Arrhenius parameters for silene insertion into silicon–hydrogen bonds

Abstract

The pyrolysis of disilane provides a clean source of singlet silene, SiH₂, which readily inserts in Si—H bonds. Co-pyrolyses of disilane with methylsilane, dimethylsilane and trimethylsilane, respectively, have been studied over a wide range of mixture compositions and total reactant pressures in the range 572–610 K. The only new product, in each case, is the corresponding alkyldisilane and the analytical data fit the proposed mechanism remarkably well. There is no reaction between disilane and tetramethylsilane.

Arrhenius parameters for the several silene insertions relative to that into disilane are evaluated. Pre-exponential factor ratios are shown to be very close to the ratios of the number of Si—H bonds in the respective molecules and rate differences are traced to activation energy differences which are taken to reflect differences in Si—H bond dissociation energies. In confirmation of previous work it is found that the sequence of these energies is secondary > primary > tertiary. The numerical differences measured accord well with previous estimates. Presently measured relative Arrhenius parameters are converted to absolute values on the basis of previously determined values for silene insertion into disilane. It is shown that the relevant absolute rate constants may then be evaluated to, at worst, a factor of three.