Ab initio study on the rate constants of SiCl₄ + H → SiCl₃ + HCl

The reaction SiCl₄ + H → SiCl₃ + HCl is studied using an ab initio dynamic method. The direct chlorine abstraction of reaction processes via a C_3v symmetry transition state is calculated by the intrinsic reaction coordinate (IRC) method at the UMP2/6-311G(d,p) level. The forward and reverse barriers are refined by UMP4(SDTQ) and UQCISD(T) single point energy calculations using the same basis set of 6-311G(d,p). The forward barrier is calculated to be 24.34 kcal mol^-1 at the UMP4(SDTQ) level and 22.62 kcal mol^-1 at the UQCISD(T) level, while the reverse barrier is 22.46 and 21.90 kcal mol^-1, respectively. The enthalpy of reaction is calculated to be 1.94 kcal mol^-1 at the UMP4(SDTQ) level and 0.72 kcal mol^-1 at the UQCISD(T) level. The forward rate constants in the temperature range 1500-1800 K are calculated by the conventional and variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The variational effect is important for the calculation of forward rate constants but the SCT correction is small. The CVT and CVT/SCT rate constants of the forward reaction at the UMP4(SDTQ) and UQCISD(T) levels are consistent with the lower limit of the experimental result. However, the theoretical activation energies in the temperature range 1500-1800 K, 28.43 kcal mol^-1 at the UMP4(SDTQ) level and 27.10 kcal mol^-1 at the UQCISD(T) level, are much higher than the experimental value, 9.54 ± 5.17 kcal mol^-1. The theoretical temperature dependence of the rate constants differs from the experimental result.