Journal oJlnclusion Phenomena and Molecular Recognition in Chemistry 26:243-251, 1996. 243 @ 1996 Kluwer Academic Publishers. Printed in the Netherlands. An NMR Study of the Stoichiometry and Stabiiity of Lithium Ion Complexes with 12-Crown-4, 15-Crown-5 and 18-Crown-6 in BinaU Acetonitrile-Nitrobenzene Mixtures ALI REZA FAKHARI Department of Chemistry, Tarbiat Modarres University, Tehran, Iran. MOJTABA SHAMSIPUR* Department of Chemistry, Razi University, Kermanshah, Iran. (Received: l 7 January 1996; in final form: l 0 May 1996) Abstract. Proton NMR spectroscopy was used to study the complexation reaction between lithium ion and 12-crown-4, 15-crown-5 and 18-crown-6 in a number of binary acetonitrile-nitrobenzene mixtures, in all cases the exchange between free and complexed crowns was fast on the NMR time scale and only a single population average IH signal was observed. Formation constants of the resulting 1 : 1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift-mole ratio data. There is an inverse relationship between the complex stability and the amount of acetonitriie in the mixed solvent. It was found that, in all solvent mixtures used, 15-crown-5 forms the most stable complex with Li + ion in the series. Key words. Crown ethers, Li + ion, mixed-solvent, complex stability, NMR. 1. Introduction Since the first synthesis of crown ether compounds by Pedersen [ l ] and his obser- vation of their selective ion binding properties, a great deaI of research work has been done on crown complexes with inorganic and organic cations [2, 3i and with neutral molecules [4!. The stoichiometry and stability of crown ether complexes depend largely on the nature of the guest, the nature of the host and, especially, the solvent system used. There is cu~entiy an increasing interest in the study ofphysic- ochemical phenomena, including complexation processes, in binary mixed solvent systems and their interpretation in terms of the solutes' preferential solvation by one of the solvent's components [5-13 i. Among a wide variety of physicochemical techniques used for such studies [2, 3], nuctear magnetic resonance of7Li, 23Na, 39K, !33Cs and 2°5T1 nuclei have been used rather extensively to study the thermodynamics [14-18i and kinetics [19- 23i of compiexation of the above cations with crown ethers in different solvents. Author for correspondence.