Gas-Phase Kinetic Study of the Prototype Silylene Addition Reaction SiH2 + C2H4 over the Temperature Range 298 - 595 K. An Example of a Third-Body Mediated Association

Article abstract of DOI:10.1021/j100071a021

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 ethene.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 298 - 595 K.The reaction of SiH2 with C2H4 is pressure dependent, consistemt with a third-body assisted association reaction to form silirane.While silirane was too labile to be detected, vinylsilane, another possible product, was ruled out by GC analysis.Extrapolation of the pressure dependent rate constants gave the Arrhenius equation: log(k<*>/cm³ molecule^-1 s^-1) = (-9.97 +/- 0.03) + (2.9 +/- 0.2) kJ mol^-1/RT ln 10.These parameters are consistent with a fast, nearly collision-controlled, association process.RRKM modeling, based on a consistent transition state for silirane decomposition, gave a good fit to the pressure dependent curves.The calculations employed a weak collisional deactivation model and confirmed (within +/- 12 kJ mol^-1) the most recent theoretical value for ΔH⁰_f(silirane) of 124 kJ mol^-1.An enthalpy surface for the reaction highlights the importance of entropy, rather than enthalpy (or energy), in determining the breakdown pathway of silirane.