THROUGH-SPACE C-13-F-19 COUPLINGS IN HIGHLY-CROWDED SYSTEMS - RADIAL DEPENDENCE AND ELECTRONIC EFFECTS

The helical molecule 1‐fluorobenzo[ c] phenanthrene shows a large (17.4 Hz) 13C–19F coupling resulting primarily from direct through‐space overlap between a fluorine lone pair and the sterically opposed CH bond. After application of corrections for estimated through‐bond contributions, comparison of the magnitude of this coupling with the analogous couplings in 4‐fluorophenanthrene, 10‐fluorobenzo[ a] pyrene, 11‐fluoro‐5‐methylchrysene and 7‐fluorobenzo[b] fluoranthene reveals an exponential decrease with increasing H … F internuclear distance, consistent with theoretical predictions. No evidence was found for an angular dependence of the coupling. HETCOR experiments showed the signs of n+1J(HF) in 1‐fluorobenzo[ c] phenanthrene, 4‐fluorophenanthrene, 10‐fluorobenzo[ a] pyrene and 11‐fluoro‐5‐methylchrysene to be opposite to those of the corresponding nJ(CF), hence negative. Substituents at both positions 4 (para to fluorine) and 9 (para to the sterically opposed CH bond) exert only a minor influence on 5J(CF), the couplings in the latter series responding largely to sterically induced distortions of the molecular framework, and in the former series to both steric distortions and an electronic effect. The difference in 5J(CF) between isomers is a measure of the electronic effect of the substituent. Electron‐withdrawing substituents para to fluorine reduce the magnitude of the through‐space coupling whereas electron‐releasing substituents increase the coupling. Copyright © 1990 John Wiley & Sons, Ltd.