Vibrational frequency-shifts of H2O caused by complex formation with a molecular cation: a density functional study

Abstract

Structure and vibrational frequencies of the benzene–water complex cation [BzH2O]+ have been calculated by means of density functional theory (B3LYP calculation). A planar structure with a Cs symmetry, in which all heavy atoms are located on a molecular plane, was obtained as the most stable form of the [BzH2O]+ cation. Hydrogen atoms of the water molecule are located above and below the molecular plane. From the present calculations, it was predicted that vibrational frequencies of symmetric and asymmetric O–H stretching modes of H2O (ν1 and ν3 modes, respectively) are red-shifted from those of a free H2O by the formation of a complex, whereas the H–O–H bending mode (ν2 mode) is blue-shifted. Infrared intensities of the three modes of H2O were significantly increased by the complex formation. Similar features were obtained for the ethylene–H2O complex cation [C2H4–H2O]+. The origin of the frequency-shifts is discussed on the basis of theoretical results. The hydrogen-hyperfine coupling constants of the complex cations were also predicted theoretically.