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No Straight Path: Roaming in Both Ground- and Excited-State Photolytic Channels of NO3 → NO + O2

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Science  02 Mar 2012:
Vol. 335, Issue 6072, pp. 1075-1078
DOI: 10.1126/science.1216911

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Exclusive Roaming

How do polyatomic molecules fall apart? The basic model, supported by centuries of chemical theory and experiment, invokes a series of internal rearrangements that lead to a fleeting high-energy transition-state geometry from which lower-energy products emerge. Over the past decade, several molecules have been shown to manifest a competing dissociation mechanism such that alongside trajectories that pass through the transition state, there are energetically accessible pathways that roam around it. Grubb et al. (p. 1075; see the Perspective by Jordan and Kable) showed that the light-induced reaction of NO3 to form NO and O2 proceeded exclusively by roaming. Although there are distinct pathways to the products in two different electronic states, neither one passes through a conventional transition state.