On the statistical behavior of the O + OH --> H + O2 reaction: a comparison between quasiclassical trajectory,...

The dynamics of the O + OH reaction on the ground state potential energy surface (PES) is investigated by means of the quasiclassical trajectory method and two statistical methods: phase space theory and statistical quantum method. Preliminary calculations with an exact quantum method are also reported. The quasiclassical trajectory calculations show evidence for a phase space bottleneck inhibiting the intramolecular energy transfer between the O-H and O-O bonds. As a result, the probability of the intermediate complex dissociating back toward the reactants is high, thereby yielding a reaction probability significantly lower than expected for a barrierless and exothermic reaction. The features of the PES, which are the cause of this dynamical effect, are identified. This is essentially the conservation of the equilibrium distance of the O-H bond, hardly changed by a close encounter with an oxygen atom. The statistical calculations, which do not take into account the PES in the complex region, yield a high reaction probability, much larger than the probability calculated from the dynamical methods, both classical and quantum. If the statistical cross sections are corrected by a scaling factor, which corresponds actually to scaling the capture probability, then a good agreement is observed between dynamical and statistical calculations of the product state distributions. The differential cross sections calculated with all the methods show a backward-forward symmetry, with sharp polarization peaks. The complex lifetime is divided into two parts by the bottleneck. During the first part, the system remains trapped in a small region of the phase space and has a high probability to dissociate back toward the reactants. This is a nonstatistical effect due to the PES shape. During the second part, fast intramolecular vibrational energy redistribution takes place, leading to a statistical distribution of energy on the rovibrational states of the products. These findings indicate that the O + OH reaction has mixed dynamics, both with statistical and nonstatistical aspects.