Low-temperature kinetics of reactions of OH radical with ethene, propene, and 1-butene

The kinetics of the reactions of the CH 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 H2O2 at 248 nm. Laser-induced fluorescence of the OH radicals excited in the (1,0) band of the A(2)Sigma(+)-X(2)Pi(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,infinity) = (8.7 +/- 0.7) x 10(-12)(T/300)((-0.85+/-0.11)) cm(3) molecule(-1) s(-1); k(2,infinity) = (2.95 +/- 0.10) x 10(-11)(T/300)((-1.06+/-0.13)) cm(3) molecule(-1) s(-1); k(3,infinity) = (3.02 +/- 0.15) x 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 H2O into C-O containing molecules.