NBO analysis and vibrational spectra of 2,6-bis(p-methyl benzylidene cyclohexanone) using density functional theory

Vibrational analysis of the 2,6-bis(p-methyl benzylidene cyclohexanone) [PMBC] compound was carried out by using NIR FT-Raman and FT-IR spectroscopic techniques. The equilibrium geometry, various bonding features and harmonic vibrational frequencies of PMBC have been investigated with the help of B3LYP/6-31 G(d) density functional theory method. The optimized geometry clearly demonstrates cyclohexanone ring chair conformation is changed into half-chair conformation. The shortening of C-H bond length and blue shifting of the CH stretching wavenumber suggest the existence of improper weak C-H***O hydrogen bonding, which is confirmed by the natural bond orbital analysis. The Mulliken population analysis on atomic charges and the HOMO-LUMO energy are also calculated.