Kinetics and mechanism of the silane decomposition

The homogeneous gas‐phase decomposition kinetics of silane has been investigated using the single‐pulse shock tube comparative rate technique ( T = 1035–1184˚K, P total ≈︁ 4000 Torr). The initial reaction of the decomposition SiH 4 {article}{empty}{document}SiH₄ → ¹ SiH₂ + H₂{document} SiH 2 + H 2 is a unimolecular process in its pressure fall‐off regime with experimental Arrhenius parameters of log k 1 (sec −1 ) = 13.33 ± 0.28–52,700 ± 1400/2.303 RT . The decomposition has also been studied at lower temperatures by conventional methods. The results confirm the total pressure effect, indicate a small but not negligible extent of induced reaction, and show that the decomposition is first order in silane at constant total pressures. RRKM‐pressure fall‐off calculations for four different transition‐state models are reported, and good agreement with all the data is obtained with a model whose high‐pressure parameters are log A 1 (sec −1 ) = 15.5, E 1(∞) = 56.9 kcal, and Δ E 0± 0 (1) = 55.9 kcal. The mechanism of the decomposition is discussed, and it is concluded that hydrogen atoms are not involved. It is further suggested that silylene in the pure silane pyrolysis ultimately reacts with itself to give hydrogen: 2SiH 2 → (Si 2 H 4 )* → (SiH 3 SiH)* → Si 2 H 2 + H 2 . The mechanism of H ⟷ D exchange absorbed in the pyrolysis of SiD 4 ‐hydrocarbon systems is also discussed.