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

Structure and vibrational frequencies of the benzene-water complex cation [BzH(2)O](+) have been calculated by means of density functional theory (B3LYP calculation). A planar structure with a C-s symmetry, in which all heavy atoms are located on a molecular plane, was obtained as the most stable form of the [BzH(2)O](+) 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 (nu (1) and nu (3) modes, respectively) are red-shifted from those of a free H2O by the formation of a complex, whereas the H-O-H bending mode (nu (2) 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.