Structural conformation and vibrational spectroscopic studies of 2,6-bis(p-N,N-dimethyl benzylidene) cyclohexanone using density functional theory

NIR-FT Raman and FT-IR spectra of the crystallized 2,6-bis (p-N,N-dimethyl benzylidene)cyclohexanone (C24H28N2O) have been recorded in the region 3200-500 and 4000-400 cm(-1), respectively. The spectral interpretation has been done following full structure optimization and vibrational wavenumber calculations based on the density functional theory (DFT) using the standard B3LYP/6-31G* basis set. The predicted vibrational spectra are in excellent agreement with the experiment, permitting an unambiguous assignment for the unusual downshifting of nu C=O caused by expanded conjugation effects in the dienone system. The optimized geometry clearly demonstrates the 'half-chair' conformation of the central nonheterocyclic ring. The ring C-H stretching vibrational modes involving aromatic hydrogen atoms participating in steric interaction have been observed with low intensities. There are valid structural and spectral bases for the enhancement of its bioactivity due to full charge transfer reaction by the substitution of the electron-donating 4-dimethylamino group into the aryl rings. The existence of intramolecular, C-H center dot center dot center dot O, improper, blue-shifted hydrogen bond was investigated by means of the natural bonding orbitals (NBO) analysis. There are also slight dissimilarities in the bond lengths and endocyclic bond angles of both phenyl rings due to the effect of the heavy substitution. The characteristic ring modes have also been assigned in detail. Copyright (C) 2006 John Wiley & Sons, Ltd.