Kinetic correlations for H2 addition and elimination reaction mechanisms during silicon hydride pyrolysis

Abstract

The mechanism of H(2) addition and elimination reactions in selected silicon hydrides (Si(x)H(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 H(2) 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 H(2) addition reactions were found to correlate reasonably well with the difference in energy between the highest occupied molecular orbital of H(2) (E(HOMO)) and the lowest unoccupied molecular orbital of the reactant silylene (E(LUMO)).