C13-F19 coupling constants have been measured inp-substituted fluorobenzenes. It has been shown that they vary linearly with Hammett’s constant (σv). Further the change in the ionic character of the C-F bond in fluorobenzenes calculated on the basis of the change in the coupling constants agrees well with that calculated from F19 chemical shifts.
The coupling constant in trifluoroacetic acid has also been measured in various solvents at several concentrations. A remarkable solvent effect on these couplings has been discovered; this appears-to be related to the dipole moment of the solvent.