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Originally published in Science Express on 21 October 2004
Science 12 November 2004:
Vol. 306. no. 5699, pp. 1158 - 1161
DOI: 10.1126/science.1104386
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Reports

The Roaming Atom: Straying from the Reaction Path in Formaldehyde Decomposition

D. Townsend,1,2 S. A. Lahankar,3 S. K. Lee,1,2,3 S. D. Chambreau,3 A. G. Suits,1,2,3* X. Zhang,4 J. Rheinecker,4 L. B. Harding,5 J. M. Bowman4*

We present a combined experimental and theoretical investigation of formaldehyde (H2CO) dissociation to H2 and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasi-classical trajectory calculations performed on a global potential energy surface for H2CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.

1 Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.
2 Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
3 Department of Chemistry, Wayne State University, Detroit, MI 48202, USA.
4 Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, GA 30322, USA.
5 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, USA.

* To whom correspondence should be addressed. E-mail: asuits{at}wayne.edu; jmbowma{at}emory.edu

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Chemical Dynamics Special Feature: Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation.
B. R. Heazlewood, M. J. T. Jordan, S. H. Kable, T. M. Selby, D. L. Osborn, B. C. Shepler, B. J. Braams, and J. M. Bowman (2008)
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Conformationally Controlled Chemistry: Excited-State Dynamics Dictate Ground-State Reaction.
M. H. Kim, L. Shen, H. Tao, T. J. Martinez, and A. G. Suits (2007)
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From the Cover: Photodissociation of acetaldehyde as a second example of the roaming mechanism.
P. L. Houston and S. H. Kable (2006)
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Skirting the transition state, a new paradigm in reaction rate theory.
J. M. Bowman (2006)
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Signatures of H2CO photodissociation from two electronic states..
H. M. Yin, S. H. Kable, X. Zhang, and J. M. Bowman (2006)
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Laser-Initiated Shuttling of a Water Molecule Between H-Bonding Sites.
J. R. Clarkson, E. Baquero, V. A. Shubert, E. M. Myshakin, K. D. Jordan, and T. S. Zwier (2005)
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