Studies of methylene chemistry by pulsed laser-induced decomposition of ketene. Part 2.—Ketene in the presence of ethylene and acetylene

Abstract

Gaseous mixtures of ketene with C₂H₄, and with C₂H₂, diluted in added argon to a total pressure of 400 Torr have been photodecomposed at room temperature by 308 nm u.v. radiation from a pulsed exciplex laser. Conversions were limited to ca. 2% and a reasonably complete hydrocarbon product analysis achieved (H₂ was also monitored). In the C₂H₄ system products up to C₅ were detected whilst in the C₂H₂ system products up to C₆ were found. The product distributions suggest the intermediacy of ethyl and vinyl radicals (C₂H₄ system) and vinyl and propynyl (C₃H₃) radicals (C₂H₂ system) as well as the involvement of methylene (CH₂, both ¹A₁ and ³B₁ states), methyl, methyne and H atoms in both systems. Most of the products arising from the C₃H₃ radicals are reported for the first time in this system.

Kinetic modelling based on the Gear algorithm has been used to predict the product distribution and its dependence on either C₂H₄ or C₂H₂ pressures. The modelling suggests the following. (a) The added gases scavenge CH₂ predominantly, if not exclusively, in its ¹A₁ state with the following rate constants (in cm³ molecule^–1 s^–1)¹CH₂+ C₂H₄→ C₃H₆ 2.1 × 10^–10, ¹CH₂+ C₂H₂→ C₃H₄ 2.5 × 10^–10. (b) The CH₂(³B₁) analogues of these processes are probably not occurring and upper limits to the rate constants (same units) are ³CH₂+ C₂H₄→ C₃H₆ <5 × 10^–16, ³CH₂+ C₂H₂→ C₃H₄ <2.6 × 10^–16. (c) C₃H₃ radicals are formed via the decomposition of vibrationally excited C₃H₄, with a rate constant, k= 4.5 × 10⁸ s^–1, consistent with an R.R.K.M. theoretical estimate. (d) Most of the remaining products in both systems are formed via radical–radical reactions with plausibly high collision efficiencies. These findings are compared and contrasted with previous flash-photolysis and other studies of ketene photodecomposition.