Association constants for the complex Na+/benzocrown-6 in acetonitrile solution were determined from C-13-NMR relaxation time measurements at different temperatures. The dipolar contributions to relaxation were separated by nuclear Overhauser enhancement measurements. The contributions are split into parts originating from uncomplexed and from complexed macrocycles assuming the 1:1 complex formation between Na+ and crown ether and by measuring solutions of different concentrations. The measured data were analysed to yield the respective relaxation times and the association constants as a function of the temperature. The results depend on the carbon site in the macrocycle thus showing preferences for binding of Na+. The dipolar relaxation times of the complex were explained on the basis of pseudorotational motion of the macrocycle which was proved with the MM+ geometry optimisation. (C) 1999 Elsevier Science B.V. All rights reserved.