NUCLEAR-SPIN SPIN COUPLING VIA NONBONDED INTERACTIONS .6. F-F COUPLING THROUGH AN INTERVENING PHENYL GROUP

Evidence is presented for a novel type of “through-space” nuclear spin-spin coupling involving two fluorine atoms that are intramolecularly crowded against opposite sides of an intervening group X. The first example of this new phenomenon of F-X-F coupling involves the interaction of F-l and F-8 through the intervening C-9 phenyl group in 1,5,8-trifiuoro-9,10-diphenylanthracene (1) with a coupling constant of 71 g= 6.4 Hz. The “through-bond” component of this coupling isestimated as 1.1 Hz on the basis that J1,8 = 1.1 Hz for 1,5,8-trifluoroanthracene (2). The fact that the magnitude of J1,8is significantly larger in 1 than in 2 is attributed to a novel coupling mechanism in 1 involving overlap interactions between the in-plane 2p lone-pair orbitals on the two fluorine atoms and the nearly isoenergetic lowest energy it π molecular orbital onthe C-9 phenyl group. To rationalize the small value of J1,8 = 0.8 Hz for 1,5,8-trifluoroanthraquinone (3), it is argued that the in-plane 2p lone-pair orbital on the oxygen atom of the C-9 carbonyl group, which would be the relevant intervening orbital for F-X-F coupling, is much higher in energy than the fluorine in-plane 2p lone-pair orbitals, and this energy mismatch allowsonly a weak interaction between these two types of orbital. © 1990, American Chemical Society. All rights reserved.