Roaming Radical Reactions

An unusual decomposition mechanism of highly vibrationally excited molecules, dubbed the roaming mechanism, has recently been discovered and is now an active area of investigation. In these reactions, a molecule undergoes partial dissociation to radical fragments by simple bond fission. When the fragments separate to 3-4 angstrom, roaming reorientation becomes feasible as the kinetic energy is low and the angular forces may be comparable to the radial forces. If this leads the system to access a distinct reactive domain, intramolecular abstraction may take place, giving molecular products with large vibrational excitation. This pathway may deviate substantially from the nominal minimum-energy path and in some cases appears to avoid the normal transition-state geometry entirely. Many of the details have come to light through high-resolution ion imaging studies of formaldehyde, in concert with quasi-classical trajectory calculations from Bowman and co-workers. Many other examples of roaming dynamics have recently been reported, both in experiment and theory. In this Perspective, we highlight key aspects of roaming reactions and point to some interesting future directions for study.