Shock wave and modelling study of the unimolecular dissociation of Si(CH3)2F2: an access to spectroscopic and kinetic properties of SiF2.

Abstract

The thermal dissociation of Si(CH3)2F2 was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF2 and CH3 were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit. Comparing modelled and experimental results, an asymmetric representation of the falloff curves was shown to be most realistic. Modelled limiting high-pressure rate constants agreed well with the experimental data. The UV absorption spectrum of SiF2 was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10-23 cm3 (between 195 and 255 nm, base e), the latter being consistent with radiative lifetimes from the literature. Experiments over the range 1900-3200 K showed that SiF2 was not consumed by a simple bond fission SiF2 →SiF + F, but by a bimolecular reaction SiF2 + SiF2 → SiF + SiF3 (rate constant in the range 1011-1012 cm3 mol-1 s-1), followed by the unimolecular dissociation SiF3 → SiF2 + F such that the reaction becomes catalyzed by the reactant SiF2. The analogy to a pathway CF2 + CF2 → CF + CF3, followed by CF3 → CF2 + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF2, analogous data for SiF are also reported.