NUCLEAR-MAGNETIC-RESONANCE STUDY OF LITHIUM ION COMPLEXES WITH SEVERAL CROWN-ETHERS IN BINARY ACETONITRILE-NITROMETHANE MIXTURES

Lithium-7 NMR measurements were used to determine the stoichiometry an d stability of Li+ complexes with 12-crown-4 (12C4). 15-crown-5 (15C5) , benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC18C6) and dibenzo-18-crown-6 (DB18C6) in various acetonitrile-nitr omethane mixtures at 27 degrees C. In all cases studied. the variation in the Li-7 chemical shift with the crown/Li+ mol ratio indicated the formation of 1:1 complexes in solution. The formation constants of th e resulting complexes were evaluated from computer fitting of the mol ratio data to an equation which relates the observed chemical shifts t o the formation constant. It was found that, in all solvent mixtures u sed, the stabilities of the resulting complexes vary in the order 15C5 >B15C5>DC18C6>12C4>18C6> DB18C6. There is an inverse relationships bet ween log K-f of the complexes and the mol fraction of acetonitrile in the solvent mixtures.