Low-Temperature Reactions of OH Radicals with Propene and Isoprene in Pulsed Laval Nozzle Expansions

The kinetics of the reactions of OH radicals with propene and isoprene in N 2 have been studied in the temperature range of 58-300 K in a Laval nozzle expansion. Laser-induced fluorescence of the OH radical that is formedin the photolysis of H 2 O 2 at 193 nm has been detected. The determined rate constants (2 × 10 - 1 1 to 2 × 10 - 1 0 cm 3 molecule - 1 s - 1 ) for the OH radicals reacting with excess propene and isoprene (2-methyl-1,3-butadiene) have been found to increase when the temperature is decreased. The room temperature rate constants are in agreement with the literature data. Below 120 K, a saturation of the rate constant for the case of propene and a turnover to a formally positive temperature dependence for isoprene have been observed. The observed negative temperature dependence and the course of the temperature dependence at very low temperatures for both reactions are discussed within the framework of the loose transition-state theory and a simple two-transition-state model.