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Abstract

The isotope dependence of the O+O₂ exchange reaction is investigated by means of kinetic experiments and classical trajectory calculations on an accurate potential energy surface. The measurements confirm the previously reported negative temperature dependence and yield the rate coefficients for both the exothermic ¹⁸O+¹⁶O₂->¹⁸O¹⁶O+¹⁶O and the endothermic ¹⁶O+¹⁸O₂->¹⁶O¹⁸O+¹⁸O reaction between 233 and 353 K: k₈=(3.4±0.6)x10^-12 (300 K/T)^1.1±0.5 cm³s^-1 and k₆=(2.7±0.4)x10^-12 (300 K/T)^0.9±0.5 cm³s^-1. In addition, the ratio of these two rates, R, has been measured with comparatively higher precision. It is 1.27±0.04 at 300 K and also shows a distinct negative temperature dependence. Four types of classical trajectory calculations are performed in order to interpret the experimental result. They differ by the way in which the quantum mechanical zero-point energy of the reactants and the differences of zero-point energies between reactants and products, ΔE_ZPEapproximate to±22 cm^-1, are phenomenologically incorporated. Only calculations which account for ΔE_ZPE are successful in reproducing the temperature dependence of R. This emphasizes that ΔE_ZPE is an important factor in low-temperature ozone kinetics. At energies slightly above threshold a dynamical effect is found which additionally favors the exothermic over the endothermic reaction.