Hydrogen bonding and dielectric effects in solvatochromic shifts

Abstract

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.