Isotope dependence of the O+O2 exchange reaction:: Experiment and theory

The isotope dependence of the O+O-2 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 O-18+O-16(2)-->(OO)-O-18-O-16+O-16 and the endothermic O-16+O-18(2)-->(OO)-O-16-O-18+O-18 reaction between 233 and 353 K: k(8)=(3.4+/-0.6)x10(-12) (300 K/T)(1.1+/-0.5) cm(3) s(-1) and k(6)=(2.7+/-0.4)x10(-12) (300 K/T)(0.9+/-0.5) cm(3) s(-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, DeltaE(ZPE)approximate to+/-22 cm(-1), are phenomenologically incorporated. Only calculations which account for DeltaE(ZPE) are successful in reproducing the temperature dependence of R. This emphasizes that DeltaE(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. (C) 2003 American Institute of Physics.