The thermolysis of tetramethylsilane has been studied in a pulsed stirred-flow system between 840 and 1055 K, yielding si&cantly different Arrhenius parameters above and below 950 K. The high temperature results relate to a non-chain mechanism, whence D(Me3Si-Me) = 355 k 6 kJ mol-I while the low temperature results relate to a short chain sequence. A mechanism for the latteriis shown by computer-aided numerical integration to be consistent with experimental results, as is a similar chain mechanism for the thermolysis of trimethylsilane.It has become apparent in recent years that the thermolysis of several methylsilanes proceeds by short-chain reactions of moderate complexity in which short-lived " double-bonded " intermediates, Si=CH2, play a prominent ~a r t . l '~ The thermolysis of hexamethyldisilane has been studied in detail over a range of pressure and temperatureY4* and has been shown to proceed by two concurrent chain sequences which differ in relative importance as the pressure is varied. Me,Si=CH, is an intermediate in the main chain sequence at low pressure, which produces trimethylsilane as the principal product. This chain could be inhibited, enabling a kinetic measurement of D(Me,Si-SiMe,) to be made.4 An earlier investigation of the thermolysis of tetramethylsilane between 8 10 and 980 K likewise produced evidence for a chain reaction involving Me,Si=CH,, but the mechanism was not fully el~cidated.~ As has recently been pointed out,'* 2* it follows from both of these studies that results obtained some years ago ' on the kinetics of thermolysis of trimethylsilane must be reinterpreted. We describe in this paper some experiments on the thermolysis of tetramethylsilane and some computer-aided calculations on the kinetics of thermolysis of tetramethylsilane, trimethylsilane and hexamethyldisilane which enable mutually consistent detailed mechanisms to be advanced for these thermolyses and which lead to a kinetic estimate of D(Me,Si-Me).
\ / EXPERIMENTAL A N D RESULTSThe thermolysis of tetramethylsilane was investigated in the same apparatus as for he~amethyldisilane,~ using the " pulsed stirred-flow " technique which has been fully described. * Analysis was by g.l.c., supplemented by mass spectrometry. The temperature range covered by the experiments was from 840 to 1055 K, with single pulses of tetramethylsilane corresponding to initial concentrations between and mol dm-3. The carrier gas was purified nitrogen,8 at above atmospheric pressure. Residence times in the reactor were varied between 13 and 120 s, partly by altering the flow rate of the carrier gas and partly by using quartz reaction vessels t Present address :
LE1 7RHA non-chain mechanism is suggested for the pyrolysis of trimethylsilane in the gas phase at low pressure in a stirred-flow system between 943 and 1031 K. Arrhenius parameters for the unimolecular dissociations Me,SiH -b Me,SiH + Meand Me,SiH --t Me,Si* + H* are given respectively by log k , / s -l = 15.9 f 0.7 -320 f 2/ 2-303 RT and log k2/s-l = 15-6 f 0-7 -336 f 2/2.303RT (activation energies in kJ mol-l). The stability of a-silylalkyl radicals is confirmed, and evidence is advanced for radical disproportionations leading ultimately to disi lacyclo butanes, WE have shown that pyrolysis of hexamethyldisilane proceeds under certain conditions by a radical nonchain mechanism rate-determined by the initial rupture of the silicon-silicon bond; the dissociation energy of this bond can then be deduced from the kinetic data. Non-chain mechanisms are likely to operate in the pyrolysis of certain other silicon compounds for reasons discussed elsewhere,l$ and trimethylsilane should be one such compound. The relative strengths of the siliconhydrogen and silicon-methyl bonds in trimethylsilane were in doubt since the high reactivity of siliconhydrogen bonds3 appeared to be at odds with recent electron-impact results 4,6 which indicate that the silicon-hydrogen bond is stronger than silicon-methyl. It was, therefore, particularly interesting to attempt to determine the strengths of these bonds kinetically by suitable pyrolysis studies of trimethylsilane, and this paper enlarges on our preliminary account of these studies, which have yielded the desired information about bond strengths in trimethylsilane and have also shed some light on the reactions of silicon-containing radicals.
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