Low-Temperature Kinetics of Reactions of OH Radical with Ethene, Propene, and 1-Butene

The kinetics of the reactions of the OH radical with ethene ( k 1 ), propene ( k 2 ), and 1-butene ( k 3 ) are studied over a temperature range of T = 96−296 K. The low-temperature environment is provided by a pulsed Laval nozzle supersonic expansion of nitrogen with admixed radical precursor and reactant gases. The OH radicals are produced by pulsed photolysis of H 2 O 2 at 248 nm. Laser-induced fluorescence of the OH radicals excited in the (1,0) band of the A 2 Σ + −X 2 Π i transition is used to monitor the OH decay kinetics to obtain the bimolecular rate coefficients. At T = 296 K, the rate constants k 1, k 2, and k 3 are also measured as a function of total pressure. The room-temperature falloff parameters are used as the basis for extrapolation of the low-temperature kinetic data, obtained over a limited range of gas number density, to predict the high-pressure limits of all three rate coefficients at low temperatures. The temperature dependence of the measured high-pressure rate constants for T = 96−296 K can be expressed as follows: k 1, ∞ = (8.7 ± 0.7) × 10 -12 ( T /300) (-0.85±0.11) cm 3 molecule -1 s -1; k 2, ∞ = (2.95 ± 0.10) × 10 -11 ( T /300) (-1.06±0.13) cm 3 molecule -1 s -1; k 3, ∞ = (3.02 ± 0.15) × 10 -11 ( T /300) (-1.44±0.10) cm 3 molecule -1 s -1 . All three high-pressure rate constants show a slight negative temperature dependence, which is generally in agreement with both low-temperature and high-temperature kinetic data available in the literature. Implications to the atmospheric chemistry of Saturn are discussed. Incorporating the new experimental data on k 1 in photochemical models of Saturn's atmosphere may significantly increase the predicted rate of photochemical conversion of H 2 O into C−O containing molecules.