Dissociation dynamics of low-lying electronic states of SiH2

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Abstract
Essential features of the potential surfaces for low‐lying electronic states of silylene, SiH2, have been characterized. Calculated transition energies between the X̃ 1A1, ã 3B1, and à 1B1 states are in agreement with previously published experimental and theoretical values. The reactions Si(1D)+H2 and SiH(2Π)+H2 leading to SiH2(X̃ 1A1) appear to be barrierless processes. On the ã 3B1 and à 1B1 surfaces, asymmetric transition states are found for SiH2 dissociation. The activation energy for dissociation, SiH2(ã 3B1)→Si(3P)+H2(1Σ+g), is calculated to be 44.7 kcal/mol, and the dissociation energy for SiH2(X̃ 1A1)→Si(1D)+H2(1Σ+g) is calculated to be 65.8 kcal/mol. Structures and vibrational frequencies are presented for the low‐lying electronic states of SiH2 and for the associated transition states. Lifetimes of individual rovibronic levels of SiH2(à 1B1) are found to decrease dramatically in v’2 =8 as compared with v2 =7. This lifetime shortening is attributed to the opening of the dissociation channel to Si(1D)+H2(1Σ+g) which establishes the relative energies of SiH2 and Si+H2. From these relative energies ΔH○f(298) and SiH2 is determined to be 63.6±2.8 kcal/mol.