Deuterium NMR study ofD2O dynamics in glassy and liquid LiCl:D2O

Deuterium NMR studies have been performed in LiCl⋅R(${\mathrm{D}}_2$O) (mole fraction R=4 and 7, respectively) over the temperature range of 15 to 170 K, i.e., below and above the glass transition temperature ($T_g$≃145 K). The motional mechanism of the four ${\mathrm{D}}_2$O molecules hydrated at each ${\mathrm{Li}}^{+}$ has been found to be a twofold reorientation jump process with a weak temperature dependence of the mean reorientation rate 〈1/${\tau }_j$〉 in the glassy state (〈1/${\tau }_j$〉∝$T^{1\pm 0.2}$) while the bulk water molecules are shown to be immobile at the time scale of the NMR experiments (${>10\mathrm{}}^{\mathrm{-}3}$ s). The observed temperature dependence can be explained in the framework of excitations of disordered modes (two-level systems) existing in the glassy state which are coupled to the hydrate ${\mathrm{D}}_2$O. Moreover, below about 50 K the ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ spin relaxation is mainly caused by the ${\mathrm{D}}_2$O motion. Close to $T_g$ the reorientation in LiCl⋅${4\mathrm{D}}_2$O becomes thermally activated with an activation energy of about 0.2 eV, whereas the activation process does not seem to occur in LiCl⋅${7\mathrm{D}}_2$O indicating an impediment of the reorientational motion of the hydrate water molecules by the bulk water molecules. At $T_g$ the mean reorientation rate increases rapidly with the increasing temperature which is shown to be accompanied by a small increase of the D-O-D bond angle of about 0.6°. At 160 K, i.e., considerably above $T_g$ a motional narrowing of the total NMR line occurs indicating a liquidlike diffusional motion of all the molecules in LiCl⋅R(${\mathrm{D}}_2$O).