AN AB-INITIO INVESTIGATION OF THE STRUCTURE AND ALKALINE-EARTH DIVALENT-CATION SELECTIVITY OF 18-CROWN-6

We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkaline earth dications Mg2+, Ca2+, Sr2 +, Ba2+, and Ru2+. Geometries, binding energies, and binding enthalpie s are evaluated at the restricted Hartree-Fock (RHF) and second-order Moller-Plesset perturbation (MP2) levels of theory using the 6-31+G b asis set and relativistic effective core potentials. The affinity of 1 8c6 for the dications is considerable, ranging from 150-300 kcal mol(- 1). The cation-18c6 interaction arises principally from the electrosta tic interaction of the dication with the nucleophilic ether backbone a nd from the polarization of the crown ether by the electric field of t he dication. Whereas Ba2+ selectivity is observed for 18c6 in aqueous environments, our calculations clearly show that the crown ether in fa ct binds Mg2+ most strongly in gas phase. Thus, solvation effects appe ar to strongly influence cation selectivity. Indeed, Ba2+ selectivity is recovered when we consider the competition of the solvent and 18c6 molecules for the dications using a simple cation exchange reaction.