Angle of Attack

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Science  17 Aug 2012:
Vol. 337, Issue 6096, pp. 779
DOI: 10.1126/science.337.6096.779-b

In qualitative terms, chemists have become adept at predicting how reaction probabilities depend on the relative dispositions of two colliding reagents. For instance, if a reactive site is bounded by a large fragment and a small one, the collision partner is more likely to make it past the small one. This steric effect is the basis for much of asymmetric catalysis. A more detailed, quantitative picture of what happens at any particular collision angle is rather harder to capture. Wang et al. have now mapped out the precise three-dimensional steric constraints guiding an elementary chemical reaction: chlorine's abstraction of a hydrogen atom in the vibrationally excited C-H bond of CHD3 to produce HCl. By varying the polarization of the vibrational excitation laser, the authors could effectively choose the alignment of the (partially deuterated) methane reagent relative to the incoming Cl atom beam, after which they used ion imaging to map out the product trajectories and their associated quantum states. The results for the production of vibrationally relaxed HCl are largely in keeping with expectations from the longstanding line-of-centers model in collision theory, whereas pathways leading to vibrationally excited HCl manifest more complex dynamics.

Nat. Chem. 4, 636 (2012).

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