Kinetics and Mechanism of the IO + ClO Reaction

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1997-05, Vol.101 (22), p.4088-4096
• Main Authors: Bedjanian, Yuri, Le Bras, Georges, Poulet, Gilles
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp963947p

The kinetics and mechanism of the reaction between IO and ClO radicals have been studied by the mass spectrometric discharge-flow method at T = 298 K and at total pressure around 1 Torr. The value of the overall rate constant k 1 = (1.1 ± 0.2) × 10-11 cm3 molecule-1 s-1 has been determined under pseudo-first-order conditions using an excess of ClO over IO radicals. The quantitative detection of the reaction products OClO, Cl, and ICl allowed for the determination of the branching ratios (0.55 ± 0.03), (0.25 ± 0.02), and (0.20 ± 0.02) for the channels producing I + OClO, I + Cl + O2, and ICl + O2, respectively. In addition, the rate coefficient (3.2 ± 0.5) × 10-13 cm3 molecule-1 s-1 has been measured at room temperature for the reaction I + Cl2O → ICl + ClO. The kinetic data obtained allow for a redetermination of the heat of formation of IO:  ΔH f(IO) = (27.7 ± 1.2) kcal mol-1 (based on the current value of ΔH f(OClO) = (22.6 ± 1.0) kcal mol-1). The data obtained indicate that the reaction IO + ClO, which could be involved in stratospheric ozone depletion if iodine is present in the stratosphere, is much less important than assumed in previous model calculations.