Gas-phase reaction of silylene with acetone: Direct rate studies, RRKM modeling, and ab initio studies of the potential energy surface

Time-resolved studies of the title reaction have been carried out over the pressure range 3-100 Torr (with SF6 as bath gas) at five temperatures in th e range 295-602 K, using laser flash photolysis to generate and monitor sil ylene, SiH2. The second-order rate constants obtained were pressure-depende nt, indicating that the reaction is a third-body-assisted association proce ss. The high-pressure rate constants, obtained by extrapolation, gave the f ollowing Arrhenius parameters: log(A/cm(3) molecule(-1) s(-1)) = -10.17 +/- 0.04 and E-a = -4.54 +/- 0.32 kJ mol(-1), where the uncertainties are sing le standard deviations. The parameters are consistent with a fast associati on process occurring at close to the collision rate. RRKM modeling, based o n a transition state appropriate to formation of a three-membered ring prod uct, 3,3-dimethylsiloxirane, and employing a weak collisional deactivation model, gives reasonable fits to the pressure-dependent curves for Delta H d egrees/kJ mol(-1) in the range -205 to -225. Ab initio calculations at the G2 level indicate the initial formation of a silacarbonyl ylid, which can t hen either form the siloxirane by ring closure or rearrange to form 2-silox ypropene. Fuller details of the potential surface are given. The energetics are consistent with siloxirane formation representing the main pathway.