The kinetics of some gas phase reactions of silyl radicals and silylenes

• Main Author: Matthews, Joseph Ioan
• Published: University of Leicester 1975
• Subjects: 541.39
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.465276

This thesis is split into two parts; the first being the abstraction of chlorine from silicon-chlorine bonds by trimethylsilyl radicals, while the second part is concerned with silylene chemistry. (i) Results from recent work8 in which RMeSiCl (R = Me, Cl) radicals are generated in the presence of chlorosilanes suggested that the abstraction of chlorine from silicon- chlorine bonds is a very rapid process despite there being no substantial thermodynamic driving force. In order to obtain quantitative results, the following reaction: was studied using both thermal and photolytic means of generating trimethylsilyl radicals. Azomethane, di-tert-butyl peroxide and mercury dimethyl, in conjunction with trimethyl- silane, were used as thermal sources of trimethylsilyl radicals, but, when azomethane and di-tert-butyl peroxide were used, no hexamethyldisilane was obtained so preventing the determination of any rate constants. However, with mercury dimethyl, hexamethyldisilane was observed, but the absence of dimethylchlorosilane suggested the presence of a non-steady state concentration of trimethylsilyl radicals, which, when coupled with the relatively small temperature range used, produced large errors in the values of the Arrhenius parameters. With the mercury photosensitised generation of trimethylsilyl radicals from trimethylsilane, a steady state concentration of trimethylsilyl radicals is set up, and rate constants obtained over a larger temperature range. Combination of the rate constants from both the thermal and photolytic methods gave the following Arrhenius parameters:- log10 and a reaction scheme which accounts for all the products and kinetic features is proposed. (ii) Experiments on the pyrolysis of pentamethyldisilane between 620 - 700 K revealed the primary decomposition step to be: with the Arrhenius parameters being Similar work on the pyrolysis of 1.1.2-trimethyldisilane between 620 - 665K gave two primary decomposition routes:- with the Arrhenius parameters being:From the products formed and the various kinetic features, reaction schemes were proposed for both disilanes. From the above results, the standard heat of formation of dimethyl- silylene was proposed to be approximately 120 kJ mole-1. The reverse reaction, i.e. the insertion of various silylenes generated both thermally and photolytically, into various monosilanes, was also studied but failed to yield Arrhenius parameters.