Kinetic correlations for H-2 addition and elimination reaction mechanisms during silicon hydride pyrolysis

The mechanism of H-2 addition and elimination reactions in selected silicon hydrides (SixHy, 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, (A) over tilde, and the activation energy, E-a, were calculated from the G3//B3LYP rate coefficients, and a group additivity scheme was developed to predict (A) over tilde and E-a. The values predicted by group additivity are more accurate than kinetic correlations currently used in the literature, which rely on representative (A) over tilde values and the Evans-Polanyi correlation. The factors that have the most pronounced effect on (A) over tilde 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).