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+ H2is both endothermic
and has high activation energies to reaction. Instead, the formation
of molecular complexes is efficient. The reaction of Si2O2with 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 Si2O2could be accelerated instead.