Nitrogen and chlorine spin-lattice relaxation in ND 4Cl

Abstract

The spin-lattice relaxation times for 14N and 35Cl in ND 4Cl as a function of temperature in the region of the order-disorder phase transition at 248.9 K are reported. The 35Cl relaxation gate is due to the quadrupolar interaction and the results are very similar to those found in NH 4Cl. In ND 4Cl the nitrogen-hydrogen nuclear dipolar interaction is reduced from that found in NH 4Cl and it is possible to separate the dipolar and quadrupolar contributions to the 14N relaxation rate. The 14N relaxation is dominated below 220 K by the dipolar mechanism and the observed relaxation rate can be adequately explained by considering only the 14N 2D interaction within a single ammonium ion. The quadrupolar mechanism determines the nitrogen relaxation in the region of the phase transition and the temperature dependence observed is similar to that found for the chlorine. It is shown that the field gradients at these nuclear sites are time dependent because of the ammonium ion switching from one orientation to another. A detailed analysis of the chlorine and nitrogen quadrupolar relaxation is carried out using the kinetic Ising model to describe the critical dynamics of the ammonium ion near the phase transition. It is found that in this particular instance the relaxation measurements are not sensitive to phenomena occurring with a critical wave vector q c = 0 so that a great deal of valuable information is not available to NMR experiments.