Time-resolved cavity ringdown measurements and kinetic modeling of the pressure dependences of the recombination reactions of SiH2 with the alkenes C2H4, C3H6, and t-C4H8.

Room-temperature rate constants for the pressure-dependent reactions SiH2 + ethene, propene, and t-butene have been determined at total pressures of 3.3 mbar < or = p < or = 300 mbar with Ar as buffer gas. SiH2 was detected by means of time-resolved cavity ringdown spectroscopy (CRDS), and the deconvolution of ringdown, kinetics, and laser bandwidth effect was accomplished with the extended simultaneous kinetics and ringdown model (eSKaR). In this way, pseudofirst-order rate constants could be extracted from nonexponential ringdown profiles. The recombination reactions, including the reaction SiH2 + i-butene, have been modeled based on the simplified statistical adiabatic channel model (SACM) and weak collision energy-grained master equation (ME) simulations. The influence of an interfering fast isomerization channel was investigated based on the Rice, Ramsperger, Kassel, Marcus theory (RRKM) and was found to be only important for the C2H4 reaction. Using ab initio energies (G3) and structures (MP2/6-311G(d,p)) as input parameters for the kinetic models, a consistent description of the pressure and temperature dependences of all four reactions was possible. In the temperature range 295 K < or = T < or = 600 K, the extrapolated limiting high-pressure rate constants, k(infinity)(C2H4)/(cm(3) x mol(-1) x s(-1)) = 1.9 x 10(14) (T/K)(-0.065), k(infinity)(C3H6)/(cm(3) x mol(-1) x s(-1)) = 1.3 x 10(14) (T/K)(0.075), k(infinity)(i-C4H8) = 1.8 x 10(14) cm(3) x mol(-1) x s(-1), and k(infinity)(t-C4H8)/(cm(3) x mol(-1) x s(-1)) = 4.6 x 10(13) (T/K)(0.21), are close to the collision number and are more or less temperature independent. In the case of ethene, probably due to the approximate treatment of rotational effects and/or the interfering isomerization process, the applied model slightly underestimates the falloff and thus yields too high extrapolated rate constants at p < 10 mbar.