Nuclear magnetic resonance study of the ligand interchange of Ba2+-18-crown-6 complex in methanol solution

Abstract

Exchange kinetics of Ba2+-18-crown-6 complex in deuterated methanol solution was studied by proton NMR line-shape analysis of a series of solutions containing equal population of free and complexed 18-crown-6, but varying concentration of the macrocycle, at various temperatures. From −33 to 37°C, the predominant mechanism for the exchange of the ligand between the two sites is a bimolecular pathway which is characterized by the following activation parameters:Ea=47±2 kJ-mol−1; ΔH≠=45±2 kJ-mol−1; ΔS≠=−8±4 J-mol−1-K−1. However, the contribution of a dissociative mechanism with activation parametersEa=36±5 kJ-mol−1, ΔH≠=33±5 kJ-mol−1 and ΔS≠=104±18 J-mol−1-K−1 becomes more important at higher temperatures.