Investigation of the Prototype Silylene Reaction, SiH2 + H2O (and D2O): Time-Resolved Gas-Phase Kinetic Studies, Isotope Effects, RRKM Calculations, and Quantum Chemical Calculations of the Reaction Energy Surface

Time-resolved kinetic studies of the reaction of silylene, SiH 2, with H 2 O and with D 2 O have been carried out in the gas phase at 296 and at 339 K, using laser flash photolysis to generate and monitor SiH 2 . The reaction was studied over the pressure range 10−200 Torr with SF 6 as bath gas. The second-order rate constants obtained were pressure dependent, indicating that the reaction is a third-body assisted association process. Rate constants at 339 K were about half those at 296 K. Isotope effects, k H / k D, were small averaging 1.076 ± 0.080, suggesting no involvement of H- (or D-) atom transfer in the rate determining step. RRKM modeling was undertaken based on a transition state appropriate to formation of the expected zwitterionic donor−acceptor complex, H 2 Si···OH 2 . Because the reaction is close to the low pressure (third order) region, it is difficult to be definitive about the activated complex structure. Various structures were tried, both with and without the incorporation of rotational modes, leading to values for the high-pressure limiting (i.e., true second-order) rate constant in the range 9.5 × 10 -11 to 5 × 10 -10 cm 3 molecule -1 s -1 . The RRKM modeling and mechanistic interpretation is supported by ab initio quantum calculations carried out at the G2 and G3 levels. The results are compared and contrasted with the previous studies.