Ab initio study of M(+):18-crown-6 microsolvation

The effect of aqueous microsolvation on the relative binding affinities of 18-crown-6 for the alkali metal cations (Li+-Cs+) was studied using second-order perturbation theory and polarized basis sets augmented with diffuse functions to minimize basis set superposition error. A cation exchange reaction involving the replacement of potassium in a cation/crown complex with a different alkali cation contained within a cation/water cluster served as the basis for modeling binding preferences in liquid water. Up through four crown ether waters of solvation (six for Li+ and K+) were considered, in conjunction with cation/water complexes including as many as nine waters. The principal impact of the added waters on the K+ <----> M(+) exchange reaction was to sharply reduce the spread in binding enthalpies among the different elements, narrowing the discrepancy between the theoretical gas phase cluster results and experimental findings obtained in aqueous solutions.