Gazing Up at a Cone

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Science  23 Jul 2010:
Vol. 329, Issue 5990, pp. 370-371
DOI: 10.1126/science.329.5990.370-d

Chemical reaction trajectories tend to be largely confined to a single electronic potential energy surface. Occasionally, though, two surfaces intersect when vibrations on the lower surface become vigorous enough to match the energy level of the higher one. Such conical intersections (so termed because the surfaces taper to a cone in their vicinity) are often invoked to explain reaction dynamics but are rarely observed directly. Lim and Kim offer an unusually direct glimpse of a conical intersection in their study of the photolytic cleavage of the PhS-CH3 bond in thioanisole (Ph is phenyl). Specifically, they find that optically populating the first excited state leads predominantly to the production of an electronically excited PhS radical, yet in the narrow vicinity of a vibrational state 722 cm−1 above the excitation threshold, there is a sudden burst of ground-state PhS product. The presence in this energy regime of a conical intersection between the first and second excited states, associated with S-C stretching vibrations, appears to account for this behavior.

Nat. Chem. 2, 10.1038/nchem.702 (2010).

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