Dissociation of C3H3I and rates for C3H3 combination at high temperatures

The decomposition of propargyl iodide, C3H3I → C3H3 + I, and the following combination of propargyl radical, C3H3 + C3H3 → C6H6, have been observed in the incident shock in mixtures of 1%, 2%, and 5% C3H3I in Kr, over 800–2000 K, for nominal post-shock pressures of 20 and 130 Torr. The iodide dissociates readily, the endothermic reaction producing an easily resolved positive density gradient whose kinetic modeling allows extraction of precise rates showing strong falloff. A simple Gorin-model RRKM treatment of the measurements results in a k∞ = 3.35 × 1018 T−0.9exp(−22192/T) s−1. The initial positive gradient is immediately followed by a strong negative, showing exothermic reaction from propargyl combination. Modeling of this gradient with an assumed theoretical product distribution, provided by J.A. Miller, results in propargyl combination rates in excellent agreement with current theory and having a strong negative temperature dependence, the rate dropping near a factor of forty over 800–2000 K. Possible errors associated with the need to assume a product distribution are discussed and it is suggested that this should not result in rate uncertainties for the C3H3 combination reaction greater than ±50%.