C-13 NMR relaxation time measurements of macrocyclic ether complexes. The association of benzo[18]crown-6 with NaClO4 center dot H2O in acetonitrile

Association constants for the complex Na+/benzo[18]crown-6 in acetonitrile solution were determined from C-13-NMR relaxation time measurements at diff erent 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 ass uming the 1:1 complex formation between Na+ and crown ether and by measurin g solutions of different concentrations. The measured data were analysed to yield the respective relaxation times and the association constants as a f unction of the temperature. The results depend on the carbon site in the ma crocycle thus showing preferences for binding of Na+. The dipolar relaxatio n times of the complex were explained on the basis of pseudorotational moti on of the macrocycle which was proved with the MM+ geometry optimisation. ( C) 1999 Elsevier Science B.V. All rights reserved.