NIR-FT Raman and FT-IR spectral investigations of the nonlinear optical chromophore p-bromoacetanilide

Vibrational spectral analysis of the hydrogen-bonded nonlinear optical (NLO) material p-bromo acetanilide (PBA) was carried out using NIR-FT-Raman and FT-IR spectroscopy. Ab initio molecular orbital computations were performed at HF/6-31G (d) level to derive equilibrium geometry, vibrational wavenumbers, intensities and first hyperpolarizability. The lowering of the imino stretching wavenumbers suggests the existence of strong intermolecular N-H center dot center dot center dot O hydrogen bonding, which was substantiated by the natural bond orbital (NBO) analysis. The vibrational spectra confirm that the charge-transfer interaction between the -NHCOCH3 group and - Br through phenyl ring is responsible for simultaneous strong IR and Raman activation of the ring mode 8a. Vibrational analysis indicates that the lowering of stretching wavenumbers of methyl group due to electronic effects simultaneously caused by induction and hyperconjugation is due to the presence of the oxygen atom. The presence of blue-shifting H-bonds of CH stretching wavenumbers, simultaneous activation of carbonyl stretching mode, the strong activity of low-wavenumber H-bond stretching vibrations and the role of intramolecular charge transfer in making the molecule NLO active have been analyzed on the basis of the vibrational spectral features. Copyright (C) 2007 John Wiley & Sons, Ltd.