NMR-STUDY OF THE STOICHIOMETRY, STABILITY, AND EXCHANGE KINETICS OF ALKALINE-EARTH COMPLEXES WITH 18-CROWN-6 IN SOME NONAQUEOUS SOLUTIONS

Proton NMR was used to study alkaline earth complexes with 18-crown-6 in nitromethane, acetonitrile, and dimethylformamide solutions. In all three solvents used, the stabilities of the resulting 1:1 complexes vary in the order Ba2+ > Sr2+ > Ca2+ > Mg2+. There is an inverse relationship between the donicity of the solvents as expressed by the Gutmann donor numbers and stability of the resulting complexes. The dissociative kinetics of the complexes were studied by proton NMR line-shape analysis. The exchange reaction rates and the activation parameters E(a), DELTA-H double-ended dagger, DELTA-S double-ended dagger, and DELTA-G double-ended dagger for the exchange have been determined and found to be strongly solvent dependent. There is a correlation between the donor number of the solvents and the logarithm of the dissociation rates as well as the activation parameters for a given cation.