Mechanisms and Thermochemistry of Reactions of SiO and Si 2 O 2 with OH and H 2 O.

This paper reports on computational studies of gas-phase reactions of SiO and Si 2 O 2 . The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H 2 O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si 2 O 2 , affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si 2 O 2 + OH reaction should be faster than SiO + OH. The reaction SiO + H 2 O → SiO 2 + H 2 is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si 2 O 2 with H 2 O, which has been suggested as efficient for producing Si 2 O 3 , might not be as efficient as previously thought. If the H 2 O molecules dissociate to form OH radicals, oxidation of SiO and Si 2 O 2 could be accelerated instead.