Experimental and theoretical study of 3-methyl-4- nitrobenzoic acid using DFT and IVP methods

The Fourier Transform Infrared (FTIR) and FT-Raman spectra of 3-methyl-4- nitrobenzoic acid have been recorded in the range 4000-400 cm^-1 and 3500-50 cm^-1, respectively. The optimized geometry of the molecule, its vibrational frequencies have been computed using the Density Functional Theory (DFT) employing B3LYP/6-311++G basis set. The scaled values of harmonic vibrational frequencies so obtained have been compared with their experimental counter parts. The scaling factors have been refined to reproduce the frequencies with an RMS error of 9.26 cm^-1 between the experimental and computed frequencies. The theoretically predicted FTIR and FT-Raman spectra agree satisfactorily with those of experimental spectra. A 89-parameter modified valence force field was evaluated by solving inverse vibrational problem (IVP) using Wilson's GF matrix method. The force constants were refined using 44 experimental frequencies of this molecule in overlay least- squares technique. The average error between observed and computed frequencies was found 11.61cm^-1. PED and eigen vectors computed in the process were used to make unambiguous vibrational assignments of all the fundamental vibrations of this molecule. The values of dipole moment and hyperpolarizability were determined to study the NLO behaviour of this molecule. The HOMO and LUMO energies were also evaluated for this molecule.