Dissociation dynamics of low-lying electronic states of SiH2

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.