HYDROGEN-BONDING AND DIELECTRIC EFFECTS IN SOLVATOCHROMIC SHIFTS

When a dipolar molecule is dissolved in a mixture of two solvents N and P of different static relative permittivity, the solvent shell becomes enriched in the more polar solvent (P). This effect of dielectric enrichment is described quantitatively by the preferential solvation index Z which relates the bulk and local mole ratios X and Y, respectively, according to Y = X exp Z where X = X(N)/X(P) and Y = y(N)/y(P). If the solute molecule can form a hydrogen-bonded complex with the polar solvent P, it is necessary to use two preferential solvation indices Z1 and Z2; Z1 is then related to the dielectric enrichment and Z2 to the energy balance of hydrogen-bond formation. This H-bond preferential solvation index is in general negative, because the energy spent in breaking the solvent structure exceeds the solute-solvent hydrogen-bond energy. The two preferential solvation indices Z1 and Z2 explain the peculiar solvatochromic shifts observed in mixtures of protic and non-protic solvents.