Relationships among solvent softness scales

Quantitative measures of solvent softness, in the sense of Pearson's hard-s oft acid-base (HSAB) concept, are considered using three scales: the mu sca le, based on the Gibbs energies of transfer of selected ions, the Raman wav enumber shift of HgBr2 (the D-s scale), and the difference between the IR w avenumber shift of the C-I stretch of ICN and the O-H stretch of phenol. Ra man 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 betwee n the three scales are obtained that permit the prediction of the softness of a wide range of solvents. A number of anomalies encountered are discusse d.