Proton NMR probing of stoichiometry and thermodynamic data for the complexation of Na + and Li + ions with 15-Crown-5 in acetonitrile–nitrobenzene mixtures

Abstract

Proton NMR was used to study the complexation reaction of Li + and Na + ions with 15-Crown-5 (15C5) in a number of binary acetonitrile (AN)–nitrobenzene (NB) mixtures at different temperatures. In all cases, the exchange between free and complexed 15C5 was fast on the NMR timescale and only a single population average 1H signal was observed. The 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 AN in the solvent mixtures. The enthalpy and entropy values for the complexation reaction were evaluated from the temperature dependence of the formation constants. In all the solvent mixtures studied, the resulting complex is enthalpy stabilized but entropy destabilized. Finally, the experimental results were compared with theoretical ones that were obtained from molecular modeling methods. Based on our results, it is most probable that Li +–15C5 in solvent stays in a rather nesting complex form with greater Log K f values, but Na +–15C5 forms a complete perching complex form with lower Log K f values.