Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane.

High-accuracy calculations were performed to study the mechanisms of the reactions between the diatomic silicon monoxide (SiO) with NH3 and CH4 . These reactions are relevant to the SiO-related astrochemistry and atmospheric chemistry as well as the activation of the N-H and C-H bonds by the SiO triple bond. Energetic data used in the construction of potential energy surfaces describing the SiO + NH3 /CH4 reactions were obtained at the coupled-cluster theory with extrapolation to the complete basis set limit (CCSD(T)/CBS) using DFT/B3LYP/aug-cc-pVTZ optimized geometries. Standard heats of formation of a series of small Si-molecules were predicted. Insertion of SiO into the N-H bond is exothermic with a small energy barrier of ∼8 kcal/mol with respect to the SiO + NH3 reactants, whereas the C-H bond activation by SiO involves a higher energy barrier of 45 kcal/mol. Eight product channels are opened in the SiO + NH3 reaction including dehydrations giving HNSi/HSiN and dehydrogenations. These reactions are endothermic by 16-119 kcal/mol (calculated at 298.15 K) with the CCSD(T)/CBS energy barriers of 21-128 kcal/mol. The most stable set of products, HNSi + H2 O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH4 reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH3 OH are endothermic by 19-118 kcal/mol with the energy barriers in the range of 71-126 kcal/mol. The formation of H2 CSiO + H2 O has the lowest energy barrier of 71 kcal/mol, whereas the most stable set of products, SiH4 + CO, is formed via a higher energy barrier of 90 kcal/mol. Accordingly, while SiO can break the N-H bond of ammonia without the assistance of other molecules, it is not able to break the C-H bond of methane.