Solvent Effects on H–F Couplings: Dipole Orientation Requirements for Solvent Dependence of Coupling Constants

Solvent-dependent variations of 2JHF, cis 3JHF, and trans 3JHF have been observed in trifluoroethylene and vinyl fluoride. These data, in conjunction with previous observations, indicate that orientation of the solute dipole strongly affects the solvent dependence of geminal coupling constants. The electric field of the solvent appears to be the primary causative factor, but strong molecular associations (e.g., H bonding) may either enhance or decrease the observed effect. The limited results available suggest that the absolute sign of 2JHF may be obtained by observing the change of 2JHF as a function of solvent.