Lithium-7 NMR study of several Li+-crown ether complexes in binary acetone-nitrobenzene mixtures

Li-7 NMR measurements were employed to monitor the stoichiometry and stabil ity of Li+ ion complexes with 12-crown-4 (12C4), 15-crown-5 (15C5), benzo-1 5-crown-5 (B15C5) l8-crown-6 (18C6), dicyclohexano-18-crown-6 (DC18C6) and dibenzo-18-crown-6 (DB18C6) in binary acetone-nitrobenzene mixtures of vary ing composition. In all cases studied, the variation of Li-7 chemical shift with the crown/Li+ mole ratio indicated the formation of 1:1 complexes. Th e formation constants of the resulting complexes were evaluated from comput er fitting of the mole ratio data to an equation that relates the observed chemical shifts to the formation constant. In all solvent mixtures used, th e stabilities of the resulting 1:1 complexes varied in the order 15C5 > B15 C5 > DC18C6 > 18C6 > 12C4 > DB18C6. It was found that, in the case of all c omplexes, an increase in the percentage of acetone in the solvent mixtures significantly decreased the stability of the complexes.