Density functional theory and Ab initio direct dynamics studies on the hydrogen abstraction reactions of SiH_4-n(CH₃)_n + H → SiH_3-n(CH₃)_n + H₂, n = 1-3

Density functional theory (DFT) and ab initio direct dynamics methods have been used to study three hydrogen abstraction reactions of SiH_4-n(CH₃)_n + H → SiH_3-n(CH₃)_n + H₂, n = 1-3. For all the reactions, the potential energy surface information is calculated at the DFT BHLYP/6-311+G** level, and energies along the minimum energy path are improved by a series of single-point ab initio PMP4/6-311+G(3df,2p)//BHLYP calculations. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction path of the reactions are discussed and compared. The rate constants of the reactions are calculated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) method in the temperature range 290-3000 K. Good agreement with experimental values is found for rate constants over the measured temperature ranges. The results show that the variational effect is small and, in the lower temperature range, the small curvature tunneling effect is important for the reactions. Methyl substitution increases the reactivity of the Si-H bond toward H atom attack, and the increase in k/n mainly stems from a corresponding increase in A/n. The activation energies for the three methyl-substituted silane reactions are nearly the same. Three-parameter fits for rate constants of the reactions within 290-3000 K are presented.