Relationships among solvent softness scales

Quantitative measures of solvent softness, in the sense of Pearson's hard-soft acid-base (HSAB) concept, are considered using three scales: the μ scale, based on the Gibbs energies of transfer of selected ions, the Raman wavenumber shift of HgBr₂ (the D_s scale), and the difference between the IR wavenumber shift of the C-I stretch of ICN and the O-H stretch of phenol. Raman and IR band shifts have been measured for a number of solvents for the neat ones (Raman) and in CCl₄ solution (IR). Good linear correlations between the three scales are obtained that permit the prediction of the softness of a wide range of solvents. A number of anomalies encountered are discussed.