Roaming Radical Reactions
An unusual decomposition mechanism of highly vibrationally excited molecules, dubbed the “roaming mechanism,” has recently been discovered and is now a very 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 A, “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 unexpected products often with 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 coworkers. Many other examples of roaming dynamics have now been reported, both in experiment and theory. I will emphasize recent results documenting “roaming-mediated isomerization” which appears to be a general feature of the decomposition of nitro compounds. In addition, I will introduce new universal probes of reaction dynamics affording isomer and even conformer selectivity.