MCSCF vibrational spectra of the symmetric and asymmetric dihydronium cations

Abstract

We report the first full geometry optimizations of the dihydronium cation performed at the MCSCF level. Large basis set effects are encountered. Depending on the basis set, we found, either the C2 symmetric or the Cs asymmetric species as global minima of the purely electronic PES. However, after harmonic ZPE correction, the C2 symmetric structure appears to be favoured. Supported by a display of MCSCF natural orbitals, we propose an analysis of the H5O2+ electronic structure in terms of a chemical-type binding. A “soft mattress” model of proton transfer, linked to this “chemical” picture, is set forward. MCSCF harmonic OH stretch frequencies, corrected for anharmonicity are compared with experimental data, and support the evidence of a C2 structure. The nature of proton motions is examined through a pictorial analysis of intermolecular vibrational motions. A tentative explanation is proposed for the very wide spectra starting near 3500 cm−1 down to 800 cm−1 in concentrated acidic solutions. Prospects related to on-going large scale MCSCF computations are sketched.