XE-129 NMR OF XENON ADSORBED IN Y-ZEOLITES AT 144-K

**1**2**9Xe NMR spectra of xenon adsorbed in Na-Y, K-Y, Mg-Y, Ca-Y, and B-Y zeolites have been measured as a function of the xenon loading at 144 and 293 K. At low xenon loadings, the **1**2**9Xe chemical shift shows two types of dependence on the xenon density in the supercage: linearly increasing and paraboliclike with a minimum. At 144 K, all the zeolites except Ca-Y have a linear dependence, whereas at 293 K, both Ca-Y and Mg-Y show a paraboliclike curvature. These dependences can be explained in terms of rapid exchange between xenon atoms adsorbed at special sites in the supercage and those remaining in the gas phase in the cages. Strong xenon adsorption at the special sites leads to the paraboliclike behavior, whereas weak adsorption yields the linear dependence.