Cation affinities of [16]starand model.: Comparison with 12-crown-4:: Crucial role of dipolar moiety orientations

[1(6)]Starand appears to be a promising ionophore because of its rigid structure with the spherical cavity into which Li+ can fit perfectly. Using ab initio calculations, we investigated if the starand model has strong affinity as well as high selectivity for Li+, compared to 12-crown-4 of almost the same cavity size. Li+ favors the external binding (binding outside of the cavity) for the [1(6)]starand model. The ion-dipolar moiety interactions are found to be the main factors affecting the preference of external binding in the starand model. When a cation is located at the center of the starand model, the out-of-plane bending angle of a cation from the plane of the ketal moiety is more than 90 degrees, resulting in unfavorable energetics. By the same reasoning, the somewhat flexible 12-crown-4 structure, upon complexation with Li+ and Na+, drastically orients itself into a volcano structure with four oxygen atoms on the top so as to have favorable ion-dipolar moiety orientations with the cation located above the volcano. Therefore, in addition to the host-guest size complementarity, the ion-dipolar moiety orientations should be very important in designing novel ionophores.