The gas-phase reaction of silylene with acetaldehyde - Part 1. Direct ratestudies, 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 photoly sis to generate and monitor both silylene species. The second order rate co nstants 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/c m(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 co llision rate. RRKM modelling, based on a transition state appropriate to fo rmation of a three-membered ring product, 3-methylsiloxirane, and employing a weak collisional deactivation model gives reasonable fits to the pressur e 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 closur e, 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 effect s are small and close to unity, indicating that secondary isotopic label sc rambling, by the reversible ring opening of the siloxirane to ethoxysilylen e is not occurring. Differences with the silirane system can be explained b y the stabilization of a silylene by an alkoxy substituent.