An extensive ab initio study of the Si + C2H2 and Si + C2H4 reactions in relation to the silicon astrochemistry

For astrophysical purposes, CCSD(T)/6-311++G(3df,2pd)//B3LYP/6-311+G(2d,p) calculations have been performed for the extensive investigation of the Si + C2H2 and Si + C2H4 processes. Both lowest singlet and triplet surfaces have been considered. This study shows that, if Si(3P) reacts with acetylene or ethylene in the cold and low density environment of the circumstellar envelopes, these reactions should be slow, since the mechanism involved is an association one. The barrier-less formation of SiCHCH(3A″) or SiC2H4(3A2) should be inhibited by the necessity for the formed complexes to release their energy excess through a radiative emission process. Within the hypotheses of efficient intersystem crossing with the singlet surface, the same conclusions can be drawn for the formation of SiC2H2(1A1), HSiCCH(1A′), SiC2H4(1A1) or SiCHCH3(1A′). From this study, it is suggested that the rapid rate constant, recently measured for the Si(3P) + C2H2 and Si(3P) + C2H4 reactions, is due to the intervention of a third body; namely the carrier gas, which efficiently stabilizes the products, by absorbing the energy released during the association process.