The thermal and electron-impact dissociation of some trimethylsilyl compounds.

1) Preliminary work was directed towards the discovery or conditions under which the thermal equilibrium Me6Si2 = 2Me3Si would be set up. Ethylpentamethyldisilane was used as a simple labelled form of hexamethyldisilane, and the symmetrical recombination product, EtMe2 SiSiMe2 Et was looked for mass-spectrometrically. Exceptional steps had to be taken to overcome rapid heterogeneous formation of siloxanes in the initial experiments, and the apparatus was extensively modified during the course of this investigatory work.;2) Once the heterogeneous behaviour had been eliminated, it became apparent that the trialkyl-silyl radicals formed by the initial dissociation were undergoing abstraction reactions instead of the expected recombination. However, the disilyl radicals formed in the abstraction reactions did combine, so a non-chain sequence was followed. The pyrolysis of ethylpentamethyldisilane was therefore studied between 528°C and 545°C, at initial pressures between 0.1 end 0.7 mm. Hg, giving D(Me3Si-SiMe3Et) = 69 +/- 2 k. cal.mole-1.;3) In the light or these results, no further work on ethylpentamethyldisilane was carried out, but attention was turned to the simpler hexamethyldisilane, which was pyrolysed to less than 5% decomposition, in a static system, between 523° and 555°C, at initial pressures of 0.2 - 0.8 mm. Hg. The product composition was determined using mass spectrometry and gas chromatography, and the rate of formation of products was found to be first order, k(sec-1) = 1013.5 +/- 1.0 exp-(67,300) +/- 2,200)/RT. The overall reaction is 11Me6Sia ? 5Me3 SiH + Me4 Si + 4Me3Si3CH2 + Me10Si4CH2, and a reaction scheme which accounts for all products and kinetic features is proposed. The activation energy is identified with the silicon-silicon bond dissociation energy, giving D(Me3Si-SiMe3) = 67 +/- 2 k.Cal.mole-1.;4) The ionisation potentials of the ethyldimethylsilyl and trimethylsilyl radicals have been determined, and the latter value, 7.1 ev., has been combined with the heats of formation and appearance potential data of other workers, to give a self-consistent set of bond dissociation energies and heats of formation for the series Me3 SiX, (X = H, Me, SiMe3, Hal). 5) The author's results, and the values derived in 4) are discussed in relation to those of other workers in the field.