Bond dissociation energies in some alkylsilanes.

Appearance Potential (A.P.) measurements were carried out on the organosilanes Me3SiX, where X = Me3Si, Me2SiH, Me, and H, using an automatic method with high sensitivity to concentrate on the threshold region of the ionisation efficiency curves. The method amounted to a direct extrapolation of the 'tails' of the curves, and did not involve the assumptions of the well-established methods. The appearance potentials have been used in conjunction with kinetic and thermodynamic data to obtain values for the bond dissociation energies and heats of formation, D (Me3Si - X) and ?H°f (Me3 Six)g. Trimethylsilane, Me3SiH, was pyrolysed in a flow system using a quartz stirred-flow reactor situated close to the ion source of an A.E.I. MS9 mass spectrometer. The system enabled the reaction to be monitored continuously, using the high sensitivity of the MS9. A simple chain scheme was proposed for the formation of the products, which were found to be: hydrogen, methane, disilanes, disilamethylenes, disilacyclobutanes and small amounts of dimethyl- and tetramethyl-silanes; no polymeric products were found. The hydrogen and methane were shown to be formed by the unimolecular dissociation of (Si - H) and (Si - C) bonds respectively, with negligible contribution from the chain sequence, and the activation energies were therefore identified with D (Me3Si - H) and D (HMe2Si - Me) respectively. There was excellent agreement between the kinetic values of bond dissociation energy and those derived from the electron impact data, enabling a reliable set of quantitative data to be put forward for the A.P., ?H°f (Me3SiX)g and D (Me3Si - X) of the organosilanes.