Quantum chemical studies and vibrational analysis of 4-acetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile--a comparative study.

The FTIR and FT-Raman vibrational spectra of 4-actetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile molecules have been recorded in the range 4000-400 and 4000-100 cm(-1), respectively. The complete vibrational assignment and analysis of the fundamental modes of the most stable geometry of the compounds were carried out using the experimental FTIR and FT-Raman data on the basis of peak positions, relative intensities and quantum chemical studies. The observed vibrational frequencies were compared with the theoretical wavenumbers of the optimised geometry of the compounds obtained from the DFT-B3LYP gradient calculations employing the standard 6-31G**, high level 6-311++G** and cc-pVDZ basis sets. The structural parameters and vibrational wavenumbers obtained from the DFT methods are in good agreement with the experimental data. With hope of providing more and effective information on the fundamental vibrations, total energy distributions of the fundamental modes have been performed by assuming C(s) point group symmetry. The effect of substituents -COCH(3), -CHO and -OH in the benzonitrile moiety have been analysed and compared. The kinetic and thermodynamic stability and chemical hardness of the molecule have been determined.