WATER DYNAMICS IN NA3PW12O40-CENTER-DOT-NH(2)O - A H-2-NMR AND P-31-NMR INVESTIGATION

The dynamics of water molecules adsorbed in the secondary lattice stru cture of sodium dodecatungstophosphate (Na3PW12O40. nH(2)O) has been i nvestigated by P-31-NMR and H-2-NMR at different temperatures and hydr ation levels (n). H-2-NMR spectra show that for T less than or equal t o 173 K only one type of adsorbed water is present. In the temperature interval between 173 and 293 K three differently adsorbed and mutuall y exchanging water components are observed. The chemical exchange rang e and population ratios of the differently chemisorbed water molecules were determined by computer simulation of H-2-NMR spectral profiles. The activation energy of the exchange process and the shape of H-2-NMR spectra indicate that the different water components correspond to tw ice hydrogen bound, mono hydrogen bound, and not bound D2O molecules. A further investigation of samples at different hydration levels has b een performed, at room temperature (293 K), by P-31-NMR spectroscopy. The line width (Delta(H)) and the longitudinal relaxation time (T-1) o f the P-31 lines have been measured using both H2O and D2O for hydrati on. The experimental data have been used to estimate the contribution to Delta(H) and T-1 due to the protons-phosphorus dipolar interactions : Delta(H) and T-1(H) respectively. These parameters, combined with th e values of population ratios of the differently adsorbed water molecu les, allow knowledge of the correlation times of the water molecules' motions.