Gas-Phase Reaction of Silylene with Acetone: Direct Rate Studies, RRKM Modeling, and ab Initio Studies of the Potential Energy Surface

Abstract

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 the range 295−602 K, using laser flash photolysis to generate and monitor silylene, SiH2. The second-order rate constants obtained were pressure-dependent, indicating that the reaction is a third-body-assisted association process. The high-pressure rate constants, obtained by extrapolation, gave the following Arrhenius parameters: log(A/cm3 molecule-1 s-1) = −10.17 ± 0.04 and Ea = −4.54 ± 0.32 kJ mol-1, where the uncertainties are single standard deviations. The parameters are consistent with a fast association process occurring at close to the collision rate. RRKM modeling, based on a transition state appropriate to formation of a three-membered ring product, 3,3-dimethylsiloxirane, and employing a weak collisional deactivation model, gives reasonable fits to the pressure-dependent curves for ΔH°/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 then either form the siloxirane by ring closure or rearrange to form 2-siloxypropene. Fuller details of the potential surface are given. The energetics are consistent with siloxirane formation representing the main pathway.