Tracking State-to-State Bimolecular Reaction Dynamics in Solution

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Science  18 Mar 2011:
Vol. 331, Issue 6023, pp. 1398-1399
DOI: 10.1126/science.1203629

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If two reactant molecules collide with sufficient energy and the geometry between them is right, then the reactants can pass through a transition state to form new chemical products. A “state-to-state” study maps out how distributing energy (such as in different bond vibrations) in the reactant states affects the likelihood of reaction, as well as the corresponding state population of the products. For gas-phase reactions, the product states, rather than being populated statistically (with population falling off exponentially with increasing energy), can instead show higher population in excited states than the ground state. This so-called state inversion is the basis for the powerful HF chemical laser (1). But does this state-to-state behavior matter at all for bimolecular reactions run in liquids? The constant fluctuations of the solvent should randomize the state of the reactant right up to the transition state, and speed the relaxation of product excited states back to a statistical distribution. On page 1423 of this issue, Greaves et al. (2) show that a vibrational state inversion can in fact be achieved in a hydrogen transfer reaction in a common liquid solvent by tuning the time scales of the relaxation processes.