POTENTIAL-ENERGY SURFACES AND VIBRATIONAL-SPECTRA OF H5O2+ AND LARGER HYDRATED PROTON COMPLEXES

This article presents calculations of the structure, binding energetics, potential energy surfaces, and vibrational spectra of the H5O2+ ion. The 15-dimensional potential energy surface for the seven nuclei in the ionic complex was computed by pointwise ab initio Moller-Plesset second-order perturbation (MP2) calculations, using the correlation-consistent pVTZ basis set augmented with diffuse basis functions on oxygen. The potential energy surface for the proton-transfer mechanism was investigated, and the effects of surrounding water molecules on the proton-transfer potential energy curve was studied. Density functional calculations for the proton-transfer potential surface are compared to the MP2 results. Geometry-optimized structures, binding energies, and harmonic vibrational spectra of H5O2+ and H9O4+ are presented. The energy-minimum structure of H5O2+ using the augmented pVTZ basis set is of C-2 symmetry, whereas for H9O4+, using the TZ2P basis set, it is of C-3 symmetry. The H-bonded OH stretching harmonic frequency of H5O2+ is very low, 913 cm(-1), whereas for H9O4+ it is 2927 cm(-1) The subspace spanned by the hydrogen-bonded OH distance and the Q-O distance were used in one- and two-dimensional calculations of the anharmonic vibrational spectrum using collocation methods. The coupling of the OH stretch with the O-O vibration causes a redshift and the anharmonicity a blueshift of the OH frequency: the resulting fundamental frequency of the H-bonded OH vibration is 1275 cm(-1). Zero-point energies of the proton vibration and pathways for exchange of protons within H5O2+ are discussed. (C) 1995 John Wiley & Sons, Inc.