Kinetic and mechanistic studies on the abstraction reactions of atomic O (3P) with (CH3)2SiH2 and (CH3)3SiH

Abstract

The reactions of atomic O (3P) with (CH3)2SiH2 and (CH3)3SiH have been studied theoretically using ab initio molecular orbital theory for the first time. Geometries have been optimized at the MP2 level with the 6-311G(d,p) and 6-311G(2d,2p) basis sets. The single-point energy calculations have been carried at the QCISD(T)/6-311+G(3df,2p) level. Theoretical analysis provides conclusive evidence that the main process occurring in each reaction is the hydrogen abstraction from the Si–H bonds leading to the formation of the H2 and silyl radical; the hydrogen abstraction from the C–H bonds has higher barrier and is difficult to react. Two nearly degenerate transition states of A″3 and A′3 symmetries have been located for each hydrogen abstraction reaction from the Si–H bonds. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths are discussed and compared. The rate constants have been deduced over a wide temperature range of 200–3000 K using canonical variational transition-state theory (CVT) with small curvature tunneling effect (SCT). The calculated CVT/SCT rate constants exhibit typical non-Arrhenius behavior, three-parameter rate-temperature formulas are fitted as follows (in units of cm3 molecule−1 s−1): k1(T)=(3.41×10−16)T1.65exp(−411.72/T) and k2(T)=(1.85×10−15)T1.42 exp(−372.57/T) for the reactions of O (3P) with (CH3)2SiH2 and (CH3)3SiH, respectively. The calculated rate constants are compared with the available experimental values.