Journal of Atomic, Molecular, and Optical PhysicsVolume 2011 (2011), Article ID 512841, 10 pagesdoi:10.1155/2011/512841

Research Article

Molecular Structure and Vibrational Analysis of 1-Bromo-2-Chlorobenzene Using ab initio HF and Density Functional Theory (B3LYP) Calculations

G. Shakila,¹ S. Periandy,² and S. Ramalingam³

¹Department of Physics, Bharathidasan Government College for Women, Puducherry 605003, India²Department of Physics, Tagore Arts College, Puducherry 605008, India³Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu 609305, India

Received 21 February 2011; Accepted 16 June 2011

Academic Editor: Amlan K. Roy

Copyright © 2011 G. Shakila et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [1]. The major use of Chlorobenzene is as an intermediate in the production of commodities such as herbicides, dyestuffs, and rubber. Chlorobenzene is also used as a high-boiling solvent in many industrial applications as well as in the laboratory. Bromobenzene can be used to prepare the corresponding Grignard reagent, phenyl magnesium bromide [2]. The combined Chlorobromobenzene is also used for the manufacture of some biological and industrial solvents. In recent years, chlorobromobenzene has been the frequent subject of experimental and theoretical work because of its significance in industry and environment. Literature survey reveals that to the best of our knowledge no ab initio HF/DFT with 6-31+G (d, p) and 6-311++G (d, p) basis sets calculations of 1-Br-2-CB have been reported so far. It is, therefore thought worthwhile to make a comprehensive vibrational analysis using both experimentally observed IR and Raman wavenumbers and theoretically calculated vibrational spectra.

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) [3] combined with gradient-corrected correlational functional of Lee et al. (LYP) [4, 5] are the best predicting results for molecular geometry and vibrational wave numbers for moderately larger molecule [6–8].

2. Experimental Details

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 μm with 200 Mw power. Both the spectra were recorded in the range of 4000 to 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

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 2 and 3, respectively. The optimized structural parameters are used in the vibrational frequency calculations at HF and DFT levels. The minimum energy of geometrical structure is obtained by using same basis sets of HF and DFT methods those which are previously mentioned. All the computations have been done by adding polarization function d and diffuse function on heavy atoms and polarization function p and diffuse function on hydrogen atoms, in addition to triple split valence basis set (6-311G (d, p)), for better treatment of polar bonds. The calculated frequencies are scaled by 0.902 and 0.884 for HF/6-311+G (d, p) [9, 10]. For B3LYP/6-31G+ (d, p) is scaled by 0.923, 0.977, 0.872, and 0.945 and for B3LYP/6-311++G (d, p) basis set, calculated frequency is scaled with 0.927, 0.976, 0.987, 0.885, and 1.008 [11]. HF/DFT calculations for 1-Br-2-CB are performed using GAUSSIAN 03 W program package on Pentium IV processor personal computer without any constraint on the geometry [12, 13]. The comparative values of IR intensities and Raman activities are presented in Table 3 and their corresponding graphs are given in Figures 4 and 5, respectively.

4. Results and Discussion

4.1. Molecular Geometry

The molecular structure of the 1-Br-2-CB belongs to CS point group symmetry. The optimized molecular structure of title molecule is obtained from GAUSSAN 03 W, and GAUSSVIEW programs are shown in Figure 1. The molecule contains Cl and Br connected with benzene ring. The structure optimization zero point vibrational energy of the title compound in HF/6-31+G (d, p), B3LYP/6-31+G (d, p), and B3LYP/6-311++G (d, p) are −226013.3, −215997.0, and −211074.9 joules/Mol, respectively. The comparative optimized structural parameters such as bond lengths, bond angles, and dihedral angles are presented in Table 1. The comparative graphs of bond lengths, bond angles, and dihedral angles of 1-Br-2-CB for four sets are presented in Figures 6, 7, and 8, respectively.

Table 1: 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
—

Figure 1: Molecular structure of 1-bromo-2-chlorobenzene.

(a)

(b)

(c)

(d)

Figure 2: Experimental (a), calculated (b), (c) and (d) FT-IR spectra of 1-bromo-2-chlorobenzene.

(a)

(b)

(c)

(d)

Figure 3: Experimental (a), calculated (b), (c) and (d) FT-Raman spectra of toluic acid.

Figure 4: Comparative graph of IR Intensities by HF and DFT (B3LYP).

Figure 5: Comparative graph of Raman intensities by HF and DFT (B3LYP).

Figure 6: Bond length differences between theoretical approach (HF and DFT).

Figure 7: Bond angle differences between theoretical approach (HF and DFT).

Figure 8: 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  [14, 15]. The C-Cl bond distance cal. 1.748 Ǻ  by B3LYP/6-311++G (d, p) is just 0.003 Ǻ  greater than the reported experimental value of 1.745 Ǻ    [16]. According to the calculated values (B3LYP/6-311++G (d, p)), the order of the optimized bond angle are as C₁–C₂–C₃ < C₂–C₁–C₆ < C₃–C₄–C₅= C₄–C₅–C₆ < C₁–C₆–C₅ < C₂–C₃–C₄. The bond angle of C₂–C₁–Br₁₁ is 0.001° (B3LYP/6-311++G (d, p)) larger than the bond angle C₃–C₄–Cl₁₂ since the substitution of chlorine and fluorine atom.

4.2. Vibrational Assignments

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′ while the out-of-plane modes as A′′. Thus the 30 normal modes of vibrations are distributed as ′′′. All the 30 fundamental vibrations are active both in Raman scattering and in IR absorption. The harmonic-vibrational frequencies calculated for 1-Br-2-CB at HF and DFT-B3LYP levels using the triple split valence basis set along with the diffuse and polarization functions, 6-311G (d, p), observed FT-IR and FT-Raman frequencies for various modes of vibrations are presented in Table 2.

Table 2: 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) υ

2
A′
3060vs
—
3388
3066
3219
3059
3202
3070
(C–H) υ

3
A′
—
3055vs
3374
3053
3207
3048
3190
3058
(C–H) υ

4
A′
—
3030w
3358
3038
3194
3035
3177
3046
(C–H) υ

5
A′
1600w
—
1774
1605
1621
1599
1610
1603
(C=C) υ

6
A′
1570vs
1570s
1757
1590
1614
1569
1604
1579
(C=C) υ

7
A′
1460vs
—
1625
1470
1490
1448
1483
1460
(C=C) υ

8
A′
1440vs
—
1586
1435
1462
1442
1457
1434
(C–C) υ

9
A′
1260vs
—
1389
1257
1326
1260
1304
1250
(C–C) υ

10
A′
1170w
—
1314
1189
1281
1169
1276
1169
(C–C) υ

11
A′
1130vs
1130
1242
1124
1187
1128
1185
1136
(C–H) δ

12
A′
1110vs
—
1230
1113
1150
1118
1147
1099
(C–H) δ

13
A′
1105w
—
1198
1084
1133
1101
1126
1108
(C–H) δ

14
A′
1040w
—
1191
1077
1098
1043
1054
1037
(C–H) δ

15
A′′
1030w
—
1130
1022
1058
1028
1028
1024
(C–H) γ

16
A′′
—
1010w
1120
1013
1030
1016
988
1013
(C–H) γ

17
A′′
970w
—
1093
989
986
973
955
969
(C–H) γ

18
A′′
940vs
—
1088
984
967
940
864
936
(C–H) γ

19
A′
860
—
946
856
849
859
761
863
(CCC) δ

20
A′
760vs
—
852
771
767
756
730
759
(C–Cl) υ

21
A′
720m
—
790
714
731
721
702
719
(CCC) δ

22
A′
—
650w
709
642
659
650
659
648
(CCC) δ

23
A′
560w
—
608
550
564
556
512
559
(C–Br) υ

24
A″
460m
460m
520
470
477
463
447
458
(CCC) γ

25
A″
—
440w
486
440
449
436
443
443
(CCC) γ

26
A″
—
380w
426
385
395
384
387
387
(CCC) γ

27
A′
—
280w
308
278
286
278
283
283
(C–Cl) δ

28
A′
—
240w
263
238
242
242
222
240
(C–Br) δ

29
A′′
—
170w
179
162
163
170
163
169
(C–Cl) γ

30
A′′
—
140w
147
133
134
139
123
139
(C–Br) γ

vs: very strong; s: strong; m: medium; w: weak; as- asymmetric; s: symmetric; υ: stretching; δ: in-plane bending; γ: out-plane bending; τ: twisting.

Table 3: 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)

IRintensity (Ai)
Ramanactivity (I)
IRintensity (Ai)
Ramanactivity (I)
IRactivity (I)
Ramanintensity (Ai)

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.

4.2.1. Computed IR Intensity and Raman Activity Analysis

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 3. The title molecule is a nonpolar molecule with CS point group. Comparison of IR intensity and Raman activity calculated by HF and DFT with B3LYP at 6-311G++ (d, p) methods with experimental values shows the variation of IR intensities and Raman activities. In the case of IR intensity, the values of HF are found to be higher than B3LYP at 6-31+G (d, P) whereas in the case of Raman activity the effect is reversed. The similar effect was also observed in the earlier paper [17]. The comparative plots of IR intensities and Raman activities for four sets are presented in Figures 4 and 5, respectively.

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), and B3LYP/6-311++G (d, p) basis sets for the 1-Br-2-CB are given in Figure 9. From the figure, it is found that the calculated (unscaled) frequencies by B3LYP with 6-311++G (d, p) basis sets are closer to the experimental frequencies than HF method with 6-311G (d, p) basis set. This observation is in line with our earlier work [18]. The standard deviation (SD) calculation made between experimental and computed frequencies (HF/DFT) for the 1-Br-2-CB is presented in Table 4. According to the SD, the computed frequency deviation decreases in going from HF/6-31+G (d, p) to B3LYP/6-31+G (d, p) to B3LYP/6-311++G (d, p). The deviation ratio between HF/6-31+G (d, p) and B3LYP/6-31+G (d, p) is 2.90 and HF/6-311+G (d, p) and B3LYP/6-31+G (d, p) is 2.88. It is also observed that the calculated frequencies by B3LYP/6-311++G (d, p) basis sets are closer to the experimental frequencies than HF method.

Table 4: 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 levels
Total values
Average
Standard deviation
Deviationratio

Experimental
34710
1157

1
HF/6-311+ (d, p)
38483
1282
88.93

2
B3LYP/6-31+ (d, p)
35961
1199
30.61
2.90

3
B3LYP/6-311++ (d, p)
35288
1176
30.82
2.88

Figure 9: Comparative graph of experimental and calculated frequencies HF and DFT (B3LYP).

4.2.3. C–H Vibrations

The aromatic organic compounds structure shows the presence of asymmetric C–H stretching vibrations in the region 3100–3000 cm⁻¹ [19, 20] which is the characteristics region for recognition on C–H stretching vibrations. In the title molecule, four bands have been observed at 3070, 3060, 3055, and 3030 cm⁻¹ assigned to C–H stretching vibrations. Due to the presence of heavy halogen atoms in the benzene ring, the assigned bands are deviated down slightly from the expected range. The C–H in-plane and out-of-plane bending vibrations normally take place as a number of strong to weak intensity sharp bends in the region of 1300–1000 cm⁻¹ and 1000–750 cm⁻¹ [21–27], respectively. The bands for C–H in-plane bending vibrations are identified at 1130, 1110, 1105, and 1040 cm⁻¹, and the C–H out-of-plane bending vibrations are found at 1030, 1010, 970, and 940 cm⁻¹. Theoretically computed frequencies for C–H in-plane and C–H out-of-plane bending vibrations by B3LYP/6-31G+ (d, p) and 6-311++G (d, p) methods showed excellent agreement with recorded spectrum. Except for some vibrations, all the bending vibrations are in line with the literature. Some of the bending vibrational modes are slightly moved away from the expected region. This is mainly due to the substitutions of halogens with different masses.

4.2.4. C=C Vibrations

Generally the C=C stretching vibrations in aromatic compounds are seen in the region of 1430–1650 cm⁻¹. According to Socrates [28], the presence of conjugate substituent such as C=C causes a heavy doublet formation around the region 1625–1575 cm⁻¹. The six ring carbon atoms which undergo coupled vibrations which are known as skeletal vibrations give a maximum of four bands in the region 1660–1420 cm⁻¹. As predicted in the earlier references [29], the prominent peaks at 1600, 1570, and 1460 cm⁻¹ are due to strong C=C stretching and 1440, 1260, and 1170 cm⁻¹ are due to strong C–C skeletal vibrations for the title compound. The peaks at 1600, 1570, and 1460 cm⁻¹ and the peaks at 1440 and 1260 cm⁻¹ are due to quadrant and semicircle stretching of CC bonds, respectively [30]. These four peaks confirm that the compound is aromatic in nature [31]. The peaks are assigned at 860, 720, and 650 cm⁻¹ due to C–C–C in-plane bending vibrations and the peaks at 460, 440, and 380 cm⁻¹ are due to C–C–C out-of-plane bending vibrations. Except C–C and CCC vibrations, all the C=C stretching vibrations are coherent with the literature data [32]. The C–C and CCC vibrations are pulled considerably to the lower region and are purely due to the substitutions.

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 [14, 15, 33]. 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 [34]. Accordingly in the present case, the C–Br stretching vibration of the 1-Br-2-CB is observed at 560 cm⁻¹ in the FT-IR spectrum. The C–Br in-plane bending and out-of-plane bending vibrations are observed at 240 cm⁻¹ and 140 cm⁻¹, respectively. This view is also supported by the above literature.

4.2.6. C–Cl Vibrations

The C–Cl stretching band is normally expected around 750–580 cm⁻¹ [35]. A strong IR band at 760 cm⁻¹ is assigned to C–Cl stretching vibration. The C–Cl deformation vibrations are expected around 460–175 cm⁻¹ [34]. A weak Raman band at 280 cm⁻¹ is assigned to the C–Cl in-plane bending vibration; a weak Raman band at 170 cm⁻¹ is assigned to the C–Cl out-of-plane bending vibration. When comparison is made with the assignments given in the literature [34–37], the presence of the bromine substituent induced a shift in the C–Cl vibrational frequencies to higher values.

5. Conclusion

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–C fundamental is very large, because of the presence of the C–Cl and C–Br bonds. The detailed anharmonic frequencies assignment of 1-B-2-CB, presented in this work, has clarified several ambiguities in the previously reported investigation of the experimental spectra. Computed vibrational analysis showed the standard deviation of computational frequencies.

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