Anharmonic vibrational spectroscopy of nitriles and their complexes with water

Three nitrile molecules, cyanamide (H(2)N-Cequivalent toN), acetonitrile (CH(3)-Cequivalent toN), and aminoacetonitrile (H(2)N-CH(2)-Cequivalent toN), and their complexes with one water molecule are studied at the ab initio level of theory (second-order Moller-Plesset perturbation theory with triple-xi basis sets). Anharmonic vibrational spectra are estimated with the Correlation Corrected Vibrational Self-Consistent Field (CC-VSCF) method that accounts for anharmonicities and couplings between different vibrational normal modes. Effects of complexation with water on geometries and vibrational spectra of nitriles are investigated. The effects are very different from those found earlier for acids (in particular, the amino acid glycine). While the intermolecular hydrogen bonding interactions lead to significant red shifts of carboxyl group (O-H and C=O) vibrational stretching frequencies of acids, Cequivalent toN stretches of nitriles shift to a much lesser extent and in the opposite direction (to the blue) upon complexation with water.