Rate Coefficient for the Reaction SiO + Si2O2 at T = 10−1000 K

Abstract

The reaction paths for the formation of Si3O3 molecules have been investigated at high level ab initio quantum chemical calculations by using the QCISD method with the 6-311++G(d,p) basis set. The cis-Si2O2 isomer does not participate in the chemical mechanism for the formation of Si3O3 molecules. Although the SiO + cis-Si2O2 reaction is exothermic and spontaneous, it is not expected to explain the growth mechanism of Si3O3 in the interstellar silicate grains of circumstellar envelopes surrounding M-type giants. The reaction of SiO with cyclic Si2O2 molecules is exothermic, is spontaneous, and has a nonplanar transition state. The Gibbs free energy for the transition state formation, (DeltaG0#), is around 5.5 kcal mol-1 at 298 K. The bimolecular rate coefficient for this reaction, kT, is about 1 x 10-12 cm3 molecule-1 s-1 at 298 K and in the collision limit, 1.5 x 10-10 cm3 molecule-1 s-1, at 500 K. The activation energy, Ea, is about 8 kcal mol-1. The enthalpy of Si3O3 fragmentation is 53.9 kcal mol-1 at 298 K. The SiO + cyclic Si2O2 reaction is expected to be the most prominent reaction path for the Si3O3 formation in interstellar environment and fabrication of silicon nanowires.