On the analysis of exit-channel effects in three-atom unimolecular reactions

The key problem of exit-channel effects in unimolecular reactions, which make Transition State Theory (TST) generally unsuitable for the calculation of product state distributions, is analyzed in the triatomic case ABC --> AB + C for a total angular momentum equal to zero. The vibrational energy of BE is supposed to be quasi conserved on the way from the transition state (TS) to the products. Moreover, classical mechanics is used for the description of rotational and translational motions. In this frame, batches of trajectories are run on model potential energy surfaces from the TS to the products. Their initial conditions on the dividing surface associated with the TS are not distributed at random but instead, they form curves the shapes of which are guided by physical considerations. The reflection of these curves on hypersurfaces orthogonal to the reaction path provides worthwhile information about the nature of exit-channel effects. It is shown that the modulus of the rotational angular momentum of AB is more likely to decrease than to increase, the amplitude of the variation being larger on the average ire the first than in the second case. As a consequence, exit-channel effects cause the rotational state distribution to be colder in the products than at the TS, as observed in the reaction O2X --> O-2 + X (X = H, D,T). In addition to that, a slight improvement of a model recently developed by the authors allows the description of exit-channel effects in a satisfying way which might be included in TST in order to go beyond the Phase Space Theory (PST) of product state distributions.