The insertion reaction of germylene into the Si–H bond of silane: absolute rate constants, temperature dependence, RRKM modelling, and quantum chemical (ab initio and DFT) calculations

Abstract

Time resolved studies of germylene, GeH₂, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, 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 SF₆ as bath gas, at 5 temperatures in the range 295–554 K. The reaction shows the characteristic pressure dependence of a third-body assisted association reaction. The high pressure rate constants, obtained by extrapolation, gave the Arrhenius equation:

These Arrhenius parameters are consistent with a moderately fast reaction occurring at approximately one thirtieth of the collision rate. Rice–Ramsperger–Kassel–Marcus (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 for a choice of the critical energy, E₀ = 138 kJ mol⁻¹, for the reverse decomposition of H₃SiGeH₃ , the reaction product. There is no previous experimental determination of this quantity. From it we derive ΔH_f⁰(GeH₂) = 233 ± 12 kJ mol⁻¹, in reasonable agreement with earlier estimates. Ab initio and DFT calculations 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 germylene fragment. As found earlier for the GeH₂ + GeH₄ reaction, the latter is lower in energy and has left and right handed forms. These complexes rearrange to H₃SiGeH₃ with low barriers. The implications of these findings and the nature of the insertion process are discussed.