Rate constants for the gas-phase reactions of silylene with methanol, deuterated methanol, and water

Abstract

Gas-phase reaction rate constants for the reaction of silylene, SiH2, with deuterated methanol, CD3OD, have been determined over the temperature range 294-423 K and at total pressures over the range 100-800 Torr of the inert bath gas, Ar. Rate constants have also been measured for the reaction of SiH2 with CH3OH at 294 K over the range 100-800 Torr, also with Ar. Rate constants for the reaction of SiH2 with H2O over the range 50-200 Torr in Ar have been determined at 294 K. The second-order rate constants are pressure-dependent up to the maximum pressures investigated. For CD3OD, for which temperature-dependent data have been obtained, the rate constants decrease with increasing temperature, indicating that the reaction proceeds via the formation of a complex. At the highest temperature studied (423 K), the experimental decay curves indicate the system approaching equilibrium, providing direct experimental evidence for the formation of the complex. Analysis of the 423 K decay curves provides an experimental determination of the equilibrium constant, Keq, and a value for the dissociation energy of the complex of 83.0 ± 1.3 kJ mol-1. The Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation modeling gives a dissociation energy for the SiH2-CD3OD complex of 83.7 kJ mol-1. Ab initio calculations, performed at the MP2/6-311+G** level of theory, give a value of 75.4 kJ mol-1, in reasonable agreement with this value. The RRKM/master equation modeling for SiH2 + CD3OD, when adjusted to account for the changes arising from deuteration, reproduces the behavior observed for SiH2 + CH3OH. The high-pressure limit predictions of the RRKM/master equation modeling are quite unusual and may indicate unusual pressure and temperature behavior in weakly bonded systems.