Four-center reactions: A computational study of collisional activation, concerted bond switching, and collisional stabilization in impact heated clusters

Molecular dynamics simulations are presented showing that four-center reactions with high activation barriers occur readily, and in a concerted manner, under conditions of cluster impact. At the high velocities which prevail inside an impact heated cluster, we show that one can conveniently examine the reactive event as a sequence of elementary, suddenlike steps. The importance of the timing of these steps is emphasized. The bond-switching step is described by a kinematic model, which is shown to account for the energetic and steric requirements and for energy disposal of the concerted four-center reaction. Three different cases are examined in detail: the bimolecular N₂ + O₂ and H₂ + I₂ reactions and the unimolecular norbornadiene → quadricyclane isomerization. The cluster is shown to have a significant role not only in activating the reactants but, equally important, in stabilization of the energy-rich nascent products. The event is over when the cluster beings to fragment, which occurs after a small number of collisions of its constituents. Comparison is made between clusters of different rare gases. In terms of the overall reactivity, all rare gases are similar and the role of the mass can be compensated by a suitable scaling of the initial velocity of impact. There are, however, differences related to the mass ratio of the reactants to that of the rare-gas atom. The high yield of the four center concerted mechanism in impact heated clusters and the very essential role of the cluster in overcoming the constraints on such a mechanism suggest that the designation "cluster catalyzed reaction" is appropriate.