The NMR spectra of some iodo‐ and bromo‐fluorobenzenes. A novel solvent effect on ortho fluorine‐fluorine couplings

The NMR spectral parameters for some bromo‐ and iodo‐substituted fluorobenzenes are presented, and the derived meta ‐ and para ‐ fluorine‐fluorine couplings are shown to be in accord with the predictions made by Abraham et al. , using an additive substituent constant scheme. While the approximation of additive substituent effects is quite good, it is shown, however, that with highly accurate experimental values there are small deviations from perfect additivity. The existence of a large solvent effect on 3 J FF is demonstrated and the mechanism of this effect is discussed. It is emphasized that certain discrepancies in the literature between various values of the same 3 J FF may be due to this large, hitherto unrecognized solvent dependence.