Quantum anharmonic frequencies of the O⋯ H⋯ O fragment of the H5O2+ ion: a model three-dimensional study

Anharmonic frequencies of the O⋯ H⋯ O fragment of H5O2+ were evaluated using the adiabatic separation of vibrational variables. The relaxed 3D potential energy surface was calculated by the MP2 method using the T(O)DZP basis set. The symmetric and asymmetric stretching frequencies obtained, 588 and 1226 cm−1, agree with previous 2D results [L. Ojamäe et al., Int. J. Quantum Chem. Symp. 29 (1995) 657] and show a modest shift from the harmonic values only. The O⋯ H⋯ O bending frequency is ca. 1025 cm−1, significantly lower than the harmonic result of 1524 cm−1. Comparison is made with results of MD simulations using ab initio forces.