The gas-phase reaction of silylene with acetaldehyde -: Part 1.: Direct rate studies, isotope effects, RRKM modelling and ab initio studies of the potential energy surface

Time-resolved studies of the title reaction, employing both SiH2 and SiD2, have been carried out over the pressure range 1-100 Torr (with SF6 as bath gas) at five temperatures in the range 297-599 K, using laser flash photolysis to generate and monitor both silylene species. 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/cm(3) molecule(-1) s(-1)) = - 10.10 +/- 0.06, E-a = - 3.91 +/- 0.47 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 modelling, based on a transition state appropriate to formation of a three-membered ring product, 3-methylsiloxirane, and employing a weak collisional deactivation model gives reasonable fits to the pressure dependent curves for DeltaH degrees /kJ mol(-1) in the range -215 to -245. Ab initio calculations at the G2 level indicate the inital formation of a silacarbonyl ylid which can then either form the siloxirane by ring closure, rearrange to form siloxyethene or give ethoxysilylene. Fuller details of the potential surface are given. The energetics are reasonably consistent with siloxirane formation representing the main pathway. The isotope effects are small and close to unity, indicating that secondary isotopic label scrambling, by the reversible ring opening of the siloxirane to ethoxysilylene is not occurring. Differences with the silirane system can be explained by the stabilization of a silylene by an alkoxy substituent.