Reaction of hydrogen atoms with hexamethyldisilane

Abstract

The reaction of hydrogen atoms with hexamethyldisilane has been studied by pulsed, mercury-sensitized photolysis experiments using Lyman a resonance absorption and resonance fluorescence detection of H atoms. At room temperature it proceeds exclusively according to H + (CH3)3SiSi(CH3)3 - (CH3)3SiH + (CH3)3Si (1) with a rate constant k(1) = (3.55 f 0.25) X cm3 molecule-’ s-’ (T = 295 K). The temperature dependence of the rate constant in molecular units can be expressed by log k(1) = (-10.9 k 0.1) - [(1770 f 50)/(2.30T)]. The pseudo-first-order rate constant for H atom disappearance, k’, shows a dependence on the initial hydrogen atom to substrate concentration ratio. This is explained by two competing reaction channels for the disappearance of the trimethylsilyl radical: bimolecular combination (k(2)) and combination of hydrogen atoms with trimethylsilyl radicals yielding trimethylsilane (k(3)). From the intensity dependence of k’ and under the assumption that only reactions 1, 2, and 3 are operative, model calculations yield k(3) = (2 f 1) X cm3 molecule-’ s-’, while for k(2) only an upper limit of 55 X lo-” cm3 molecule-’ s-l can be given. Thermochemical calculations suggest that the true value for k(2) is lower, probably close to the liquid phase values.