Résumé
The first two doublet and quartet states of H3 O have been surveyed by multiconfiguration self-consistent field/first-order configuration interaction (MCSCF/FOCI) calculations in C3v symmetry. Geometries of the minima on the doublet surfaces have been optimized by MCSCF gradient techniques and energies obtained by large-scale multireference single and double excitation CI calculations. The correlation diagram linking the minima to different dissociation limits has been established. A local minimum in C3v symmetry of ion-pair character is shown to be unstable with respect to dissociation to H2 +H+O. The lowest state, 1 2 A1 in C3v , (1 2 A′ in Cs ) may be characterized as an H3 O+ core surrounded by an oxygen 3s Rydberg electron. Transition states for dissociation of the 1 2 A′ state to H2 O+H and to OH+H2 have been investigated. An extremely low barrier height, 3.58 kcal/mol without vibration, 0.4 kcal/mol for H3 O, an estimated 1.3 kcal/mol for D3 O, with zero-point energy, is found for dissociation of the 1 2 A′ state to H2 O+H. Within the uncertainty of the calculation it is not possible to predict whether the lowest state of H3 O should be observable experimentally. The relationship of these results to experimental observations for H3O is discussed.