Reactions of silylene with unreactive molecules. I: Carbon dioxide; Gas-phase kinetic and theoretical studies

Time-resolved studies of the reaction of silylene, SiH2, with CO2 have been carried out over the temperature range 298-681 K, using laser flash photolysis to generate and monitor SiH2. Between 339 and 681 K the reaction obeys second-order kinetics and the derived rate constants gave the following Airlienius parameters: log(A/cm(3) molecule(-1) s(-1)) = -11.89 +/- 0.13, E-a, = +16.36 +/- 1.23 kJ mol(-1), where the uncertainties are single standard deviations. The reaction shows no overall pressure dependence. This reaction is unusual for SiH2 in being extremely slow and having a positive activation energy. Ab-initio calculations at the G2 level suggest a mechanism occurring via the intermediacy of siloxiranone (formed via addition of SiH2 to one of the C=O double bonds) leading to overall formation of H2SiO + CO, consistent with kinetic findings. Direct abstraction of an O atom appears to be ruled out, as are other potential pathways. The mechanism resembles that of CH2((1)A(1)) + CO2.