Time-resolved studies of the temperature dependence of the gas-phase reactions of methylsilylene with silane and the methylsilanes

Abstract

The 193 nm laser flash photolysis of gas-phase 1,2-dimethyldisilane has been found to give a broad-band absorption in the wavelength range 454–515 nm, which is plausibly shown to be due to the transient species methylsilylene, MeSiH. By carrying out the title studies, the first direct kinetic studies of MeSiH, second-order rate constants have been obtained for reactions of MeSiH with SiH₄, MeSiH₃, Me₂SiH₂ and Me₃SiH, in the temperature range 360–580 K. The reactions are fast but show negative activation energies, increasing from –7.5 kJ mol^–1 for SiH₄ to –18.4 kJ mol^–1 for Me₃SiH. The data are interpreted as proceeding via an intermediate complex, whose rearrangement becomes rate-determining at higher temperatures. Comparisons of reactivity of MeSiH with those of other silylenes reveal the general pattern of methyl substituent effects of these complexes. In conjunction with ab initio theory (for the reaction of SiH₂ with SiH₄) these show that the electrophilic interaction probably precedes the nucleophilic interaction, although the latter is important in the rate-determining (second) step for the insertion reactions of both MeSiH and SiMe₂. Combination of MeSiH insertion rate constants with the reverse unimolecular decomposition rate constants of the product disilanes enable the calculation of an improved value of 202 ± 6 kJ mol^–1 for Δ_fH^⊖(MeSiH).