Anisotropic mobility of water in hydrated layered chalcogenides A+x(H2O)y[TaS2]x− and A+x(H2O)y [NbS2]x− as studied by NMR

Wideline 1H NMR studies have been carried out in various hydrated layered chalcogenides of the type A+x(H2O)y [TaS2]x− at temperatures from 148 to 300 K, where A=Na, K, NH4, Cs, and Ca. Some measurements have been made at 77, 363, 398, and 425 K, respectively, and a sample with Nb instead of Ta has also been examined. Depending upon the temperature range and upon the particular compound, three different types of spectra have been observed. The rigid lattice H2O spectra are characterized by intra- and intermolecular dipole–dipole interactions. At elevated temperatures, the spectrum equals rather perfectly the theoretical shape of the proton spectrum for an isolated two-spin system. This suggests the occurance of a range of activated ordered mobility which is explained by a two-dimensional diffusion of water inside the interlayer space. For compounds with bilayers rather than monolayers of water, rotation of H2O about its C2 axis is also activated. This is supported by measurements of the spin–lattice relaxation time T1. Finally, an additional narrow line, which appears at high temperature, is attributed to a subphase of water undergoing random motion.