Vibrational spectral investigation, NBO, first hyperpolarizability and UV–Vis spectral analysis of 3,5-dichlorobenzonitrile and m-bromobenzonitrile by ab initio and density functional theory methods

•Optimized geometries of 3,5-dichlorobenzonitrile and m-bromobenzonitrile have been carried out.•The vibrational frequencies were calculated by HF and DFT methods.•The complete assignments are performed on the basis of the TED.•First hyperpolarizability, NBO, HOMO–LUMO, UV–Vis spectra and thermodynamic properties were performed.

The FT-IR and FT-Raman spectra of 3,5-dichlorobenzonitrile and m-bromobenzonitrile have been recorded in the region 4000–400 cm−1 and 3500–50 cm−1, respectively. The optimized geometry, wave numbers and intensity of vibrational bonds of title molecules are obtained by ab initio and DFT level of theory with complete relaxation in the potential energy surface using 6-311++G(d, p) basis set. A complete vibrational assignments aided by the theoretical harmonic frequency, analysis have been proposed. The harmonic vibrational frequencies calculated have been compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. The UV–Vis spectral analysis of the molecules has also been done which confirms the charge transfer of the molecules. Furthermore, the first hyperpolarizability and total dipole moment of the molecules have been calculated.

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