Ab initio studies of molecular structures, conformers and vibrational spectra of heterocyclic organics: I. Nicotinamide and its N-oxide.

FTIR spectra of nicotinamide and its N-oxide have been recorded and analyzed in the range 400-4000 cm(-1). The stabilities, optimized molecular geometries, APT charges and vibrational characteristics for the two possible conformers of nicotinamide and its N-oxide have been studied theoretically using restricted Hartree-Fock (RHF) and density functional theory (DFT) methods. The E (trans) conformers of nicotinamide and its N-oxide are found to be more stable and less polar than their respective Z (cis) conformers. Due to addition of an O atom at the N(1) site in the NA molecule the magnitudes of atomic charges on all the H atomic sites are found to increase. For all the studied molecules, magnitude of the wagging mode of the NH(2) group is found to be higher than its torsion mode, which is in the reverse order as compared to that for the aniline molecule. Most of the vibrational frequencies have nearly the same magnitude for the two conformers of nicotinamide and its N-oxide, however, significant changes are noticed in their IR intensities, Raman activities and depolarization ratios of the Raman bands. The frequency of the ring breathing mode for the NA molecule is found to decrease by 100 cm(-1) in going to the NANO molecule for both the conformers. The IR intensity for the scissoring mode of the CON(H(2)) group is found to decrease significantly for the NA-II conformer as compared to that for the NA-I conformer.