Fluorine Diverted

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Science  27 Nov 2009:
Vol. 326, Issue 5957, pp. 1164
DOI: 10.1126/science.326.5957.1164-b

The advent of frequency tunable laser sources briefly enticed chemists with the prospect of performing a sort of molecular surgery, whereby energy input at the precise resonance frequency of a bond could be used to break that bond, while leaving the rest of the molecule undisturbed. It quickly became clear, though, that most molecules wouldn't cooperate; the energy tended to spread across the whole molecular framework, despite being introduced at a specific site. Still, selective vibrational excitation would at worst have no impact. Surely it wouldn't hinder subsequent reactivity, would it? Very recently, precisely such a baffling outcome was observed in the reaction of fluorine atoms with trideuterated methane (CHD3): Exciting the C-H bond actually reduced the likelihood of fluorine's breaking it, instead leading to an increased proportion of D-F product (see Zhang et al., Reports, 17 July 2009, p. 303). Czakó and Bowman have now analyzed this reaction system theoretically. Preliminary examination of the potential energy surface suggested that vibration should lower the barrier toward breaking the excited bond, in keeping with intuition. Yet in quasiclassical trajectory simulations of low-energy collisions, inducing C-H bond vibration appeared instead to steer the F atom toward the deuterium centers, thus bolstering the experimental findings.

J. Am. Chem. Soc. 131, 10.1021/ja906886z (2009).

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