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 S1 provide binding energies of the complexes M *(SO, (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 CI-(CH,CN) and K+(CH3CN) shows that the weaker bonding to C1-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 CI-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.The present work utilizes data obtained from gas phase ion equilibria measure-ments1S2 to provide insights into ion solvation by protic and aprotic solvents. The equilibria (n, n -1) can be measured for various anions Alike F-, C1-, PhO-and cations C + like Na+, K+, H30+, PhNHf and diverse solvent molecules S1 like HzO, NH3, CH3CN etc., A-(Sl),The apparatus used employs mass spectrometric detection of ions which have reached equilibrium in a reaction chamber containing known pressures of the neutral reactants S1. The temperature dependence of the equilibrium constant Knsn-provides, via van't Hoff plots, the desired binding energies AHan,n-l and Early observations that organic ions are very reactive in dipolar aprotic solvents like dimethylsulphoxide (DMSO) and dimethylf~rmamide~ led to the s ~g g e s t i o n ~* ~ that these solvents solvate negative ions less well than positive ions. Studies of the enthalpies of tran~fer,~ activity coefficients and Volta potentials' have confirmed this supposition. When searching for an explanation of this behaviour one encounters the question whether the effect is due to the interactions of the ion with the " closest " solvent molecules or to bulk properties of the solvent. As shown in the Results and Discussion section, gas phase measurements of (n, n -1) equilibria involving positive

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