Molecular mechanics study of 18-azacrown-6 and its binding interactions in 1 : 1 host–guest complexes with neutral and anionic species

Abstract

The stereochemical preferences of 18-azacrown-6 have been studied by molecular mechanics and semiempirical quantum mechanics. Eight possible conformations were considered. Molecular mechanics results showed that several of these conformations had similarly low energies but that the order of preferences were dependent upon the charge dipole used for the C—N and N—H bonds. Calculations were also carried out for the [H₃(18-azacrown-6)]³⁺ cation in the presence of silicic and boric acids and carbonate ion. All three guests could be attached to the host via the formation of intermolecular hydrogen bonds. The energy of the combined species was considerably less than that of the sum of the individual species in all three cases. For boric acid and the carbonate anion the difference was ca.– 13.0 kcal mol^–1, but for silicic acid where three strong hydrogen bonds could be formed the difference was – 26.4 kcal mol^–1.