Rate Constants for the Gas-Phase Reactions of Silylene with Methanol, Deuterated Methanol, and Water

Gas-phase reaction rate constants for the reaction of silylene, SiH 2, with deuterated methanol, CD 3 OD, 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 SiH 2 with CH 3 OH at 294 K over the range 100−800 Torr, also with Ar. Rate constants for the reaction of SiH 2 with H 2 O 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 CD 3 OD, 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, K eq, 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 SiH 2 −CD 3 OD 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 SiH 2 + CD 3 OD, when adjusted to account for the changes arising from deuteration, reproduces the behavior observed for SiH 2 + CH 3 OH. 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.