The effects of π-electron distribution and intramolecular electric fields on the19F N.M.R. shielding in substituted perfluorobenzenes

Several workers have attempted to correlate 19F chemical shifts in fluorinated benzene derivatives with the π-electron charge densities and bond orders as calculated by the Hückel molecular orbital method. In all cases chemical shifts of the fluorine nuclei in meta and para positions to substituents could be predicted successfully, but for fluorine nuclei at ortho positions large deviations were encountered. We have shown that such ortho effects in haloperfluorobenzenes can be accounted for satisfactorily in terms of intramolecular electric field contributions.