Investigation of the structural and harmonic vibrational properties of 2-nitro-, 4-nitro- and 5-nitro-m-xylene by ab initio and density functional theory.

The Fourier transform infrared (FTIR) and FT-Raman spectra of 2-nitro-m-xylene (2NMX), 4-nitro-m-xylene (4NMX) and 5-nitro-m-xylene (5NMX) have been recorded in the range 4000-400 and 4000-100 cm(-1), respectively. The experimental vibrational frequency was compared with that obtained theoretically by ab initio HF and DFT-B3LYP gradient calculations employing the standard 6-31G(d,p) basis set for the optimised geometries of the compounds. The complete vibrational assignment, analysis and correlation of the fundamental modes of the compounds were carried out using the experimental FTIR and FT-Raman data, and ab initio and DFT quantum chemical studies. The geometrical parameters and the wavenumbers of normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental values. The potential energy distribution of the fundamental modes was calculated with ab initio force fields utilising Wilson's FG matrix method. The influence of bulky methyl groups on the nitro group fundamental modes and on the ring skeletal vibrations are investigated.