Slow motional processes for which there is no motional narrowing can be important in spin-lattice relaxation. These relaxation mechanisms cannot be treated by perturbation theory. It is shown that the relaxation times are directly proportional to the motional correlation times. It is suggested that they can be used very effectively to measure the rate of motional processes in solids. A theory of the direct effect of molecular rotations on longitudinal spin relaxation in pure quadrupole resonance is described. The theory is applied to the ${\mathrm{N}}^{14}$ resonance in hexamethylene tetramine. Measurement of the longitudinal relaxation times in this molecule were carried out by a steady-state pulse method, and the rotational correlation times were determined. The results are in agreement with previous nmr measurements.

• Received 7 December 1964

DOI:https://doi.org/10.1103/PhysRev.138.A845