A gas-phase kinetic study of the silylene addition reaction SiH 2 + C 2 D 4 as a function of temperature and pressure: isotope effects and mechanistic complexities

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with C2D4. The reaction was studied in the gas phase, over the pressure range 1–100 Torr (in SF6 bath gas) at five temperatures in the range 291–595 K. The reaction of SiH2 + C2D4 was pressure dependent but significantly less so than that of SiH2 + C2H4 studied earlier. The high pressure limiting rate constants were the same for the two reaction systems within experimental error. RRKM modelling studies were carried out but the calculated inverse isotope effects, kD/kH, were poorly reproduced by a mechanism consisting of a simple third body assisted association reaction. When this was extended to include a reversible isotopic scrambling process, the RRKM modelling of the isotope effects gave better agreement with experiment, although not perfect agreement with the pressure dependence. This mechanism, involving the reversible isomerisation of silirane-d4 to ethylsilylene-d4, is consistent with thermochemical and ab initio calculations of the energy surface.