The FT-Raman and FT-IR spectra for 1-bromo-2-chlorobenzene (1B2CB) have been recorded in the region 4000–100 cm−1 and compared with the harmonic vibrational frequencies calculated using HF/DFT (B3LYP) method by employing 6-31+G (d, p) and 6-311++G (d, p) basis set with appropriate scale factors. IR intensities and Raman activities are also calculated by HF and DFT (B3LYP) methods. Optimized geometries of the molecule have been interpreted and compared with the reported experimental values of some substituted benzene. The experimental geometrical parameters show satisfactory agreement with the theoretical prediction from HF and DFT. The scaled vibrational frequencies at B3LYP/6-311++G (d, p) seem to coincide with the experimentally observed values with acceptable deviations. The theoretical spectrograms (IR and Raman) have been constructed and compared with the experimental FT-IR and FT-Raman spectra. Some of the vibrational frequencies of the benzene are affected upon profusely with the halogen substitutions in comparison to benzene, and these differences are interpreted.
Aromatic compounds such as benzene derivative compounds are commonly used for chronic inflammation treatment products in pharmaceutical products. Benzene is frequently used as an industrial solvent, especially for degreasing metal. Chlorobenzene is an important industrial solvent and a widely used intermediate in production of commodities such as herbicides, dyestuffs, and rubber [
In this study, molecular geometry, optimized parameters, and vibrational frequencies are computed and the performance of the computational methods for ab initio (HF), hybrid density functional methods B3LYP at 6-31G+ (d, p) and 6-311G++ (d, p) basis sets are compared. 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, Becke’s three parameter exact exchange-functional (B3) [
The spectroscopic grade 1-Br-2-CB was purchased from Sigma Aldrich chemicals, USA, 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 66 V spectrometer in the range of 4000 to 100 cm−1. The spectral resolution is ±2 cm−1. The FT-Raman spectrum of 1-Br-2-CB was also recorded in the same instrument with FRA 106 Raman module equipped with Nd: YAG laser source operating at 1.064
HF/DFT calculations for 1-bromo-2-chlorobenzene are performed using GAUSSIAN 03 W program package on Pentium IV processor personal computer without any constraint on the geometry. The molecular structure of the title compound in the ground state is computed both ab initio HF with 6-311G (d, p) and DFT (B3LYP) with 6-311++G (d, p) and 6-31+G (d, p) basis sets. The comparative IR and Raman spectra of experimental and calculated (HF/B3LYP) are given in Figures
The molecular structure of the 1-Br-2-CB belongs to C
Optimized geometrical parameters for 1-bromo-2-chlorobenzene computed at HF/6-31+G (d, p), B3LYP/6-31+G (d, p), and B3LYP/6-31G++ (d, p) basis sets.
Geometrical Parameters | Methods | |||
HF/6-31+G (d, p) | B3LYP/6-31+G (d, p) | B3LYP/6-311++G (d, p) | Experimental value | |
Bond length (Å) | ||||
C1–C2 | 1.3865 | 1.400 | 1.397 | — |
C1–C6 | 1.388 | 1.397 | 1.395 | — |
C1–Br11 | 1.8837 | 1.895 | 1.908 | 1.867 |
C2–C3 | 1.388 | 1.398 | 1.395 | — |
C2 –Cl12 | 1.7355 | 1.749 | 1.748 | 1.745 |
C3–C4 | 1.3824 | 1.394 | 1.390 | — |
C3–H7 | 1.0735 | 1.084 | 1.082 | 1.080 |
C4–C5 | 1.3868 | 1.396 | 1.393 | — |
C4–H8 | 1.0748 | 1.085 | 1.083 | 1.080 |
C5–C6 | 1.3828 | 1.394 | 1.390 | — |
C5–H9 | 1.0747 | 1.085 | 1.083 | 1.080 |
C6 –H10 | 1.0735 | 1.084 | 1.082 | 1.080 |
Bond angle (°) | ||||
C2–C1–C6 | 119.3229 | 119.25 | 119.79 | — |
C2–C1–Br11 | 122.2402 | 121.88 | 121.87 | 114.4 |
C6–C1–Br11 | 118.4369 | 118.85 | 118.32 | — |
C1–C2–C3 | 120.2134 | 120.28 | 119.73 | — |
C1–C2–Cl12 | 121.8473 | 121.44 | 121.86 | 116.3 |
C3–C2–Cl12 | 117.9394 | 118.26 | 118.40 | 116.3 |
C2–C3–C4 | 120.1128 | 120.01 | 120.26 | — |
C2–C3–H7 | 119.1113 | 119.08 | 118.83 | — |
C4–C3–H7 | 120.7759 | 120.90 | 120.89 | — |
C3–C4–C5 | 119.9138 | 119.96 | 119.98 | — |
C3–C4–H8 | 119.6402 | 119.56 | 119.51 | — |
C5–C4–H8 | 120.4459 | 120.46 | 120.49 | — |
C4–C5–C6 | 119.8645 | 119.92 | 119.98 | — |
C4–C5–H9 | 120.5147 | 120.54 | 120.52 | — |
C6–C5–H9 | 119.6208 | 119.53 | 119.48 | — |
C1–C6–C5 | 120.5726 | 120.56 | 120.23 | — |
C1–C6–H10 | 119.185 | 119.07 | 119.16 | — |
C5–C6–H10 | 120.2424 | 120.35 | 120.60 | — |
Dihedral angle (°) | ||||
C6–C1–C2–C3 | 0.0 | 0.0 | 0.0 | — |
C6–C1–C2–Br11 | 180.0 | 180.0 | 180.0 | — |
Br11–C1–C2–C3 | 180.0 | 180.0 | 180.0 | — |
Br11–C1–C2–Br11 | 0.0 | 0.0 | 0.0 | — |
C2–C1–C6–C5 | 0.0 | 0.0 | 0.0 | — |
C2–C1–C6–H10 | 180.0 | 180.0 | 180.0 | — |
Br11–C1–C6–C5 | 180.0 | 180.0 | 180.0 | — |
Br11–C1–C6–H10 | 0.0 | 0.0 | 0.0 | — |
C1–C2–C3–C4 | 0.0 | 0.0 | 0.0 | — |
C1–C2–C3–H7 | 180.0 | 180.0 | 180.0 | — |
Cl12–C2–C3–C4 | 180.0 | 180.0 | 180.0 | — |
Cl12–C2–C3–H7 | 0.0 | 0.0 | 0.0 | — |
C2–C3–C4–C5 | 0.0 | 0.0 | 0.0 | — |
C2–C3–C4–H8 | 180.0 | 180.0 | 180.0 | — |
H7–C3–C4–C5 | 180.0 | 180.0 | 180.0 | — |
H7–C3–C4–H8 | 0.0 | 0.0 | 0.0 | — |
C3–C4–C5–C6 | 0.0 | 0.0 | 0.0 | — |
C3–C4–C5–H9 | 180.0 | 180.0 | 180.0 | — |
H8–C4–C5–C6 | 180.0 | 180.0 | 180.0 | — |
H8–C4–C5–H9 | 0.0 | 0.0 | 0.0 | — |
C4–C5–C6–C1 | 0.0 | 0.0 | 0.0 | — |
C4–C5–C6–H10 | 180.0 | 180.0 | 180.0 | — |
H9–C5–C6– C1 | 180.0 | 180.0 | 180.0 | — |
H9–C5–C6– H10 | 0.0 | 0.0 | 0.0 | — |
Molecular structure of 1-bromo-2-chlorobenzene.
Experimental (a), calculated (b), (c) and (d) FT-IR spectra of 1-bromo-2-chlorobenzene.
Experimental (a), calculated (b), (c) and (d) FT-Raman spectra of toluic acid.
Comparative graph of IR Intensities by HF and DFT (B3LYP).
Comparative graph of Raman intensities by HF and DFT (B3LYP).
Bond length differences between theoretical approach (HF and DFT).
Bond angle differences between theoretical approach (HF and DFT).
Dihedral angle differences between theoretical approach (HF and DFT).
From theoretical values, it is found that most of the optimized bond lengths are slightly larger than the experimental values, because the theoretical calculations belong to isolated molecules in gaseous phase and the experimental results belong to molecules in solid state. Comparing bond angles and lengths of B3LYP with those of HF, as a whole the formers are bigger than the laters and the B3LYP calculated values correlate well compared with the experimental data. Despite the differences, calculated geometrical parameters represent a good approximation, and they 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 and chlorine atom in the place of H atoms. According to the calculated values (B3LYP/6-311++G (d, p)), the order of the optimized bond lengths of the six C–C bonds of the ring 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 compressed exactly in the substitutional place The C-Br bond distance cal. 1.908 Ǻ by B3LYP/6-311++G (d, p) is just 0.041 Ǻ lower than the reported experimental value of 1.867 Ǻ [
The 1-Br-2-CB consists of 12 atoms, and belongs to CS symmetry. Hence the number of normal modes of vibrations for 1-Br-2-CB works to 30. Of the 30 normal modes of vibrations, 21 modes of vibrations are in plane and the remaining 9 are out of plane. The bands that belong to the in-plane modes are represented as A
Observed and HF /6-31+G (d, p), B3LYP/6-31+G (d, p), and B3LYP/6-311++G (d, p) level calculated vibrational frequencies of 1-bromo-2-chlorobenzene.
S. no. | Symmetry species CS | Observed | Calculated frequency (cm−1) with | Vibrational assignments | ||||||
frequency | HF/6-31+G (d, p) | B3LYP/6-31+G (d, p) | B3LYP/6-311++G (d, p) | |||||||
FTIR | FTRaman | Unscaled value | Scaled value | Unscaled value | Scaled value | Unscaled value | Scaled value | |||
1 | A′ | — | 3070vs | 3392 | 3069 | 3222 | 3062 | 3206 | 3073 | (C–H) |
2 | A′ | 3060vs | — | 3388 | 3066 | 3219 | 3059 | 3202 | 3070 | (C–H) |
3 | A′ | — | 3055vs | 3374 | 3053 | 3207 | 3048 | 3190 | 3058 | (C–H) |
4 | A′ | — | 3030w | 3358 | 3038 | 3194 | 3035 | 3177 | 3046 | (C–H) |
5 | A′ | 1600w | — | 1774 | 1605 | 1621 | 1599 | 1610 | 1603 | (C=C) |
6 | A′ | 1570vs | 1570s | 1757 | 1590 | 1614 | 1569 | 1604 | 1579 | (C=C) |
7 | A′ | 1460vs | — | 1625 | 1470 | 1490 | 1448 | 1483 | 1460 | (C=C) |
8 | A′ | 1440vs | — | 1586 | 1435 | 1462 | 1442 | 1457 | 1434 | (C–C) |
9 | A′ | 1260vs | — | 1389 | 1257 | 1326 | 1260 | 1304 | 1250 | (C–C) |
10 | A′ | 1170w | — | 1314 | 1189 | 1281 | 1169 | 1276 | 1169 | (C–C) |
11 | A′ | 1130vs | 1130 | 1242 | 1124 | 1187 | 1128 | 1185 | 1136 | (C–H) |
12 | A′ | 1110vs | — | 1230 | 1113 | 1150 | 1118 | 1147 | 1099 | (C–H) |
13 | A′ | 1105w | — | 1198 | 1084 | 1133 | 1101 | 1126 | 1108 | (C–H) |
14 | A′ | 1040w | — | 1191 | 1077 | 1098 | 1043 | 1054 | 1037 | (C–H) |
15 | A′′ | 1030w | — | 1130 | 1022 | 1058 | 1028 | 1028 | 1024 | (C–H) |
16 | A′′ | — | 1010w | 1120 | 1013 | 1030 | 1016 | 988 | 1013 | (C–H) |
17 | A′′ | 970w | — | 1093 | 989 | 986 | 973 | 955 | 969 | (C–H) |
18 | A′′ | 940vs | — | 1088 | 984 | 967 | 940 | 864 | 936 | (C–H) |
19 | A′ | 860 | — | 946 | 856 | 849 | 859 | 761 | 863 | (CCC) |
20 | A′ | 760vs | — | 852 | 771 | 767 | 756 | 730 | 759 | (C–Cl) |
21 | A′ | 720m | — | 790 | 714 | 731 | 721 | 702 | 719 | (CCC) |
22 | A′ | — | 650w | 709 | 642 | 659 | 650 | 659 | 648 | (CCC) |
23 | A′ | 560w | — | 608 | 550 | 564 | 556 | 512 | 559 | (C–Br) |
24 | A | 460m | 460m | 520 | 470 | 477 | 463 | 447 | 458 | (CCC) |
25 | A | — | 440w | 486 | 440 | 449 | 436 | 443 | 443 | (CCC) |
26 | A | — | 380w | 426 | 385 | 395 | 384 | 387 | 387 | (CCC) |
27 | A′ | — | 280w | 308 | 278 | 286 | 278 | 283 | 283 | (C–Cl) |
28 | A′ | — | 240w | 263 | 238 | 242 | 242 | 222 | 240 | (C–Br) |
29 | A′′ | — | 170w | 179 | 162 | 163 | 170 | 163 | 169 | (C–Cl) |
30 | A′′ | — | 140w | 147 | 133 | 134 | 139 | 123 | 139 | (C–Br) |
vs: very strong; s: strong; m: medium; w: weak; as- asymmetric; s: symmetric;
Comparative values of IR intensity and Raman activity between HF/6-31+G (d, p), B3LYP/6-31+G (d, p), and B3LYP/6-311++G (d, p) of 1-bromo-2-chlorobenzene.
S. no. | Observed frequency (cm−1) | Calculated with HF/6-31+G (d, p) | Calculated with B3LYP/6-31+G (d, p) | Calculated with B3LYP/6-311++G (d, p) | |||
IR | Raman | IR | Raman | IR | Raman | ||
1 | 3070 | 5.51 | 234.74 | 4.11 | 280.35 | 3.34 | 241.41 |
2 | 3060 | 3.84 | 23.15 | 3.04 | 29.11 | 2.55 | 62.85 |
3 | 3055 | 8.89 | 104.63 | 7.76 | 121.31 | 6.52 | 114.39 |
4 | 3030 | 1.71 | 49.57 | 1.56 | 53.97 | 1.32 | 50.37 |
5 | 1600 | 9.54 | 28.55 | 11.09 | 26.93 | 10.37 | 23.82 |
6 | 1570 | 4.72 | 10.62 | 4.55 | 11.42 | 4.385 | 11.71 |
7 | 1460 | 59.39 | 2.89 | 53.84 | 1.95 | 55.74 | 1.80 |
8 | 1440 | 23.59 | 0.07 | 19.83 | 0.08 | 20.07 | 0.06 |
9 | 1260 | 5.37 | 0.20 | 0.56 | 9.05 | 0.56 | 9.31 |
10 | 1170 | 0.73 | 1.96 | 4.53 | 0.15 | 4.37 | 0.09 |
11 | 1130 | 17.33 | 6.72 | 0.04 | 0.54 | 0.07 | 5.27 |
12 | 1110 | 12.72 | 5.38 | 3.81 | 2.10 | 3.90 | 2.33 |
13 | 1105 | 3.30 | 16.07 | 33.39 | 18.7 | 33.63 | 16.32 |
14 | 1040 | 0.02 | 1.08 | 0.03 | 1.32 | 13.09 | 42.11 |
15 | 1030 | 12.96 | 36.80 | 12.10 | 42.04 | 41.75 | 2.78 |
16 | 1010 | 36.93 | 7.61 | 40.70 | 2.29 | 0.03 | 0.09 |
17 | 970 | 1.78 | 0.18 | 0.01 | 0.32 | 1.34 | 0.07 |
18 | 940 | 0.32 | 0.70 | 1.36 | 0.27 | 0.00 | 0.04 |
19 | 860 | 0.03 | 0.47 | 0.02 | 0.15 | 68.52 | 0.00 |
20 | 760 | 84.39 | 0.62 | 66.42 | 0.34 | 21.66 | 1.43 |
21 | 720 | 17.54 | 0.79 | 18.95 | 1.43 | 0.33 | 0.49 |
22 | 650 | 13.15 | 9.17 | 12.70 | 7.44 | 12.97 | 7.01 |
23 | 560 | 0.00 | 0.03 | 0.02 | 0.04 | 0.04 | 0.28 |
24 | 460 | 7.97 | 0.02 | 7.32 | 0.04 | 6.08 | 13.53 |
25 | 440 | 5.38 | 15.45 | 6.01 | 13.69 | 4.78 | 0.07 |
26 | 380 | 0.64 | 4.54 | 0.66 | 3.69 | 0.07 | 4.05 |
27 | 280 | 0.48 | 3.24 | 0.66 | 2.50 | 0.74 | 2.65 |
28 | 240 | 1.45 | 0.80 | 1.41 | 0.95 | 1.50 | 0.62 |
29 | 170 | 0.15 | 1.08 | 0.19 | 1.24 | 0.15 | 1.44 |
30 | 140 | 0.04 | 2.19 | 0.04 | 2.59 | 0.08 | 2.24 |
Although basis set are marginally sensitive as observed in the HF and DFT values using 6-31+G+ (d, p) and 6-311++G (d, p), reduction in the computed harmonic vibrational frequencies are noted. Without affecting the basic level of calculations, it is customary to scale down the calculated harmonic frequencies in order to get an agreement with the experimental values. The scaled calculated frequencies minimize the root mean square difference between calculated and experimental frequencies for bands with definite identifications.
Computed vibrational spectral IR intensities and Raman activities of the 1-Br-2-CB for corresponding wavenumbers by HF and DFT methods with B3LYP at 6-311G ++(d, p) basis sets have been collected in Table
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), and B3LYP/6-311++G (d, p) basis sets for the 1-Br-2-CB are given in Figure
Standard deviation of frequencies by HF/DFT (B3LYP/) at 6-31+G (d, p) and 6-311++G (d, p) basis sets.
S. no. | Basic set | Total | Average | Standard | Deviation |
Experimental | 34710 | 1157 | |||
1 | HF/6-311+ | 38483 | 1282 | 88.93 | |
2 | B3LYP/6-31+ | 35961 | 1199 | 30.61 | 2.90 |
3 | B3LYP/6-311++ | 35288 | 1176 | 30.82 | 2.88 |
Comparative graph of experimental and calculated frequencies HF and DFT (B3LYP).
The aromatic organic compounds structure shows the presence of asymmetric C–H stretching vibrations in the region 3100–3000 cm−1 [
Generally the C=C stretching vibrations in aromatic compounds are seen in the region of 1430–1650 cm−1. According to Socrates [
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 [
The C
Complete vibrational analysis has been made in the present work for proper frequency assignments for 1-bromo-2-chlorobenzene. The equilibrium geometries have been determined and compared with experimental data. Anharmonic frequencies are determined and analyzed by DFT level of theory utilizing 6-31G+ (d, p) and 6-311++G (d, p) basis sets. Good agreement between the calculated and experimental spectra was obtained. The HF/DFT spectra showed better agreement with experimental spectra. However, the difference between the observed and scaled wavenumber values of C