Effects of interactions between hopping particles onT1relaxation rates at low concentrations

We calculate the correlation function for a pair of specific particles making nearest-neighbor jumps in a simple-cubic lattice. Short-ranged interactions are included by allowing one jump rate when the particles are separated and arbitrary jump rates into and out of configurations where the particles are nearest neighbors. This correlation function is then used to calculate the effects of particle repulsion (or attraction) on $T_1\mathrm{}(I-I)\mathrm{}$, the motionally altered spin relaxation time due to the dipolar interaction between the hopping particles. The frequency or magnetic-field dependence of this relaxation time is not what one would expect from simply doctored second-moment arguments.