4-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 concer ted manner, under conditions of cluster impact. At the high velocities which prevail inside an impact heated cluster, we show that one can c onveniently examine the reactive event as a sequence of elementary, su ddenlike steps. The importance of the timing of these steps is emphasi zed. 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 N-2 + O-2 and H-2 + I-2 reactions and the unimolecular norbornadiene --> quadricyclane isomer ization. The cluster is shown to have a significant role not only in a ctivating the reactants but, equally important, in stabilization of th e energy-rich nascent products. The event is over when the cluster bei ngs to fragment, which occurs after a small number of collisions of it s 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 t he 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 clu sters and the very essential role of the cluster in overcoming the con straints on such a mechanism suggest that the designation ''cluster ca talyzed reaction'' is appropriate.