Thermal Rate Constants for SiH4→←SiH3+ H and CH4→←CH3+ H by Canonical Variational Transition State Theory

The thermal rate constants for SiH4 SiH3 + H and CH4 CH3 + H are calculated by the Canonical Variational Transition State Theory (CVTST). The minimum energy path potential and the deformation path potential along the reaction coordinate are calculated by the MCSCF-CI and UHF methods, respectively. The transitional bending mode which connects the bending vibrations of the parent molecules with the translational and rotational motions of the products of the dissociation is treated in terms of a two-dimensional quantum hindered rotor. The saddle points and the CVTST rate constants are determined by the maximum free energy criterion. Comparative discussions are made for the two systems. The calculated association rate constants for the SiH4 system are 8.4 × 1010 l mol−1 s−1 at 300 K and 12.5 × 1010 l mol−1 s−1 at 2000 K, while those for the CH4 system are 12.5 × 1010 l mol−1 s−1 and 18.6 × 1010 l mol−1 s−1 at the two temperatures. At all the temperatures studied the rate constant of the former system is smaller by 30—35% than the latter. The dissociation rate constants are evaluated to be 7.2 × 105 s−1 at 2000 K and 1.2 × 109 s−1 at 3000 K for the SiH4 system, while the constant for the CH4 system at 2000 K is about one order of magnitude smaller. This difference diminishes at higher temperatures. The results are also compared with experimental and other theoretical studies.