Proton NMR study of the orientation and motion of H2O in Na β′′ alumina

Abstract

Proton NMR measurements in nearly dry Na β\ alumina crystals display a Pake doublet due to H₂O molecules. At 300 K the angular dependence of the Pake splitting shows the time average orientation of the water HH vectors is parallel to the crystal c axis. Measurements at 133 K, where motional averaging is removed, demonstrate that HH is tilted from c by 5° to 13°. The frequency splitting of the doublet shows that the HH distance is between 1.65 and 1.71 Å, longer than in free H₂O. Above 250 K, line narrowing indicates the H₂O are translationally diffusing while maintaining their orientation. A model of this motion is suggested. The chemical shift anisotropy of the H₂O is found to be in agreement with previous measurements of H₂O in ice and gypsum.