Roaming Atoms and Radicals: A New Mechanism in Molecular Dissociation

The detailed description of chemical reaction rates is embodied in transition state theory (TST), now recognized as one of the great achievements of theoretical chemistry. TST employs a series of simplifying assumptions about the dynamical behavior of molecules to predict reaction rates based on a solid foundation of quantum theory and statistical mechanics. The study of unimolecular decomposition has long served as a test bed for the various assumptions of TST, foremost among which is the very notion that reactions proceed via a single well-defined transition state. Recent high-resolution ion imaging studies of formaldehyde unimolecular decomposition, in combination with quasiclassical trajectory calculations from Bowman and coworkers, have shown compelling evidence, however, for a novel pathway in unimolecular decomposition that does not proceed via the conventional transition state geometry. This “roaming” mechanism involves near dissociation to radical products followed by intramolecular abstraction to give, instead, closed shell products. This phenomenon is significant for a number of reasons: it resists easy accommodation with TST, it gives rise to a distinct, highly internally excited product state distribution, and it is likely to be a common phenomenon. These imaging studies have provided detailed insight into both the roaming dynamics and their energy-dependent branching. The dynamics are dominated by the highly exoergic long-range abstraction of H from HCO by the “roaming” hydrogen atom. The energy-dependent branching may be understood by considering the roaming behavior as being descended from the radical dissociation; that is, it grows with excess energy relative to the conventional molecular dissociation because of the larger A-factor for the radical dissociation. Recent work from several groups has identified analogous behavior in other systems. This Account explores the roaming behavior identified in imaging studies of formaldehyde and considers its implications in light of recent results for other systems.