Mechanistic Investigation of Thermal and Photoreactions between Boron and Silane

Density functional theory and high-level ab initio calculations were performed to elucidate the detailed reaction mechanism from B and SiH₄ to a structure with two bridging H atoms (Si(μ-H₂)BH₂, silicon tetrahydroborate). On the basis of the calculated results, this reaction mechanism includes both thermal and photochemical reactions. Especially, thermal conversion of silylene dihydroborate (H₂B═SiH₂) to Si(μ-H₂)BH₂ is not feasible because two high energetic barriers must be overcome. In contrast, the reverse reaction is feasible because it is effectively only necessary to overcome a single barrier. The characteristics of the excited states of H₂B═SiH₂ and Si(μ-H₂)BH₂ have been identified. Two successive conical intersections (CIs) are involved in the photochemical reaction. The BSiH₄ bending coordinate is almost parallel to the reaction coordinate near the regions from the second CI to Si(μ-H₂)BH₂. The activated BSiH₄ bending mode lift the degeneracy of the second CI, thereby the reaction readily proceeds to Si(μ-H₂)BH₂. All calculated results in this work reasonably well describe the recent experimental observations.