Reacting By Roaming

Abstract

Transition-state theory is one of the bedrocks of chemistry. Even introductory students might see a plot of potential energy versus reaction coordinate, with reactants on the left, products on the right, and a high energy barrier in between. But researchers increasingly see dissociation or decomposition reactions bypassing their conventional transition-state routes in favor of what’s called a “roaming” mechanism. In roaming, an atom or group pulls away from the rest of a molecule, as if to dissociate the molecule into two radical species. But instead of fully separating, the mobile piece moves out just enough to give itself some freedom. It can then move about in the vicinity of the remaining atoms. In formaldehyde photodissociation, for example, a C–H bond may stretch out to about 3 to 4 A—more than three times its normal length—allowing the hydrogen to start roaming around. If it then encounters the other hydrogen and abstracts ...