The solvent dependence of fluorine chemical shifts. Part II. Anisotropic and polar solvents

Abstract

The ¹⁹F chemical shifts of a number of fluorinated compounds have been measured in anisotropic and polar solvents, and thence the gas to solution chemical shifts obtained for these systems. For the non-polar fully fluorinated solutes studied (CF₄, C₄F₈, C₆F₁₄) these gas to solution shifts are shown to agree with the van der Waals mechanism and to result from a product of solvent (S^v) and solute (S^u) contributions. This also allows the solvent anisotropic contribution to be obtained. For benzene and carbon disulphide these are –0·25 and +0·5 p.p.m., comparable with those obtained from ¹H n.m.r. studies. The polar solutes in these solvents deviat from this formulation indicating more specific effects. In the case of the polar solvents large systematic upfield deviations from the solvent × solute contributions are observed for the other solutes. These deviations, which may be defined as the electric field contribution are shown to depend on the solvent dielectric constant but to have no relationship with the solute dipole moment. Previous interpretations of ¹⁹F polar solvent shifts in terms of a time independent electric field dependence plus a reaction field model are shown to be inadequate. It is suggested that the dipolar fields of the solvent molecules may contribute directly to these large shifts via the 〈E²〉 mechanism.