Kinetic correlations for H2 addition and elimination reaction mechanisms during silicon hydridepyrolysis

Abstract

The mechanism of H₂ addition and elimination reactions in selected silicon hydrides (Si_xH_y, x = 1–10, y = 4–20) was modeled using quantum chemical calculations, statistical thermodynamics, transition state theory and transition state group additivity. Rate coefficients for 25 H₂ addition reactions were calculated using G3//B3LYP. For nearly every reaction, the overall conversion exhibits two steps. In the addition direction, the reactants first meet to form an adduct which then converts into a saturated silicon hydride via homolytic H–H bond cleavage. Values for the single-event Arrhenius pre-exponential factor, Ã, and the activation energy, E_a, were calculated from the G3//B3LYP rate coefficients, and a group additivity scheme was developed to predict à and E_a. The values predicted by group additivity are more accurate than kinetic correlations currently used in the literature, which rely on representative à values and the Evans-Polanyi correlation. The factors that have the most pronounced effect on à and E_a were investigated, and stabilization of the divalent silicon atom of the unsaturated silicon hydride with electron-donating substituents was found to influence kinetic parameters considerably. The rate coefficients for H₂ addition reactions were found to correlate reasonably well with the difference in energy between the highest occupied molecular orbital of H₂ (E_HOMO) and the lowest unoccupied molecular orbital of the reactant silylene (E_LUMO).