DFT and ab initio quantum chemical studies on p-cyanobenzoic acid

The Fourier transform infrared (FTIR) and FT-Raman spectra of p-cyanobenzoic acid (CBA) 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 compound were carried out using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with theoretical wavenumbers obtained from ab initio HF and DFT-B3LYP gradient calculations employing 6-31G**, 6-311++G** and cc-pVTZ basis sets for the optimised geometry of the compound. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method. The interactions of cyano and carboxylic acid groups with the skeletal vibrational modes were investigated.

Research highlights
► Complete vibrational assignment and analysis of the fundamental modes of p-cyanobenzoic acid were performed.
► The observed vibrational frequencies were compared with theoretical wavenumbers obtained from HF and B3LYP methods.
► The interactions of cyano and carboxylic acid groups with the skeletal vibrational modes were investigated.
► The mixing of fundamental modes were analysed by normal coordinate analysis.