John Patrick Cannady

The gas-phase reaction of silylene with acetaldehyde Part 1. Directrate studies, isotope effects, RRKM modelling and ab initio studiesof the potential energy surface

Time-resolved studies of the title reaction, employing both SiH2 and SiD2, have been carried out over the pressure range 1–100 Torr (with SF6 as bath gas) at five temperatures in the range 297–599 K, using laser flash photolysis to generate and monitor both silylene species. The second order rate constants obtained were pressure dependent indicating that the reaction is a third-body assisted association process. The high pressure rate constants, obtained by extrapolation, gave the following Arrhenius parameters: log(A/cm3 molecule−1 s−1) = − 10.10 ± 0.06, Ea = − 3.91 ± 0.47 kJ mol−1, where the uncertainties are single standard deviations. The parameters are consistent with a fast association process occurring at close to the collision rate. RRKM modelling, based on a transition state appropriate to formation of a three-membered ring product, 3-methylsiloxirane, and employing a weak collisional deactivation model gives reasonable fits to the pressure dependent curves for ΔH°/kJ mol−1 in the range − 215 to − 245. Ab initio calculations at the G2 level indicate the inital formation of a silacarbonyl ylid which can then either form the siloxirane by ring closure, rearrange to form siloxyethene or give ethoxysilylene. Fuller details of the potential surface are given. The energetics are reasonably consistent with siloxirane formation representing the main pathway. The isotope effects are small and close to unity, indicating that secondary isotopic label scrambling, by the reversible ring opening of the siloxirane to ethoxysilylene is not occurring. Differences with the silirane system can be explained by the stabilization of a silylene by an alkoxy substituent.

A mechanistic study of cyclic siloxane pyrolyses at low pressures

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethyl cyclopentasiloxane (D5), and the results interpreted in the context of various kinetic models. In particular, it is shown that the significant pyrolysis products--which include CH3, CH4, C2H2, C2H4, C2H6 and SiO--may be satisfactorily accounted for by radical reactions involving dimethylsiloxane (D1), and estimates are made of the various chain lengths for the proposed reactions based on a range of ambient conditions.

A mechanistic study of the low pressure pyrolysis of linear siloxanes

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of 1,1,3,3-tetramethyldisiloxane (L1), 1,1,3,3,5,5-hexamethyltrisiloxane (L2) and 3H,5H-octamethyltetrasiloxane (L3) at ca. 1000 K in a flow reactor at low pressures. The hydrocarbons CH3, CH4, C2H2, C2H4, and C2H6 were observed as prominent pyrolysis products in all three systems, and amongst the weaker features are bands arising from the methylsilanes Me2SiH2 (for L1 and L2) and Me3SiH (for L3). The fundamental of SiO was also observed very weakly. By use of quantum chemical calculations combined with earlier kinetic models, mechanisms have been proposed involving the intermediacy of silanones Me2Si=O and MeSiH=O. Model calculations on the decomposition pathways of H3SiOSiH3 and H3SiOSiH2OSiH3 show that silanone elimination is favoured over silylene extrusion.