Vibrational spectral analysis of 2,6 Bis (p-methoxy benzylidene) cyclohexanone using density functional theory

Abstract

The geometry optimization and the computation of vibrational wavenumbers have been carried out for 2,6 Bis (p-methoxy benzylidene) cyclohexanone using density functional theory at B3LYP/6-31G(d) level. The optimized geometry reveals that the central cyclohexanone ring possesses a nearly 'half chair' conformation. The steric interaction causes the twisting of phenyl rings and the twisting of both Ph1 and Ph2 are energetically favored by 6.66 kJ mol$^{-1}$ and 6.86 kJ mol$^{-1}$ respectively. The relaxed PES scan studies have been conducted to expose the changes in the geometrical parameters caused by ring twisting. The stretching vibrational modes involving aromatic hydrogen atoms participating in steric interaction have been observed to possess higher vibrational wavenumbers. The unusual lowering of C=O stretching vibrational wavenumber has been noticed, attributable to the mesomeric interaction of the group -CH=C-(C=O)-C=CH-. The methylene groups involving steric interaction have been found to possess different spectral behavior compared to free CH$_{2}$ groups. The computed geometry and vibrational spectrum of cyclohexanone at B3LYP/6-31G(d) level have been used to analyze, the spectral distinction between chaired cyclohexanone and half chaired central cyclohexanone ring in BMBC and also the bond angle effect and conjugation effects on carbonyl stretching modes.