Nuclear magnetic resonance and ab initio theoretical studies of 18-crown-6, benzo- and dibenzo-18-crown-6 and their alkali-metal complexes

The formation of alkali-metal ion complexes with the crown ethers, 18-crown-6(18c6), benzo-18c6 and dibenzo-18c6 has been investigated. Theoretical studies of structural changes on complexation, binding energies and changes in electron distribution have been carried out using ab initio quantum theoretical methods. A complementary study of the NMR chemical shifts and coupling constants has also been used to obtain information on complexation and structural changes. The overall views of the structural changes and their relation to ion selectivity as determined by the theoretical and NMR investigations are in agreement. In most cases the structural parameters found bear a close relation to those obtained in crystallographic work on the complexes, where such results are available. An attempt has been made to include solvent effects in aqueous solution by computing the energy of the reaction in which some water molecules in the coordination shell of the metal ion are replaced by the crown ether. When this is done, the ion selectivity sequence predicted from the theoretical work is in conformity with existing (incomplete) experimental data.