High‐Temperature Rate Constants for H + Tetramethylsilane and H + Silane and Implications about Structure–Activity Relationships for Silanes

The shock‐tube technique has been used to investigate the reactions H + SiH₄ → H₂ + SiH₃ (R1) and H + Si(CH₃)₄ → Si(CH₃)₃CH₂ + H₂ (R2) behind reflected shock waves. C₂H₅I was used as a thermal in situ source for H atoms. For reaction (R1), the experiments covered a temperature range of 1170–1251 K and for (R2) 1227–1320 K. In both cases, the pressures were near 1.5 bar. In these experiments, H atoms were monitored with atomic resonance absorption spectrometry. Fits to the H‐atom temporal concentration profiles applying postulated chemical kinetic reaction mechanisms were used for determining the rate constants k₁ and k₂. Experimental rate constants were well represented by the Arrhenius equations k₁(T) = 2.75 × 10⁻⁹ exp(−37.78 kJ mol⁻¹/RT) cm³ s⁻¹ and k₂(T) = 1.17 × 10⁻⁷ exp(−86.82 kJ mol⁻¹/RT) cm³ s⁻¹. Transition state theory (TST) calculations based on CBS‐QB3 and G4 levels of theory show good agreement with experimentally obtained rate constants; the experimental values for k₁ and k₂ are ∼40% lower and ∼50% larger than theoretical predictions, respectively. For the development of a mechanism describing the thermal decomposition of tetramethylsilane (Si(CH₃)₄; TMS), also TST‐based rate constants for reaction CH₃ + Si(CH₃)₄ → Si(CH₃)₃CH₂ + CH₄ (R3) were calculated. A comparison between experimental and theoretical rate constants k₂ and k₃ with available rate constants from the literature indicates that Si(CH₃)₄ has very similar reactivity toward H abstractions like neopentane (C(CH₃)₄), which is the analog hydrocarbon to TMS. Based on these results, the possibility of drawing reactivity analogies between hydrocarbons and structurally similar silicon‐organic compounds for H‐atom abstractions is discussed.