Kinetic Study of OH + OH and OD + OD Reactions

Abstract

The kinetics of the disproportionation reactions of OH and OD radicals, OH + OH → O + H2O (1), OD + OD → O + D2O (3), have been studied by the mass spectrometric discharge-flow method at temperatures between 233 and 360 K and at total pressure of 1 Torr of helium. The following Arrhenius expressions were obtained: k 1 = (7.1 ± 1.0) × 10-13exp[(210 ± 40)/T] and k 3 = (2.5 ± 0.5) × 10-13exp[(170 ± 60)/T] cm3 molecule-1 s-1. Reaction 4 between OH and OD radicals was also investigated and the same rate constant as for reaction 1 was measured. Both the observed temperature dependence of k 1 and the measured kinetic isotopic effect (k 1/k 3) are compatible with the mechanism proposed in a previous theoretical study (Harding, L. B.; Wagner, A. F. 22nd International Symposium on Combustion, 1988). In addition, a temperature-independent rate coefficient of (1.20 ± 0.25) × 10-10 cm3 molecule-1 s-1 was measured for the reaction D + NO2 → OD + NO in the temperature range 230−365 K.

Remaining uncertainties in the kinetic mechanism of hydrogen combustion

“…suggested by Wooldridge et al [59], has strong nonArrhenius dependence, supported by the measurements at high [60] and at low [61] temperatures. Baulch et al [12] proposed a new expression almost indistinguishable from the expression of Wooldridge et al [59] and estimated uncertainty of a factor of 1.5.…”

Kinetics and mechanism of the reaction of Cl atoms with HO2 radicals

“…The possible influence of secondary chemistry on this procedure of absolute calibration of HOBr (and OH) signals was discussed in details in previous articles [17,21]. ClO radicals, which were found as the products of reaction (1), were detected at their parent peak (ClO ϩ , m/e ϭ 51).…”

Kinetics and mechanism of the reaction of OH with ClO

“…The concentration of Br 2 was determined from the measured flow rate of known Br 2 /He mixtures. The possible influence of secondary chemistry on this detection method and on the OH calibration procedure was discussed in detail in previous papers [15,16]. In the experiments conducted on the mechanistic study of reaction (1), where high concentrations of OH were used, OH radicals were directly detected at their parent peak (m/e = 17) with subtraction of the contribution of H 2 O.…”

“…The good agreement (typically within a few percent) between the experimental and simulated data (for all the species except HCl) was achieved without varying any kinetic parameters. The only variable parameter, the rate constant of reaction (1b), was determined from the best fit to the kinetics All rate constants are from [2], except HO 2 + Cl [22], OH + OH [16], H + Cl 2 [19], and wall loss rates measured in this work.…”