MRD-CI study on the isomers SiOH and HSiO

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Abstract
Large scale MRD-CI calculations have been carried out for various low-lying electronic states of SiOH and HSiO. The non-linear ground state equilibrium geometries were optimized at the CI level. The more stable isomer SiOH lies 61·9 ± 4·6 kJ mol-1 below HSiO (without zero point corrections). SiOH is a π-type radical, while HSiO shows σ-character, in agreement with recent E.S.R. measurements. The electronic spectra of the two isomers are quite different. The 1 2 A″ state of SiOH (T vert. = 0·37 eV) is found to be bent, in variance with Walsh's rules, while 2 2 A′ (T vert. = 2·87 eV) is a semidiffuse state. The Rydberg series starts at 4·0 eV. The low-lying X 2 A′ and 1 2 A″ states of HSiO can be derived from two different configurations depending on geometry. The transition X 2 A′ → 1 2 A″ (T vert. = 2·59 eV) is expected to have a complex spectrum since the excited state can relax either to a linear geometry or to a strongly bent structure. The semidiffuse 42 A′ state is placed at 5·00 eV vertically. The low-lying Rydberg states of HSiO are perturbed by various valence states.