibrations and three bands in the region 1980–1430 cm−1 [33,34] due to C–C vibrations. In the present work, the prominent peaks are found at 1700, 1600 and 1580 cm−1 due to C C stretching and at 1572, 1450 and 1350 cm−1 due to C–C stretching vibrations. Except for last one, all the bands are observed within the characteristic region. One band is suppressed in C–C which is purely due to the Br. The peaks at 950, 850 and 750 cm−1 are assigned to C–C–C in-plane bending vibrations and at 520, 390 and 380 cm−1 are assigned to C–C–C out-of-plane bending vibrations. According to the literature [34], all the CCC vibrations are pulled considerably to the higher region and are purely due to the substitutions. Thus the stretching of the ring bonds is affected by the substitutions whereas the ring bending modes are favoured by the same. 4.2.5. C–Br vibrations The vibration belonging to the bond between the ring and the bromine atom is important as mixing of vibrations is possible due to the presence of heavy atom [35–37]. C–Br bond shows lower absorption frequency as compared to C–H bond due to the decreased force constant and increase in reduced mass. Bromine compounds absorb strongly in the region 650–485 cm−1 due to the C–Br stretching vibrations [38]. Accordingly, in the present case, the C–Br stretching vibration of the present compound is observed at 700 cm−1 and 610 cm−1 in the IR spectrum. The peaks are observed at 300 and 250 cm−1 and at 220 and 130 cm−1 are observed for C–Br in-plane and out-of-plane bending vibrations. From the vibrational bands of C–Br, it is observed that, in the case of stretching, first
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band is asymmetric and second is symmetric. All the vibrations are in line with the literature.
[12] [13] [14] [15]
5. Conclusions [16]
In the present work, complete vibrational analysis has been made for proper frequency assignments of 1,2-dibromobenzene. The FT-IR and FT-Raman frequencies of the title compound have been theoretically computed using HF and DFT methods (B3LYP and LSDA) by employing 6-31 +G (d, p) and 6-311 ++G (d, p) basis sets. IR intensities and Raman activities are also calculated by HF and DFT (B3LYP and LSDA) methods. The optimized parameters have also been determined and compared with experimental data. The HF/DFT spectra showed better agreement with experimental spectra. However, it is found that the observed wavenumber values of C–C fundamentals are moved away from the expected region due to the presence of the C–Br bonds. It is also found that the calculated (unscaled) frequencies by LSDA with 6-311 ++G (d, p) basis sets are closer to the experimental frequencies than the other methods. Thus, it is concluded that all the fundamental vibrational bands of benzene ring are affected appreciably due to the couple of Br. References [1] Benzene Wikipedia; The Free Encyclopedia. [2] C. Long, Q. Li, Y. Li, Y. Liu, A. Li, Q. Zhang, Chem. Eng. J. 160 (2010) 723–728. [3] M. Rossberg, et al., Chlorinated Hydrocarbons in Ullmann’s Industrial Chemistry, Wiley-VCH, Weinheim, 2006. [4] F. Zucchi, G. Trabanelli, N.A. Gonzalez, J. Archaeol. Modern Chem. 132 (1995) 4579. [5] B.T. Khan, S.R.A. Khan, K. Annapoorna, Indian J. Chem. Soc. 34 (1995) 11878. [6] B. Suzuki, M. Ito, Spectrochim. Acta 25 A (1969) 1017–1021. [7] G. Schultz, M. Kolonits, I. Hargittai, G. Portalone, A. Domenicano, J. Mol. Struct. 176 (1988) 71–80. [8] A.D. Becke, Phys. Rev. A.38 (1988) 3098. [9] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [10] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [11] Z. Zhengyu, D. Dongmei, J. Mol. Struct. (THEOCHEM) 505 (2000) 247–249.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectroscopic investigation (FT-IR and FT-Raman) on 1,2-dibromobenzene by HF and hybrid (LSDA and B3LYP) calculations G. Shakila a,∗ , S. Periandy b , S. Ramalingam c a b c
Department of Physics, Bharathidasan Government College for Women, Puducherry, India Department of Physics, Tagore Arts College, Puducherry, India Department of Physics, A.V.C. College, Mayiladuthurai, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 29 June 2011 Received in revised form 17 October 2011 Accepted 29 October 2011 Keywords: FT-IR FT-Raman HF DFT Vibrational pattern 1,2-dibromobenzene
a b s t r a c t The FT-IR and FT-Raman spectra of the compound 1,2-dibromobenzene have been recorded in the region 4000–100 cm−1 . The vibrational analysis has been made using HF and DFT (B3LYP and LSDA) level of theory by employing 6-31 +G (d, p) and 6-311 ++G (d, p) basis sets. Optimized geometrical parameters have been calculated, interpreted and compared with the reported experimental values of some halogensubstituted benzene. The experimental geometrical parameters show satisfactory agreement with the theoretical prediction of HF and DFT. The geometrical structure of the compound is fractured by the substitutions of couple of Br in the ring. From the vibrational assignments it is observed that, the vibrational pattern of the fundamental modes is realigned slightly with respect to the substitutions. The simulated FT-IR and FT-Raman spectra of the compound for different methods are compared with the experimental spectra. The impact of Br in the vibrational assignments of the molecule is also investigated. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction The aromatic benzene derivative, bromobenzene is organic halogen compounds, formed by replacing hydrogen atoms in benzene by atoms of bromine. It is a pale yellow liquid with a pleasant, sweet smell and they are practically insoluble in water and denser than water [1]. The water solubility is decreasing if more brominated. Benzene is mostly commonly used as an additive to other chemicals and is used in the manufacturing processes of drugs, detergents, pesticides and explosives [2]. The commercial bromobenzenes are used as heavy liquid solvents, motor oil additives and as an intermediate to manufacture organic chemicals including pharmaceuticals, pesticides and flameretardants for polymeric materials [3]. It is used to make styrene, which is used to make plastics and polymers, and in the manufacturing process of nylon. Investigations on the structure of benzene and its derivative have been a subject of great interest because of their peculiar physical properties and industrial uses [4,5]. Suzuki and Ito [6] have studied the polarized Raman spectra of p-dichlorobenzene and p-dibromobenzene single crystals. Schultz et al. [7] has studied the molecular structure and ring distortions of p-dibromobenzene as determined by electron diffraction.
∗ Corresponding author. Tel.: +91 9444811389, fax: +91 9444811389. E-mail address: [email protected] (G. Shakila).
Literature survey reveals that no HF and DFT (LSDA/B3LYP) calculations with different basis sets of 1,2-dibromobenzene have been reported so far. Therefore, the purpose of the present study is to make a comprehensive vibrational analysis using both experimentally observed IR and Raman wave numbers and theoretically calculated vibrational spectra. The ab initio (HF) and DFT calculations are performed to obtain the ground state optimized geometries and the vibrational wave numbers of the different normal modes as well as to predict the corresponding intensities for the different modes of the molecule. These methods predict relatively accurate molecular structure and vibrational spectra with moderate computational effort. In particular, for polyatomic molecules the DFT methods lead to the prediction of more accurate molecular structure and vibrational frequencies than the conventional ab initio Hartree-Fock calculations. In DFT methods, the LSDA and Becke’s three parameter exact exchange-functional (B3) [8] combined with gradient-corrected correlational functional of Lee, Yang and Parr (LYP) [9,10] are the best predicting results for molecular geometry and vibrational wave numbers for moderately larger molecule [11–13]. 2. Experimental details The spectroscopic grade 1,2-dibromobenzene was purchased from Sigma Aldrich chemicals, U.S.A. and used as such for recording spectra without further purification. The FT-IR spectrum of the 1-Br-4-CB was recorded in Bruker IFS 66V spectrometer in
1386-1425/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.10.066
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Table 1 Zero point vibrational energies in J/Mol and kcal/Mol based on HF/6-31 +G, B3LYP/631 +G, 6-311 ++G and LSDA/6-311 ++G basis sets. Type
J/Mol
kcal/Mol
HF/6-31G B3LYP/6-31G B3LYP/6-311G LSDA/6-311G
231661.0 217058.0 209448.7 205590.8
55.36831 51.87810 50.05944 49.13737
the range of 4000–100 cm−1 . The spectral resolution is ±2 cm−1 . The FT-Raman spectrum of same compound was also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG laser source operating at 1.064 m with 200 Mw power. Both the spectra were recorded in the range of 4000–100 cm−1 with scanning speed of 30 cm−1 min−1 of spectral width 2 cm−1 . The frequencies of all sharp bands are accurate to ±1 cm−1 . 3. Computational methods The molecular geometry optimization, energy and vibrational frequency calculations are carried out with Gaussian 03W software package [14,15] using the HF, B3LYP and LSDA functional combined with standard 6-31G and 6-311G basis sets. The optimized structural parameters have been evaluated for the calculations of vibrational frequencies at different level of theories and a variety of basis sets by assuming Cs point group symmetry. As a result, the calculated frequencies reduced mass, force constant infrared intensity, Raman activity, and depolarization ratios are obtained. All the computations have been done by adding polarization function (p) and diffuse function (d) on heavy atoms and on hydrogen atoms, in addition to triple split valence basis set; 6-311G (d, p), for better treatment of polar bonds. In order to fit the theoretical wave numbers to the experimental wave numbers, overall scaling factors have been introduced. The calculated frequencies are scaled by 0.9113, 0.9209, 0.9413, 0.9644 and 0.9924 for HF/6-31 +G (d, p) [16,17]. The B3LYP with 6-31 +G (d, p) set is scaled with 0.9604, 0.9754, 0.997, 1.0248and 1.0781 and B3LYP/6-311 ++G (d, p) basis set is scaled with 0.9628, 1.0388 1.0620, 1.08, 1.1583 and 1.203 and LSDA/6-311 ++G basis set is scaled with 0.9837, 0.9988, 1.0367, 1.0575, 1.0819, 1.102, 1.1180 and 1.221 [18]. After scaling by suitable scale factors, the deviation from the experiments is less than ±10 cm−1 with a few exceptions. The assignments of the calculated normal modes have been made and Gauss view program [15] has been considered to get visual animation and also for the verification of the normal modes assignment.
Fig. 1. Molecular structure of 1,2-dibromobenzene.
lengths of B3LYP with those of HF, as a whole the formers are bigger than later and the LSDA/B3LYP calculated values correlates well compared with the experimental data. Although there are differences, calculated geometrical parameters are the bases for the calculating other parameters, such as vibrational frequencies and thermodynamics properties. The benzene ring appears little distorted and angles slightly out of perfect hexagonal structure. It is due to the substitutions of the bromine atom in the place of H atoms. According to the calculated values for LSDA and B3LYP with 6-311 ++G (d, p) set, the order of the optimized bond lengths of the six C–C bonds of the ring lie as C3–C4 C5–C6 < C4–C5 < C1–C6 C2–C3 < C1–C2. From the order, it is clear that the C–C bond length is extended exactly in the substitution place. The bond length of C–Br 1.9095 A˚ calculated by B3LYP/6-311 ++G (d, p) is 0.0425 A˚ higher than the reported experimental value 1.867 A˚ [19,20]. Like bond
4. Results and discussion 4.1. Molecular geometry The zero point vibrational energy obtained by the HF and DFT structure optimization based on B3LYP/6-31G, 6-311G and LSDA/6311G basis sets of 1-2 DBB is presented in Table 1.The structure of the title compound belonging to Cs point group symmetry is shown in Fig. 1. The title compound consists of two Bromine atoms connected to a benzene ring. The comparative optimized geometrical parameters namely bond lengths, bond angles and dihedral angles at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6311 ++G (d, p) levels are given in Table 2. The comparative graphs of bond lengths, bond angles and dihedral angles of 1-2 DBB for four sets are presented in Figs. 2–4, respectively. From the theoretical values, it is found that all of the optimized bond lengths are slightly larger than the experimental values especially for B3LYP and LSDA methods. Comparing bond angles and
Fig. 2. Bond length differences between theoretical approach [HF and DFT].
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Table 2 Optimized geometrical parameters for 1,2-dibromobenzene computed at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6-311 ++G (d, p) basis sets. Methods Geometrical parameters
HF/6-31 +G (d, p)
˚ Bond length (A) C1 –C2 C1 –C6 C1 –Br7 C2 –C3 C2 –Br8 C3 –C4 C3 –H9 C4 –C5 C4 –H10 C5 –C6 C5 –H11 C6 –H12
B3LYP/6-31 +G (d, p)
B3LYP/6-311 ++G (d, p)
LSDA/6-311 ++G (d, p)
1.3857 1.3883 1.883 1.3883 1.883 1.3821 1.0748 1.3863 1.0748 1.3821 1.0748 1.0734
1.3991 1.3979 1.8949 1.3979 1.8949 1.3938 1.0844 1.3961 1.0855 1.3938 1.0855 1.0844
1.3982 1.3955 1.9095 1.3955 1.9095 1.3911 1.0824 1.3929 1.0836 1.3911 1.0836 1.0824
1.3917 1.3873 1.874 1.3873 1.874 1.3873 1.0927 1.3862 1.093 1.3837 1.093 1.0927
Bond angle (◦ ) C2 –C1 –C6 C2 –C1 –Br7 C6 –C1 –Br7 C1 –C2 –C3 C1 –C2 –Br8 C3 –C2 –Br8 C2 –C3 –C4 C2 –C3 –H9 C4 –C3 –H9 C3 –C4 –C5 C3 –C4 –H10 C5 –C4 –H10 C4 –C5 –C6 C4 –C5 –H11 C6 –C5 –H11 C1 –C6 –C5 C1 –C6 –H12 C5 –C6 –H12
119.6843 122.4515 117.8642 119.6843 122.4515 117.8642 120.5022 119.2624 120.2354 119.8135 119.6396 120.5469 119.8135 120.5469 119.6396 120.5022 119.2624 120.2354
119.6876 122.0092 118.3033 119.6876 122.0092 118.3033 120.4603 119.1845 120.3552 119.8522 119.5743 120.5735 119.8522 120.5735 119.5743 120.4603 119.1845 120.3552
119.7501 122.3275 117.9225 119.7501 122.3275 117.9225 120.2695 119.2278 120.5027 119.9805 119.5221 120.4975 119.9805 120.4975 119.5221 120.2695 119.2278 120.5027
119.8001 121.5044 118.6955 119.8001 121.5044 118.6955 120.1831 118.9347 120.8822 120.0168 119.5176 120.4656 120.0168 120.4656 119.5176 120.1831 118.9347 120.8822
Dihedral angle (◦ ) C6 –C1 –C2 –C3 C6 –C1 –C2 –Br8 Br7 –C1 –C2 –C3 Br7 –C1 –C2 –Br8 C2 –C1 –C6 –C5 C2 –C1 –C6 –H12 Br7 –C1 –C6 –C5 Br7 –C1 –C6 –H12 C1 –C2 –C3 –C4 C1 –C2 –C3 –H9 Br8 –C2 –C3 –C4 Br8 –C2 –C3 –H9 C2 –C3 –C4 –C5 C2 –C3 –C4 –H10 H9 –C3 –C4 –C5 H9 –C3 –C4 –H10 C3 –C4 –C5 –C6 C3 –C4 –C5 –H11 H10 –C4 –C5 –C6 H10 –C4 –C5 –H11 C4 –C5 –C6 –C1 C4 –C5 –C6 –H12 H11 –C5 –C6 –C1 H11 –C5 –C6 –H12
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0 0.0 180.0 180.0 0.0
length, according to the calculated values of B3LYP/LSDA/6-311 ++G (d, p), the order of the optimized bond angle are lie as C2–C1–C6 C1–C2–C3 < C3–C4–C5 C4–C5–C6 < C2–C3–C4 C1–C6 –C5. 4.2. Vibrational assignments The title molecule consists of 12 atoms, which undergoes 30 normal modes of vibrations. Of the 30 normal modes of vibrations, 21 modes are in-plane and remaining 9 are out-of-plane. The bands that are in the plane of the molecule are represented as A and out of
Experimental values 1.384 1.396 1.867 1.395 1.867 1.384 1.090 1.387 1.090 1.383 1.090 1.090 121.8 124.9 114.4 119.2 121.8 – 119.7 – – 120.1 – – 121.4 – – 117.7 – – – – – – – – – – – – – – – – – – – – – – – – – –
plane as A . Thus the 30 normal modes of vibrations are distributed as vib = 21 A + 9 A In agreement with CS symmetry all the 30 fundamental vibrations are active in both Raman scattering and IR absorption. The detailed vibrational assignment of the experimental wave numbers is based on comparison with theoretically scaled wave numbers by HF and B3LYP/LSDA methods. The scaled wave numbers following LSDA/6-311G method are found closest to experimental data than the results obtained using other methods.
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Table 3 Assignments of fundamental vibrations of 1,2-dibromobenzene computed at HF/6-31 +G (d, p), B3LYP/6-31 +G, 6-311 ++G (d, p) and LSDA/6-31 ++G (d, p) basis sets. S. No.
Symmetry species Cs
Observed fundamentals (cm−1 )
Calculated frequencies (cm−1 )
FT-IR
HF/6-31 +G(d, p)
FT-Raman
Unscaled 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
– 3080m – 3060w 1700vw 1600w – 1572s 1450vs 1350w – 1160w – 1100vs 1080w – 1020s 1010vs 950m 850w 750vs 700s 610w
3120w – 3070m – – – 1580w – – – 1280vw – 1150w – 1080w 1030s – – – – – – – 520w 390s 380w 300w 250w 220w 130m
3391 3387 3373 3358 1773 1755 1624 1584 1463 1393 1315 1234 1225 1200 1136 1127 1108 1096 977 858 771 703 636 572 410 402 278 275 149 141
B3LYP/6-31 +G (d,p)
Scaled 3122 3086 3073 3060 1709 1599 1575 1571 1451 1343 1275 1161 1153 1093 1095 1027 1020 1009 942 851 743 697 613 521 395 378 299 250 219 129
Vibrational assignments
Unscaled 3223 3219 3207 3194 1622 1613 1491 1461 1409 1327 1286 1188 1149 1130 1058 1013 1009 986 874 774 719 654 593 537 383 374 259 256 138 130
Scaled 3119 3091 3080 3067 1662 1608 1580 1575 1443 1359 1282 1158 1145 1102 1084 1038 1029 1010 942 835 736 705 607 515 392 383 299 249 219 129
B3LYP/6-311 ++G(d, p) Unscaled 3205 3201 3189 3175 1605 1602 1477 1455 1301 1277 1186 1143 1115 1052 1018 988 956 866 760 710 689 653 502 439 375 359 252 211 128 113
Scaled 3108 3081 3070 3056 1704 1599 1580 1571 1444 1356 1280 1154 1148 1092 1081 1026 1015 1003 950 852 744 705 603 526 389 381 302 253 218 130
LSDA/6-31 ++G(d,p) Unscaled 3135 3129 3122 3111 1598 1596 1442 1417 1378 1225 1138 1116 1112 1045 997 953 916 826 727 705 672 641 490 424 386 358 252 205 127 112
Scaled 3119 3077 3071 3060 1689 1594 1589 1561 1457 1350 1278 1156 1152 1105 1078 1031 1024 1008 945 860 751 706 598 517 389 378 307 250 219 136
(C–H) (C–H) (C–H) (C–H) (C C) (C C) (C C) (C–C) (C–C) (C–C) (C–H) ␦ (C–H) ␦ (C–H) ␦ (C–H) ␦ (C–H) ␥ (C–H) ␥ (C–H) ␥ (C–H) ␥ (CCC) ␦ (CCC) ␦ (CCC) ␦ (C–Br) (C–Br) (CCC) ␥ (CCC) ␥ (CCC) ␥ (C–Br) ␦ (C–Br) ␦ (C–Br) ␥ (C–Br) ␥
VS – very strong; S – strong; m – medium; w – weak; as – asymmetric; s – symmetric; – stretching; ␦ – In-plane bending; ␥ – out-plane bending.
The calculated frequencies are usually higher than the corresponding experimental quantities, due to the combination of electron correlation effects and basis set deficiencies. After applying, the overall uniform scaling factor theoretical calculations reproduce the experimental data well in agreement. The observed and scaled theoretical frequencies using HF/6-31G and DFT method (B3LYP/LSDA) with 6-31G (d, p) and 6-311G (d, p) basis sets are listed in Table 3. The comparative experimental and calculated (HF/B3LYP) IR and Raman spectra of 1-2 DBB are given in Figs. 5 and 6, respectively.
Fig. 3. Bond angle differences between theoretical [HF and DFT] approaches.
4.2.1. Computed IR intensity and Raman activity analysis Computed vibrational spectral IR intensities and Raman activities of the 1-2 DBB for corresponding wave numbers by HF and DFT methods with B3LYP/LSDA at 6-311G ++(d, p) basis sets have been collected in Table 4. The title molecule is a non-polar molecule with CS point group. Comparison of IR intensity and Raman activity calculated by HF and DFT with B3LYP/LSDA at 6-31 +G (d, P) and 6-311G ++(d, p) methods with experimental values show the variation of IR intensities and Raman activities. The IR intensity values predicted by HF methods are found to be larger when compared
Fig. 4. Dihedral angle differences between theoretical [HF and DFT] approaches.
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Table 4 Comparative values of IR intensity and Raman activity between HF/6-31 +G (d, p), B3LYP/6-31 +G (d, p), B3LYP/6-311 ++G (d, p) and LSDA/6-311 ++G (d, p) of 1,2dibromobenzene. S. No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Observed frequency
3120 3080 3070 3060 1700 1600 1580 1572 1450 1350 1280 1160 1150 1100 1080 1030 1020 1010 950 850 750 700 610 520 390 380 300 250 220 130
Calculated with HF/6-31G (d, p)
Calculated with B3LYP/6-31G (d, p)
Calculated with B3LYP/6-311G (d, p)
Calculated with LSDA/6-311G (d, p)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
IR intensity (Ai)
Raman activity (I)
6.3798 3.2177 8.7778 1.6888 10.1928 4.4998 53.8106 23.1789 0.0000 6.5696 0.3102 2.6026 15.6389 3.8835 0.0000 9.0801 2.0908 42.3649 0.0000 77.7322 14.2803 10.6193 0.0000 11.5513 3.1297 0.0080 1.2943 0.4583 0.0000 0.0298
232.1062 25.2160 109.0976 48.5400 30.9332 10.5016 3.5747 0.0170 0.0979 0.0564 2.3524 0.0096 10.1546 18.9813 0.4185 44.6663 0.2207 4.8043 0.3363 0.8631 0.1078 8.7333 0.0107 0.0967 17.0610 2.4043 1.2346 2.3574 2.4450 0.8753
4.6495 2.7077 7.8334 1.5507 12.3129 4.8115 50.3028 19.6923 0.0000 0.3465 5.9495 0.0209 1.3283 24.0613 11.7462 0.0000 42.5158 1.7490 0.0000 59.2364 14.5742 10.2231 0.0000 11.4732 3.4783 0.0017 1.2403 0.5785 0.0000 0.0726
275.8349 32.3917 128.6354 52.8351 28.3299 11.6239 2.4148 0.0021 0.0205 11.7678 0.0142 6.5198 0.2769 16.4839 47.4668 0.3778 3.1646 0.1303 0.0984 0.5718 1.0814 7.0696 0.0153 0.1694 14.4991 1.8225 1.4993 1.8222 2.9302 0.9753
4.0340 1.7724 6.6471 1.4574 11.8282 4.0740 50.4415 20.6861 0.1881 5.2381 0.0004 0.8995 23.5199 13.4031 43.9301 0.0000 1.3877 0.0000 66.3556 20.6338 0.0000 10.6326 0.0000 4.9487 3.9428 0.0029 0.6802 2.0427 0.0627 0.0000
271.9115 31.5187 124.7663 49.5878 24.7710 11.1154 2.2211 0.0239 11.9279 0.0048 6.2674 0.2586 12.3473 48.7047 3.5102 0.1913 0.0542 0.0049 0.0045 0.6218 0.1180 6.2209 0.2203 0.0125 14.5876 2.5737 1.8856 1.0324 1.2778 2.6101
0.9979 0.0240 1.3193 0.6707 13.886 6.7268 62.2552 24.6051 2.0658 3.1309 1.4582 0.0784 27.1955 8.4835 38.4494 0.0000 1.1351 0.0000 70.8925 17.1974 0.0000 10.2303 0.0000 5.4950 3.1056 0.0346 0.4569 2.5398 0.0829 0.0000
327.3245 41.5121 101.1985 39.5847 26.1101 9.8646 3.5142 0.0000 11.4381 0.0002 4.2208 0.6443 37.5469 29.6461 2.1610 0.1364 0.1612 0.1332 0.0422 1.6911 0.7551 5.6702 0.4858 0.0204 12.9113 2.3456 1.3396 1.1521 1.1680 2.8892
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 Figs. 7 and 8, respectively. The similar effect was also observed in the earlier work [21].
4.2.2. Computed vibrational frequency analysis The comparative graph of calculated vibrational frequencies by HF and DFT methods at HF/6-31 +G (d, p), B3LYP/6-31 +G (d, p), B3LYP/6-311 +G (d, p) and LSDA/6-311 ++G (d, p) basis sets for the 1-2 DDB are given in Fig. 9. From the figure, it is found that the calculated (unscaled) frequencies by LSDA with 6-311 ++G (d, p) basis sets are closer to the experimental frequencies than the
Fig. 5. Experimental [A], calculated [B], [C], [D] and [E] FT-IR spectra of 1,2dibromobenzene.
Fig. 6. Experimental [A], calculated [B], [C], [D] and [E] FT-Raman spectra of 1,2dibromobenzene.
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other three methods. This observation is in line with our earlier work [22]. According to the SD, the computed frequency deviation decreases in going from LSDA/6-311 ++G (d, p) to HF/6-31 +G (d, p) to B3LYP/6-311 ++G (d, p) to B3LYP/6-31 +G (d, p). The deviation ratio between HF/6-31 +G (d, p) and B3LYP/6-31 +G (d, p) is 1.72, HF/6-31 +G (d, p) and B3LYP/6-311 +G (d, p) is 1.04 and HF/6-31 +G (d, p) and LSDA/6-311 +G (d, p) is 0.99.
Fig. 7. Comparative graph of IR intensities by HF and DFT (LSDA/B3LYP).
Fig. 8. Comparative graph of Raman activities by HF and DFT [B3LYP and LDSA).
Fig. 9. Comparative graph of experimental and calculated frequencies HF and DFT [LSDA and B3LYP].
4.2.3. C–H vibrations The aromatic compounds particularly benzene derivative commonly exhibit multiple bands in the region 3100–3000 cm−1 [23,24] due to aromatic C–H stretching vibrations. In the present study, the C–H vibrations are observed at 3120 and 3070 cm−1 in Raman spectrum and at 3080 and 3060 cm−1 in IR. The calculated wave numbers HF and B3LYP/LSDA methods for C–H aromatic stretching lie within the range 3122–3060 cm−1 which are in good agreement with the literature. Some of the assigned bands are moved to the top of the expected range which is due to the presence of heavy halogen atoms in the benzene ring. The bands due to C–H in-plane and out-of plane bending vibrations are normally observed as a number of bands in the region 1300–1000 cm−1 and 950–800 cm−1 [25–32], respectively. In this compound, four in-plane bending vibrations are observed at 1280, 1160, 1150 and 1110 cm−1 . The theoretically scaled vibrations by HF and B3LYP/LSDA methods also show good agreement with experimentally recorded data. The vibrations are identified at 1080, 1030, 1020 and 1010 cm−1 are assigned to C–H out-of plane bending. Most of the vibrations are observed in the IR spectrum except one. All C–H in-plane bending vibrations are line with the literature whereas all C–H out-of-plane bending vibrations are lie above the expected region. This is mainly due to the substitutions of heavy halogens atoms. 4.2.4. C–C vibrations Generally the CC stretching vibrations in aromatic compounds are seen in the region of 1430–1650 cm−1 . The six ring carbon atoms undergo coupled vibrations which are known as skeletal vibrations give a maximum of three bands in the region 1700–1580 cm−1 due to C C vibrations and three bands in the region 1980–1430 cm−1 [33,34] due to C–C vibrations. In the present work, the prominent peaks are found at 1700, 1600 and 1580 cm−1 due to C C stretching and at 1572, 1450 and 1350 cm−1 due to C–C stretching vibrations. Except for last one, all the bands are observed within the characteristic region. One band is suppressed in C–C which is purely due to the Br. The peaks at 950, 850 and 750 cm−1 are assigned to C–C–C in-plane bending vibrations and at 520, 390 and 380 cm−1 are assigned to C–C–C out-of-plane bending vibrations. According to the literature [34], all the CCC vibrations are pulled considerably to the higher region and are purely due to the substitutions. Thus the stretching of the ring bonds is affected by the substitutions whereas the ring bending modes are favoured by the same. 4.2.5. C–Br vibrations The vibration belonging to the bond between the ring and the bromine atom is important as mixing of vibrations is possible due to the presence of heavy atom [35–37]. C–Br bond shows lower absorption frequency as compared to C–H bond due to the decreased force constant and increase in reduced mass. Bromine compounds absorb strongly in the region 650–485 cm−1 due to the C–Br stretching vibrations [38]. Accordingly, in the present case, the C–Br stretching vibration of the present compound is observed at 700 cm−1 and 610 cm−1 in the IR spectrum. The peaks are observed at 300 and 250 cm−1 and at 220 and 130 cm−1 are observed for C–Br in-plane and out-of-plane bending vibrations. From the vibrational bands of C–Br, it is observed that, in the case of stretching, first
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band is asymmetric and second is symmetric. All the vibrations are in line with the literature.
[12] [13] [14] [15]
5. Conclusions [16]
In the present work, complete vibrational analysis has been made for proper frequency assignments of 1,2-dibromobenzene. The FT-IR and FT-Raman frequencies of the title compound have been theoretically computed using HF and DFT methods (B3LYP and LSDA) by employing 6-31 +G (d, p) and 6-311 ++G (d, p) basis sets. IR intensities and Raman activities are also calculated by HF and DFT (B3LYP and LSDA) methods. The optimized parameters have also been determined and compared with experimental data. The HF/DFT spectra showed better agreement with experimental spectra. However, it is found that the observed wavenumber values of C–C fundamentals are moved away from the expected region due to the presence of the C–Br bonds. It is also found that the calculated (unscaled) frequencies by LSDA with 6-311 ++G (d, p) basis sets are closer to the experimental frequencies than the other methods. Thus, it is concluded that all the fundamental vibrational bands of benzene ring are affected appreciably due to the couple of Br. References [1] Benzene Wikipedia; The Free Encyclopedia. [2] C. Long, Q. Li, Y. Li, Y. Liu, A. Li, Q. Zhang, Chem. Eng. J. 160 (2010) 723–728. [3] M. Rossberg, et al., Chlorinated Hydrocarbons in Ullmann’s Industrial Chemistry, Wiley-VCH, Weinheim, 2006. [4] F. Zucchi, G. Trabanelli, N.A. Gonzalez, J. Archaeol. Modern Chem. 132 (1995) 4579. [5] B.T. Khan, S.R.A. Khan, K. Annapoorna, Indian J. Chem. Soc. 34 (1995) 11878. [6] B. Suzuki, M. Ito, Spectrochim. Acta 25 A (1969) 1017–1021. [7] G. Schultz, M. Kolonits, I. Hargittai, G. Portalone, A. Domenicano, J. Mol. Struct. 176 (1988) 71–80. [8] A.D. Becke, Phys. Rev. A.38 (1988) 3098. [9] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [10] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [11] Z. Zhengyu, D. Dongmei, J. Mol. Struct. (THEOCHEM) 505 (2000) 247–249.
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