Mechanistic study of the Me2SiH–Me3Si radical system

Abstract

Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me₂Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.