Pressure and temperature dependence of the gas-phase reaction of silylene with dimethyl ether

Abstract

Gas-phase reaction rate constants for the reaction of silylene, SiH₂, with dimethyl ether, CH₃OCH₃ have been determined over the temperature range 294–441 K and at total pressures of inert bath gas (Ar or SF₆) over the range 30–850 Torr. The second-order rate constants are pressure dependent, even up to the maximum pressure investigated of 850 Torr, and the rate constants decrease with increasing temperature, indicating that the reaction proceeds ia the formation of a complex. At the highest temperature studied (441 K), the experimental decay curves do not return to the baseline. This is attributed to the system reaching equilibrium, with SiH₂ being produced by dissociation of the complex, and provides direct experimental evidence for the formation of the complex. Analysis of the decay curves provided an experimental determination of the equilibrium constant, K_eq, at 441 K. The high-pressure rate constants, obtained by extrapolation of the experimental data using RRKM/master equation modelling, yield the Arrhenius parameters log (A/cm³ molecule⁻¹ s⁻¹) = − 7.6 ± 0.4 and E_a = 9.3 ± 2.8 kJ mol⁻¹. The RRKM/master equation modelling gives a well depth for the SiH₂–CH₃OCH₃ complex of 87 kJ mol⁻¹. This compares with a value of 88.4 ± 1.7 kJ mol⁻¹ determined from K_eq at 441 K. Ab initio calculations, performed at the MP2/6-311 + G** level of theory, give a well-depth for the complex of 82.9 kJ mol⁻¹, in excellent agreement with this value.