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

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.


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][7][8].

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 .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.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

Results and Discussion
Figure 6: Bond length differences between theoretical approach (HF and DFT).
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 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.    2.
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 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 C S 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.

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][22][23][24][25][26][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.

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.

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.

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][35][36][37], the presence of the bromine substituent induced a shift in the C-Cl vibrational frequencies to higher values.

Conclusion
Complete vibrational analysis has been made in the present work for proper frequency assignments for 1-bromo-2chlorobenzene.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.

4. 1 .
Molecular Geometry.The molecular structure of the 1-Br-2-CB belongs to C S 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 −211074.9joules/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.

Figure 9 :
Figure 9: Comparative graph of experimental and calculated frequencies HF and DFT (B3LYP).
S. no.Symmetry species CS Observed Calculated frequency (cm −1