MECHANISTIC STUDY OF THE ME2SIH-ME3SI RADICAL SYSTEM

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 dis proportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si-H bonds of the reactants. From a quantitati ve determination of the disilane products as a function of the reactan t ratio, a value for the branching ratio of cross-disproportionation o f 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 la rger 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 ex tent and are responsible for the formation of six trisilanes. Secondar y reactions, mainly initiated by H-atom abstraction from tetra- and pe ntamethyldisilane by silyl radicals, also take place. The relative rat e constants estimated for these reactions are in agreement with a prev ious determination.