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MO: Molekülphysik
MO 18: Stossprozesse, Energietransfer
MO 18.7: Talk
Thursday, March 25, 2004, 18:00–18:15, HS 355
Temperature and Isotope Dependence of the O + O2 Exchange Reaction — •Christof Janssen and Dieter Krankowsky — Max-Planck-Institut für Kernphysik, Postfach 103980, 69029 Heidelberg
Oxygen isotope effects in the formation of ozone are unconventional and very large. Presently a complete and undisputed understanding of these isotope effects is not available, but differences in zero point energy (ZPE) are suggested to play a pivotal role. Isotope exchange reactions, such as the exothermic 18O + 16O2 → 16O + 16O18O (1) and the endothermic 16O + 18O2 → 18O + 16O18O (2) reactions, are closely linked to the dynamics of ozone formation. The study of isotope and/or ZPE effects in these reactions may help to understand the kinetics of O + O2 collisions.
Previous studies, however, have neglected differences between isotopic variants. We have studied the temperature dependence of reactions (1) and (2) between 233 and 353 K. Both exchange rates show a strong positive temperature dependence and agree well with previous measurements: k1 = (3.4±0.6) × 10−12 (300 K/T)1.1± 0.5 cm3 s−1, k2 = (2.7± 0.4)× 10−12 (300 K/T)0.9± 0.5 cm3 s−1. The ratio of these rates k1/k2 = (1.27± 0.4) exp[(50±40) (K/T−1/300)] demonstrates that the exothermic reaction not only is strongly favoured at room temperature, but also that with decreasing temperature its rate coefficient advantage increases. This observation is in agreement with the interpretation that ZPE effects play a crucial role in the isotope selective association/dissociation dynamics of excited ozone. The observed negative activation energy of 50 ± 40 K roughly corresponds to the difference of reaction enthalpies of reactions (1) and (2), which is 65 K.