NMR-STUDY OF CROWN-ETHER MOTION IN 2 ALKALIDES

Nuclear spin-lattice relaxation times (T1) of both 133Cs and 23Na were measured as a function of temperature for two cesium sodides, Cs+( 18-crown-6)2Na- and Cs+( 15-crown-5)2Na-. A logarithmic dependence of the T1, values of these two compounds on the inverse temperature is interpreted as the result of crown ether rotational/conformational reorientation with average activation barriers of 31.2 and 35.4 kJ mol-1, respectively. This interpretation is supported by the line-width reduction of the 23Na resonances, reduction of the observed 23Na quadrupole coupling constants, and the disappearance of peaks in the powder X-ray diffraction patterns. The line-width data are used with the approximate formula of Waugh (Waugh, J. S.; Fedin, E. I. Sov. Phys. Solid State 1963, 4, 1633) to yield activation barriers in good agreement with the nuclear spin-lattice relaxation analysis. © 1992 American Chemical Society