A study of the mechanism of the reaction between ozone and the chlorine atom using density functional theory

Density functional calculations of the potential energy surface for the reactions Cl-2 + O-1(3) (-->) 2ClO + O-3(2) and 2ClO + O-3 (-->) Cl-2 + O-3(2) have been carried out using the Perdew--Wang functionals for exchange and for correlation with a variety of basis sets. For the former reaction, a number of stationary points have been identified corresponding to both a trans and a cis pathway. Both pathways have an early transition state, followed by a weakly bound complex, and are connected through a late transition state leading to the products. All stationary points on the surface lie at energies lower than those of the initial reactants. The early transition state and complex are planar in both paths, whereas the late, common transition state is nonplanar. The early transition state is not a transition state for the reaction path but connects two alternative but equivalent ways in which the complex distorts out of the plane. The region between the late transition state and the products is complicated by a mixing of O-2 spin states. The latter reaction has a much simpler reaction surface, involving an early transition state with a very low barrier leading directly to the products with high exothermicity. These DFT results for the former reaction fail to identify an early transition state found by previous investigators (14) using QCISD and CASSCF methods, raising questions about the applicability of DFT methods for transition state calculations. There are, however, some areas of agreement between this and the prior study. However, neither study satisfactorily replicates the experimentally determined classical barrier for the reaction or unambiguously determines whether an intermediate complex ClO-O-2 is present.