AN AB-INITIO INVESTIGATION OF THE STRUCTURE AND ALKALI-METAL CATION SELECTIVITY OF 18-CROWN-6

We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li+, Na+, K+, Rb+, and Cs+. Geometries, binding energies, and binding enthalpies are eval uated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31+G) and relativistic effective core potentials. E lectron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NBO) and a ssociated methods. The affinity of 18c6 for the alkali metal cations i s quite strong (50-100 kcal mol(-1), depending on cation type), arisin g largely from the electrostatic (ionic) interaction of the cation wit h the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as th e electrostatic interaction. Agreement of the calculated binding entha lpies and experimentally determined quantities is rather poor. For exa mple, the binding energy for K+/18c6 (-71.5 kcal mol(-1)) is about 30 kcal mol(-1) stronger than that determined by experiment, and it is no t clear how to reconcile this difference. Our calculations clearly sho w that solvation effects strongly influence cation selectivity. Gas-ph ase 18c6 preferentially binds Li+, not K+ as found in aqueous environm ents. We show, however, that K+ selectivity is recovered when even a f ew waters of hydration are considered.