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Author Notes:

To whom correspondence may be addressed: Scott H. Kable (Email: kable_s@chem.usyd.edu.au), David L. Osborn (Email: dlosbor@sandia.gov), or Joel M. Bowman (Email: joel.bowman@emory.edu)

Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved May 15, 2008.

Author contributions: S.H.K., D.L.O., B.J.B., and J.M.B. designed research; B.R.H., M.J.T.J., T.M.S., D.L.O., B.C.S., and B.J.B. performed research; B.R.H., M.J.T.J., S.H.K., D.L.O., B.C.S., B.J.B., and J.M.B. analyzed data; and M.J.T.J., S.H.K., D.L.O., and J.M.B. wrote the paper.

Mr. Howard Johnsen is gratefully acknowledged for excellent technical support.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under Contract DE-AC04-94-AL85000.

The authors declare no conflict of interest.


Research Funding:

This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

J.M.B. and B.C.S. thank the Department of Energy (DE-FG02-97ER14782) and BJB thanks the Office of Naval Research (N00014-05-1-0460) for financial support.

The Sydney group thanks the Australian Research Council (DP0772006) for financial support and an Australian Postgraduate Award (to B.R.H.).


  • reaction dynamics
  • roaming mechanisms
  • photochemistry
  • quasiclassical trajectories
  • transition state

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation


Journal Title:

Proceedings of the National Academy of Sciences


Volume 105, Number 35


, Pages 12719-12724

Type of Work:

Article | Post-print: After Peer Review


Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed “roaming,” has been described in the photodissociation of H2CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH3CHO to CH4 + CO. The CH4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at ≈95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH4 + CO products is via a roaming pathway where a CH3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH4 + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH3 roaming mechanism is attributed to the fact that the CH3 + HCO radical asymptote and the TS saddle-point barrier to CH4 + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H2CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.

Copyright information:

© 2008 by The National Academy of Sciences of the USA

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