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 H5O ion. The 15-dimensional potential energy surface for the seven nuclei in the ionic complex was computed by pointwise ab initio Møller-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 H5O and H9O are presented. The energy-minimum structure of H5O using the augmented pVTZ basis set is of C2 symmetry, whereas for H9O, using the TZ2P basis set, it is of C3 symmetry. The H-bonded OH stretching harmonic frequency of H5O is very low, 913 cm−1, whereas for H9O it is 2927 cm−1. The subspace spanned by the hydrogen-bonded OH distance and the O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 H5O are discussed.