High-temperature kinetics of propyne and allene: Decomposition vs. isomerization

The kinetics of the unimolecular dissociation of propyne and allene, C ₃H₄ + M → C₃H₃ + H + M, was investigated behind reflected shock waves at temperatures between 1400 and 2150 K and at pressures near 0.3, 1.3, 2.6 (propyne only), and 4.0 bar with argon as bath gas. Rate coefficients were obtained from the initial slope of the hydrogen-atom concentration-time profiles monitored with atomic resonance absorption spectroscopy at the Lyman α wavelength (121.6 nm). Within the experimental uncertainty (±30%), identical rate coefficients for propyne and allene decomposition were obtained, indicating a fast mutual isomerization. The dissociation reactions are shown to be in the low-pressure limit with a bimolecular rate coefficient kbim(T)=(3.4±1.0)×1012exp[-(36, 300±400)K/T]bar-1s-1. From a combination of our experimental results with kinetic data from the literature, we infer the following temperature and pressure dependence of the rate coefficient, which reproduces most of the experimental data at temperatures between 1200 and 2400 K and pressures between 0.1 and 5 bar better than within a factor of two: k(T,P)=2.58×1041(T/K)-7. 81exp(-50,590K/T)(P/bar)s-1. This corresponds to a bimolecular rate coefficient in concentration units of kbim(T)=3.56×1019(T/K)-6.81exp(-50,590K/T) cm3s^-1.