The <sup>13</sup>C dipole-dipole relaxation times of tetraoxa-, pentaoxa-, hexaoxa-cyclooctadecane, and dioxohexaoxacyclooctadecane were obtained in acetone-d<sub>6</sub> by the aid of the inversion recovery method at 15.0 MHz. The dependence of dipolar relaxation time on the molecular size and the conformational aspects of cyclo-polyoxa molecules, (CH<sub>2</sub>OCH<sub>2</sub>)<sub>n</sub>, are discussed from the point of view of the cation-binding properties of such compounds. It is concluded that the <i>T</i><sub>1</sub><sup>DD</sup> values reflect the rate of pseudorotation and therefore the flexible nature of the ring which causes the preference of the cation binding; accordingly, the rigidity determines the degree of stability of ion-molecule complexes.

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