Mechanisms and Thermochemistry of Reactions of SiO and SiO with OH and HO

This paper reports on computational studies of gas-phase reactions of SiO and SiO. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and HO molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, SiO, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the SiO + OH reaction should be faster than SiO + OH. The reaction SiO + HO → SiO + H is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of SiO with HO, which has been suggested as efficient for producing SiO, might not be as efficient as previously thought. If the HO molecules dissociate to form OH radicals, oxidation of SiO and SiO could be accelerated instead.