Vibrational spectroscopic properties of hydrogen bonded acetonitrile studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C=N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstrom -Delta E range = 13-48 kJ/mol) hydrogen-bonded 1: 1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-3I++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C=-N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman v(C=-N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C=N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated v(C=N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.