Hydrogen bond descriptors and other properties of ion pairs

Descriptors, including those for hydrogen bond acidity and hydrogen bond basicity, have been obtained for some 80 ion pairs, including those for the alkali halide series, sodium carboxylates, and protonated base chlorides. These descriptors are on the same scales as those for ions and neutral molecules, so that equations can be constructed that include all these species together. It is shown that ion pairs are both strong hydrogen bond acids and strong hydrogen bond bases. The descriptors for the ion pairs can be used to predict ion pair partition coefficients from water to a number of solvents, there being good agreement with experiment. It is also shown that for partition into polar phases or into the polar parts of phases such as liposomes and membranes, it is likely that ions and not ion pairs are involved. Only for partition into rather nonpolar membranes will ion pairs be partitioned preferentially over the corresponding ions. The diffusion coefficient in water for an ionic species is about the same as for the neutral species, and diffusion coefficients for a pair of ions are calculated to be slightly greater than for the ion pair.