Gas-Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory

Reactions of the hydroxyl radical with propene and 1‐butene are studied experimentally in the gas phase in a continuous supersonic flow reactor over the range 50≤T/K≤224. OH radicals are produced by pulsed laser photolysis of H2O2 at 266 nm in the supersonic flow and followed by laser‐induced fluorescence in the (1, 0) A2Σ+←X2Π3/2 band at about 282 nm. These reactions are found to exhibit negative temperature dependences over the entire temperature range investigated, varying between (3.1–19.2) and (4.2–28.6)×10−11 cm3 molecule−1 s−1 for the reactions of OH with propene and 1‐butene, respectively. Quantum chemical calculations of the potential energy surfaces are used as the basis for energy‐ and rotationally resolved Rice–Ramsperger–Kassel–Marcus calculations to determine the rate constants over a range of temperatures and pressures. The negative temperature dependences of the rate constants are explained by competition between complex redissociation and passage to the adducts by using a model with two transition states. The results are compared and contrasted with earlier studies and discussed in terms of their potential relevance to the atmosphere of Saturn. High‐pressure limiting rate constants for the OH+propene and OH+ 1‐butene reactions are determined by experimental and theoretical methods and are found to increase monotonically with decreasing temperature (see picture).