Roaming Around the Saddle

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Science  10 Nov 2006:
Vol. 314, Issue 5801, pp. 895-897
DOI: 10.1126/science.314.5801.895d

Transition state theory has proven to be a powerful framework for understanding and predicting chemical reaction kinetics. A central tenet of the theory is passage of each productive reaction trajectory through a specific configuration, or transition state, corresponding to a potential energy saddle point. Rate models therefore tend to focus on determination of this configuration. Recently, however, experimental and theoretical analyses of formaldehyde (CH2O) dissociation implicated a pathway that skirted the transition state and instead relied on the roaming or wandering motion of one H atom about the HCO core.

Houston and Kable have observed evidence of a similar roaming mechanism in the photo-induced dissociation of an acetaldehyde (CH3CHO) sample to CO and CH4. By resolving the rotational states of the CO product using laser-induced fluorescence, they found that ∼15% of the dissociation events distributed an unexpectedly large proportion of the excitation energy to the methane co-product. To explain this outcome, the authors favor a mechanism involving a roaming methyl group, though they note that theoretical simulations will be necessary to rule out an alternative higher-energy H-atom roaming mechanism. — JSY

Proc. Natl. Acad. Sci. USA 103, 16079 (2006).

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