Theoretical study of low-lying states of H3O

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Abstract
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.

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