Solvation of negative ions by protic and aprotic solvents. Information from gas phase ion equilibria measurements

Abstract

Measurement of the gas phase ion equilibria between ions M and solvent molecules Sl provide binding energies of the complexes M^±(S1)_n(for n= 1 to 6). Comparison of these data with single ion energies of solvation shows that differences in ion solvation in solution are reflected in the binding energies of ion-molecule complexes in the gas phase. The weaker solvation of negative ions (relative to positive ions) observed in liquid aprotic solvents is reflected in the binding energies of negative ion aprotic molecule complexes, a weaker binding being found for the first and subsequent few aprotic molecules. An analysis of the bonding in Cl^–(CH₃CN) and K⁺(CH₃CN) shows that the weaker bonding to Cl^– is due to the very diffuse distribution of the positive pole of the dipole in acetonitrile. In effect the dipole can not come close to the negative ion. Analysis shows a similar picture also for acetone. Experimental results for the bonding between Cl^–HR are given for a variety of compounds HR. These show that for RH = protic compounds, like oxygen acids, the hydrogen bond in Cl^–HR increases with the acidity of HR. For aprotic compounds, i.e., carbon acids, no relationship between the bond in the complex and the acidity of HR is found. An examination of the solvation of substituted phenoxide ions by protic and aprotic solvents shows that solvation by protic solvents is adversely affected by charge dispersal in the ion, while aprotic solvents are much less sensitive to charge dispersal. The reasons for this important difference in behaviour are examined.