Quasi-two-dimensional motion and proton exchange in electrolyte layers of hydrated chalcogenides

Molecular motion and proton exchange has been studied by 1H NMR in various hydrated layered chalcogenides of the type Ax+(H2O)y{MS2}x−, where A is an alkalimetal and M is Nb or Ta. Information about the anisotropy of the translational motion was deduced from temperature and frequency dependent measurements of the 1H spin-lattice relaxation rate. In the monolayer hydrates, the motion of the water molecules is essentially two-dimensional in character; significant contributions from three-dimensional motion have to be taken into account, however, for the bilayers. Proton exchange has been shown to exist even in the monolayers. It causes characteristic powder line shapes observed at elevated temperatures, which are analyzed quantitatively. At lower temperatures, values for the exchange rate were obtained from 1H spin alignment measurements. The data are in accord with the activation energy for the autoprotolysis of water.