Gas phase kinetic and quantum chemical studies of the reactions of silylene with the methylsilanes. Absolute rate constants, temperature dependences, RRKM modelling and potential energy surfaces

Abstract

Time resolved studies of silylene, SiH₂, generated by the 193 nm laser flash photolysis of phenylsilane, have been carried out to obtain rate coefficients for its bimolecular reactions with methyl-, dimethyl- and trimethyl-silanes in the gas phase. The reactions were studied over the pressure range 3–100 Torr with SF₆ as bath gas and at five temperatures in the range 300–625 K. Only slight pressure dependences were found for SiH₂ + MeSiH₃ (485 and 602 K) and for SiH₂ + Me₂SiH₂ (600 K). The high pressure rate constants gave the following Arrhenius parameters:

ReactionLog(A/cm³ molecule⁻¹ s⁻¹)E_a/kJ mol⁻¹SiH₂ + MeSiH₃−10.05 ± 0.11−3.91 ± 0.81SiH₂ + Me₂SiH₂−10.07 ± 0.05−3.48 ± 0.35SiH₂ + Me₃SiH−10.37 ± 0.10−4.65 ± 0.76These are consistent with fast, near to collision-controlled, association processes. RRKM modelling calculations are consistent with the observed pressure dependences (and also the lack of them for SiH₂ + Me₃SiH). Ab initio calculations at both second order perturbation theory (MP2) and coupled cluster (CCSD(T)) levels, showed the presence of weakly-bound complexes along the reaction pathways. In the case of SiH₂ + MeSiH₃ two complexes, with different geometries, were obtained consistent with earlier studies of SiH₂ + SiH₄. These complexes were stabilised by methyl substitution in the substrate silane, but all had exceedingly low barriers to rearrangement to product disilanes. Although methyl groups in the substrate silane enhance the intrinsic SiH₂ insertion rates, it is doubtful whether the intermediate complexes have a significant effect on the kinetics. A further calculation on the reaction MeSiH + SiH₄ shows that the methyl substitution in the silylene should have a much more significant kinetic effect (as observed in other studies).