Search

Duality of roaming mechanism in H 2 CO The phenomenon of roaming in chemical reactions (that is, bypassing the minimum energy pathway from unlikely geometries) has attracted a great deal of attention in the chemical reaction dynamics community over the past decade and still demonstrates unexpected results. Using velocity-map imaging of state-selected H 2 products of H 2 CO photodissociation, Quinn et al. discovered the bimodal structure of rotational distribution of the other product fragment, CO. Quasiclassical trajectories showed that this bimodality originates from two distinctive reaction pathways that proceed by the trans or cis configuration of O–C–H⋯H, leading to high or low rotational excitations of CO, respectively. Whether such a mechanism is present in the many other chemical reactions for which roaming reaction pathways have been reported is yet to be determined. Science , this issue p. 1592

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.