Determination of solvation descriptors for ionic species: hydrogen bond acidity and basicity

Literature values of Gibbs energies of transfer of ions from water to other solvents have been used in conjunction with our solvation equation to obtain descriptors for univalent ions. It is suggested that descriptors used for nonelectrolytes are not adequate to describe transfers of single ions, and that two specific ionic descriptors (J(+) and J(-)) for cations and anions, respectively, are required. The ions studied include the alkali metal and tetraalkylammonium cations, halide and other anions, and the tetraphenylarsonium, tetraphenylphosphonium, and tetraphenylborate ions. It is shown that simple cations such as Na(+) act as very strong hydrogen bond acids and that the R(4)N(+) ions are only weak hydrogen bond acids. The halide anions are very strong hydrogen bond bases, as is also the acetate anion. Other anions, such as azide, cyanide, and nitrate, are again very strong hydrogen bond bases. The tetraphenylarsonium, tetraphenylphosphonium, and tetraphenylborate ions have no hydrogen bond acidity but are quite strong hydrogen bond bases. It is suggested that this is due to the basic properties of the phenyl groups.