The solvent dependence of fluorine chemical shifts. Part I

Abstract

The ¹⁹F chemical shifts of a variety of organofluorine compounds (including the usual ¹⁹F reference compounds) have been measured in a range of non-polar solvents. These allow elucidation of ¹⁹F reference shifts from CFCl₃(int) in these solvents. The solvent dependence of the ¹⁹F shifts in non-polar media is shown to be a product function of solute and solvent terms in the general case, and the experimental data are transformed into these solvent and solute numbers. The solvent numbers are discussed in the light of current theories of the van der Waals interaction, which are shown to require modification in order to fully explain the solvent shifts. The solvent sensitivity of the ¹⁹F shift in any solute molecule (the solute number) is shown to be due to the extent of the electronic screening of the flurorine from the solvent by the other substituents in the solute molecule. A semi-empirical relationship is presented which quantitatively accounts for the observed results.