Iain M. T. Davidson

Enthalpies of formation of silanes

Abstract Bensons electrostatic model for calculating enthalpies of formation has been applied to the methylsilanes and some disilanes, conforming recent conclusions from electron-impact measurements that these silanes essentially follow bond additivity rules, with very small interaction corrections. These findings are shown to be consistent with recently measured bond dissociation energies.

The role of silylenes in the direct synthesis of methylchlorosilanes

Abstract From butadiene trapping experiments in a batch flow reactor, the silylene intermediates SiMeCl and SiCl2 are shown to be formed during the Direct Synthesis. Two types of silylene intermediate are believed to be involved. Silylenoids are formed on the surface where they react with methyl chloride yielding methylchclorosilanes (SiMeCl gives Me2SiCl2, SiCl2 gives MeSiCl3) in accordance with the van den Berg mechanism. Free silylenes are released into the gas phase, where they may be trapped by butadiene, but are not directly involved in methylchlorosilane production. The addition of Me3SiH to the methyl chloride promotes radical reactions; the major product is Me3SiCl. Me3SiCl is believed to result from an efficient chain sequence proceeding mainly on the surface involving Me3Si· radicals which scavenge surface-bound chlorine.

Quantitative aspects of some unimolecular isomerization reactions of organosilicon intermediates

Abstract Estimates of rate constants are deduced for several reactions of silylenes. These estimates, applied to experimental results for some intriguing silylene isomerization reactions, are shown to reconcile some apparent conflicts in the literature and to draw attention to hitherto unnoticed mechanistic features.

A gas kinetic study of the pyrolysis of octamethyl-1,2-disilacyclobutane

Abstract Pyrolysis of octamethyl-1,2-disilacyclobutane(I) has been shown to proceed by a unimolecular isomerisation involving a biradical which undergoes internal hydrogen-abstraction. Kinetic data were obtained for this pyrolysis and for that of the oxidation product of I, octamethyl-1,3-disila-2-oxacyclopentane(II). A few additional experiments indicated that intermediates with π-bonds to silicon may be generated by photolysis of I. The energetics of these reactions were discussed.

Silicon knocked into a cocked hat

Abstract The propensity of silicon to form a three-membered ring intermediate (the “cocked hat” of the title) in preference to larger cyclic intermediates in many thermal reactions of organosilicon compounds is discussed in the light of recent developments in organosilicon kinetics and thermochemistry.

Factors influencing the formation of silylenes in the pyrolysis of silicon compounds

Abstract Pyrolysis of silicon compounds may yield silyl radicals or silylenes. The thermodynamic and kinetic features of these two modes of decomposition are discussed and the importance of kinetic factors in determining whether any particular silicon compound gives silyl radicals or silylenes on pyrolysis is established. Some pyrolyses which have not yet been fully investigated are discussed in the light of the generalizations which have been developed.

Some examples of the contribution of gas-kinetic studies to the understanding of organosilicon reaction mechanisms

Abstract Several cases in which gas kinetic studies have elucidated mechanistic problems in organosilicon chemistry are described and discussed, in the hope of encouraging wider use of simple gas kinetic techniques.

Gas-phase photochemical reactions of dodecamethylcyclohexasilane with silicon compounds. kinetics of some insertion reactions of dimethylsilylene

Attempts to measure the kinetics of gas-phase insertion reactions of dimethylsilylene, generated by photolysis of dodecamethylcyclohexasilane, are described. Insertion of dimethylsilylene into silicon—hydrogen bonds was the main reaction with trimethylsilane, pentamethyldisilane, and sym-tetramethyldisilane; in all cases the activation energy for insertion was zero, and the rate constants were in the ratio of 1 : 3.1 : 4.3. Dimethylsilylene also inserted cleanly into hydrogen chloride with an activation energy of 28 kJ mol−1. Photochemical reactions with methylchlorosilanes were much more complex, involving little or no silylene chemistry; such reactions as did occur appeared to proceed almost entirely by radical mechanisms.

The Kinetics and Mechanism of the Pyrolysis of Manganese and Manganese Silicide CVD Precursors

The gas-phase pyrolysis of methyl pentacarbonyl manganese and acetyl pentacarbonyl manganese, both alone and in the presence of trimethyl silane, has been investigated using stirred flow reactor kinetic measurements and IR laser powered pyrolysis, together with electron spin resonance (ESR) detection of radical intermediates. For methyl pentacarbonyl manganese alone, all observations support a mechanism initiated by methyl–manganese bond homolysis, whereas the initial step in the acetyl compound involves CO loss and methyl migration. The addition of trimethylsilane sensitizes decomposition of MeMn(CO)5, yielding methane and trimethylsilyl pentacarbonyl manganese; on the other hand, Me3SiH has little effect on the rate of decomposition of MeCOMn(CO)5. A mechanism based on methyl migration is proposed to account for these observations.

A simple technique for gas kinetic studies in organosilicon chemistry

Abstract A batch stirred-flow technique, although simple and inexpensive to build and operate, is shown to be capable of yielding useful kinetic data, even from pyrolyses of considerable mechanistic complexity. Details are given in the hope of encouraging organosilicon chemists to make wider use of kinetics in elucidating reaction mechanism.