Linear solvation energy relationships. A scale describing the "softness" of solvents

The μ-scale of solvent softness is defined as the difference between the mean of the Gibbs free energies of transfer of sodium and potassium ions from water to a given solvent and the corresponding quantity for silver ions (in kJ mol^-1) divided by 100. Values are tabulated for 34 solvents. The μ-scale is compatible with a combination of the vibration frequencies of the C-I bond in ICN and the O-H bond in phenol, providing semiquantitalive μ-values for a further 60 solvents. The μ-scale can be used in linear solvation energy correlations, for instance for the transfer of ions into soft solvents, such as N,N-dimethylthioformamide, tetrahydrothiophene, or pyridine.