Mechanism of the Gas-Phase Thermolysis of Monomethylsilane

Article abstract of DOI:10.1021/ja00253a033

The thermolysis of monomethylsilane (MMS) has been studied as a function of pressure (33-400 Torr), temperature (340-440 deg C), and conversion.Under conditions of very low (tipically, 0.5percent) conversion and in a carefully seasoned vessel the major products are H2 and dimethyldisilane (DMDS).Dimethylsilane (DMS) comprises ca. 5percent of the major products.MMS-d₃ generates D2 exclusively.In the presence of ca. 10percent C2H4 the yields of H2 and DMDS are considerably reduced and both products follow first-order kinetics in their formation.Also, the formation of DMS is completely suppressed, and the Arrhenius parameters for the molecular process CH3SiH3 -> CH3H + H2 (1a) when determined from the rate of H2 production and from (CH3H + CH3SiH3 -> DMDS) production are log k_1a = (15.02 +/- 0.10) - (63270 +/- 310) / 2.3RT and (14.87 +/- 0.12) - (63150 +/- 350) / 2.3RT, respectively.The molecular rate constant for H2, however, includes a small contribution from radical processes that cannot be completely suppressed.When the latter expression for k_1a is used, the rate data for H2 is the unscavenged reaction can be fitted to a mechanism incorporating a second primary step, a slow, surface-catalyzed reaction generating H^. and CH3SiH2^. radicals , which then set up a short chain: CH3SiH2^. + CH3SiH3 -> DMDS + H H + CH3SiH3 -> H2 + CH3SiH2^. .On the basis of kinetic analysis of the data it is concluded that the chain is terminated linearly by CH3SiH2^. radicals at the surface , with log A(s^-1) = 11.7 and E_a ca. 32.3 kcal mol^-1.The derived rate expression for the surface-catalyzed radical initation step CH3SiH3 -> Ch3SiH2^. + H (1b) is log k_1b = 12.7 - 57900/2.3RT.From the measured kinetic data the following themochemical values were derived: D(CH3H-H) = 73.5 kcal mol^-1 and ΔH_f(CH3H) = 51.9 kcal mol^-1.