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

Townsend, D.
Lahankar, S. A.
Lee, S. K.
Chambreau, S. D.
Suits, A. G.
Zhang, X.
Rheinecker, J.
Harding, L. B.
Bowman, J. M.

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

*To whom correspondence should be addressed. E-mail: [email protected]; [email protected]

Supporting Online Material: www.sciencemag.org/cgi/content/full/1104386/DC1; Movies S1 and S2

23 August 2004; accepted 7 October 2004

Published online 21 October 2004; 10.1126/science.1104386

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Science 306(5699):p 1158-1161, November 12, 2004.

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. Quasiclassical 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.