Prototype Si—H insertion reaction of silylene with silane. Absolute rate constants, temperature dependence, RRKM modelling and the potential-energy surface

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH4. The reaction was studied in the gas phase over the pressure range 1–100 Torr, with both Ar and SF6 as bath gases, at six temperatures in the range 298–665 K. The reaction of SiH2 with SiH4 to form disilane, Si2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule–1 s–1)=(–9.91 ± 0.04)+(3.3 ± 0.3 kJ mol–1)/RT In 10. These Arrhenius parameters are consistent with a fast, nearly collision-controlled, process. RRKM modelling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure-dependent curves. The step sizes (energies removed in a down collision) corresponded to collisional efficiencies (βc) of ca. 0.5 for SF6 and ca. 0.2 for Ar.The rate constants for the insertion and reverse decomposition (of Si2H6) have been combined to obtain a precise value of the equilibrium constant Kp at 552 K. Using the third-law method, a value for ΔfH°(SiH2)= 273 ± 2 kJ mol–1 is derived which represents the most precise experimental value for this quantity yet obtained. Ab initio calculations at the correlated level, reveal the presence of two weak complexes (local-energy minima) on the potential-energy surface corresponding to either direct or inverted geometry of the inserting silylene fragment. Surprisingly, the latter is the lower in energy, lying 51.5 kJ mol–1 below the unassociated reactants. These complexes rearrange to disilane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.