Roaming Reaction Pathways Along Excited States

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Science  02 Mar 2012:
Vol. 335, Issue 6072, pp. 1054-1055
DOI: 10.1126/science.1218767

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Transition state theory (TST) describes chemical reactions in terms of “reaction coordinates,” usually the coordinates of atoms involved in breaking and forming bonds. Typically, there is an energetic barrier, the transition state (TS), between reactants and products. In 2004, a reaction mechanism was reported that seemingly defied the tenets of TST (1). In the photodissociation of H2CO, one of the hydrogen (H) atoms “roamed” around the periphery of the HCO core, with no apparent reaction coordinate, and abstracted the other H atom to form H2 and CO. The roaming products showed characteristic product-state distributions, distinct from those arising from a standard TS mechanism. On page 1075 of this issue, Grubb et al. (2) use detailed state-selective correlated experiments, together with theoretical calculations, to show that the photodissociation of NO3 into NO and O2, an important reaction in the atmosphere, occurs via roaming reactions on both the electronic excited state and the ground state of NO3.