High-Temperature Rate Constants for the Reaction of Hydrogen Atoms with Tetramethoxysilane and Reactivity Analogies between Silanes and Oxygenated Hydrocarbons.

The shock-tube technique has been used to investigate the H-abstraction reaction H + Si(OCH3)4 → H2 + Si(OCH2)(OCH3)3 behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. The experiments covered a temperature range of 1111-1238 K, and pressures of 1.3-1.4 bar. H atom concentrations were monitored with atomic resonance absorption spectrometry (ARAS). Fits to the temporal H atom concentration profiles based on a developed chemical kinetics reaction mechanism were used for determining bimolecular rate constants. Experimental total H-abstraction rate constants were well represented by the Arrhenius equation ktotal( T) = 10-9.16±0.24 exp(-25.5 ± 5.6 kJ mol-1/ RT) cm3 s-1. Transition state theory (TST) calculations based on the G4 level of theory show excellent agreement with experimentally obtained rate constants, i.e., the theory values of k( T) deviate by less than 25% from the experimental results. Regarding H abstractions, we have compared the reactivity of C-H bonds in Si(OCH3)4 with the reactivity of C-H bonds in dimethyl ether (CH3OCH3). Present experimental and theoretical results indicate that at high temperatures, i.e., T > 500 K, CH3OCH3 is a good reactivity analog to Si(OCH3)4, i.e., kH+Si(OCH3)4( T) ∼ 1.5 × kH+CH3OCH3( T). On the basis of these results, we discuss the possibility of drawing reactivity analogies between oxygenated silanes and oxygenated hydrocarbons.