On the solid-state conformations of 18-crown-6 complexes

A procedure for displaying macrocylic torsion angles as a map on polar coordinates is discussed with reference to the solid-state conformations of l8-crown-6 and its complexes. The maps aid in comparisons of related structures, in the perception of pseudo-symmetry elements, and in the classification of the conformations of 18-crown-6. Only four conformational groups are found in the 1 : 1 complexes of 18-crown-6 with sodium, potassium, rubidium, cesium, thallium(I), calcium and strontium cations. The relationship of donor number, mean cavity radius and effective ionic radius combined with skeletal drawings of the donors and the polar map of the torsion angles provide a composite picture of the structures and insight into the balance between cation-donor interaction energy and conformational energy.This paper is dedicated to the memory of the late Dr C. J. Pedersen.