A theoretical investigation on the Wurster's crown analogue of 18-crown-6

An ab initio, quantum mechanical study of the Wurster's crown analogue of 18-crown-6 and its interactions with the alkali metal cations are presented. This study explores methods for accurately treating large, electron-rich species while providing an understanding of the molecular behavior of a representative member of this class of crowns. The molecular geometries, binding energies, and binding enthalpies are evaluated with methods similar to those reported for the analysis of 18-crown-6 and its alkali metal complexes to facilitate direct comparison. Hybrid density functional methods are applied to gauge the effects of electron correlation on the geometries of the electron-rich phenylenediamine moiety present in the Wurster's crowns. While the structure of the crown ether backbone is largely unperturbed by the incorporation of the redox active functionality, the alkali metal binding enthalpies are uniformly stronger for the Wurster's crown complexes, adding 1.8 to 5.1 kcal/mol to the strength of the interaction, depending on cation type. The additional strength, due to the exchange of an oxygen donor atom in the crown ether backbone by a nitrogen donor supplied by the redox group, is tightly coupled to the rotation of the dimethylaminophenyl group with respect to the plane of the macrocycle. Gas-phase selectivities favor the more highly charge-dense cations, while the explicit addition of only a few waters of hydration in the calculations recovers the selectivities expected in solution. The alkali metal binding affinity to the singly oxidized Wurster's crown is significantly diminished, while it is completely eliminated for the doubly oxidized ligand.