The reactions of atomic chlorine with acrolein, methacrolein and methyl vinyl ketone

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 u p to the maximum pressure investigated of 850 Torr, and the rate constants decrease with increasing temperature, indicating that the reaction proceeds via the formation of a complex. At the highest temperature studied (441 K) , the experimental decay curves do not return to the baseline. This is attr ibuted to the system reaching equilibrium, with SiH2 being produced by diss ociation of the complex, and provides direct experimental evidence for the formation of the complex. Analysis of the decay curves provided an experime ntal determination of the equilibrium constant, K-eq, at 441 K. The high-pr essure rate constants, obtained by extrapolation of the experimental data u sing RRKM/master equation modelling, yield the Arrhenius parameters log (A/ cm(3) molecule(-1) s(-1))=-7.6 +/-0.4 and E-a=9.3 +/-2.8 kJ mol(-1). The RR KM/master equation modelling gives a well depth for the SiH2-CH3OCH3 comple x of 87 kJ mol(-1). This compares with a value of 88.4 +/-1.7 kJ mol(-1) de termined 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.