Vibronic origin of the H3O metastability

The vibronic origin of the metastability of the hydronium radical H3O in its planar D3h and pyramidal C3v nuclear configurations is revealed by solving the three-mode (A1′+A2″+E′)⊗(a2″+e′+e′) Jahn-Teller problem. The vibronic constants in this model were estimated by fitting of the ab initio calculated adiabatic potential in the vicinity of the high-symmetry D3h and C3v nuclear configurations to its analytical expression. It was shown that the planar D3h configuration is unstable with the respect to the out-of-plane distortions (leading to the C3v nuclear configuration) because of the pseudo Jahn-Teller effect (PJTE). The barriers to dissociation H3O → H2O + H from both metastable nuclear configurations (D3h and C3v) are due to the common action of the PJTE in the ground state and the Jahn-Teller effect (JTE) in the first degenerate excited state.