Ionization potentials and electron affinities in aqueous solution

Abstract

By combining gas-phase proton affinities and aqueous pK/sub a/ values, it is possible to obtain free energies of hydration for a large number of cations and anions. With some corrections these numbers can be used to convert gas-phase ionization potentials, I, and electron affinities, A, into the corresponding aqueous solution values, I' and A'. An assumption must be made about the free energy of hydration of H/sup +/, taken as -259.5 kcal/mol. The absolute potential of the hydrogen electrode is then 4.50 V, and values of I' and A' can be converted to standard oxidation or reduction potentials. The absolute electronegativity, (I' + A')/2, in solution is estimated and found to be little different from the gas-phase value, (I + A)/2. The absolute hardness, (I'-A')/2, is much smaller than the gas-phase value, but it is shown that the solution value has little utility. The gas-phase value should be used even in solution.