Mechanisms and Thermochemistry of Reactions of SiO and Si2O2 with OH and H2O

This paper reports on computational studies of gas-phase reactions of SiO and Si2O2. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H2O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si2O2, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si2O2 + OH reaction should be faster than SiO + OH. The reaction SiO + H2O → SiO2 + H2 is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si2O2 with H2O, which has been suggested as efficient for producing Si2O3, might not be as efficient as previously thought. If the H2O molecules dissociate to form OH radicals, oxidation of SiO and Si2O2 could be accelerated instead.