Quantum statistical study of O+O2 isotopic exchange reactions:: Cross sections and rate constants

Using a wave packet based statistical model, we compute cross sections and thermal rate constants for various isotopic variants of the O + O-2 exchange reaction on a recently modified ab initio potential energy surface. The calculation predicts a highly excited rotational distribution and relatively cold vibrational distribution for the diatomic product. A small but important threshold effect was identified for the 160 + 1802 reaction, which is suggested to contribute to the experimentally observed negative temperature dependence of the rate ratio, k(O-18 + O-16(2))/k(O-16+ O-18(2)). Despite reasonable agreement with quasiclassical trajectory results, however, the calculated thermal rate constants are smaller than experimental measurements by a factor from 2 to 5. The experimentally observed negative temperature dependence of the rate constants is not reproduced. Possible reasons for the theory -experiment discrepancies are discussed.