ln order to investigate the cation binding power of macrocyclic ethers the (13)C NMR relaxation times, T(1), of free benzocrown-5 and benzocrown-4 as well as their 1/1 Na(+) complexes were determined at different temperatures in acetonitrile. The dipolar contributions to NMR relaxation were separated by nuclear Overhauser enhancement measurements. The dipolar relaxation times of free and complexed crown ethers were explained on the basis of intramolecular [(1)H]- (13)C interactions of the molecules CH(2)-O-CH(2) backbone. From (13)C NMR relaxation times observed at extreme narrowing conditions the association constants, K(a), of the sodium-crown ether complexes were obtained as a function of temperature; the Na(+) associations were shown to depend on the carbon-oxygen sites reducing directly the O-C-H relaxation times, T(1). Benzocrown-5 displayed different T(1) for different carbon segments on the ring, while benzocrown-4 showed less T(1) changes among the macrocyclic backbone carbons. Complexed macrocyclic ethers exhibited higher barrier energies for internal motion than the uncomplexed macrocycles because the oxygen charges of the former are attracted by the cation in a complex. (C) 2004 Elsevier B.V. All rights reserved.