Science 335, 1075–1078 (2012)

Transition-state theory has given chemists the framework with which to understand and explain most chemical reactions. Sometimes, however, incongruous experimental results can force chemists to consider other more exotic possibilities. In the past decade, details have emerged of a new mechanism for photodissociation reactions in which an atom, on attempting to break its bond with a small molecule, does not have enough energy to escape. It 'roams' around the remaining fragment before reacting with it further and abstracting another atom to form the final products.

Previously, such reactions were observed to occur on the electronic ground-state potential-energy surface and were always accompanied by a competing reaction pathway that followed a more standard route: that is, through a traditional transition state. Now, Simon North of Texas A&M University and co-workers — studying the photo-dissociation of NO3 to NO and O2 — have observed roaming on the excited-state surface and have shown that there is no route that proceeds via a standard transition state. The reaction was previously understood to occur through two pathways, one of which was shown by North in 2011 to involve roaming in the ground state and one of which, until now, remained uncharacterized. Previous calculations, however, have suggested that this route involved roaming on the excited-state surface.

To confirm this theoretical prediction, North and co-workers looked for signs that the reaction products came from two different electronic surfaces. Such a signature was predicted (using ab initio calculations) to be visible in the orbital symmetry of the NO products and was later observed using ion imaging.