Nonexponential Spin-Lattice Relaxation of Protons in Solid C${\mathrm{H}}_3$CN and Solid Solutions of C${\mathrm{H}}_3$CN in C${\mathrm{D}}_3$CN

The proton spin-lattice relaxation in solid C${\mathrm{H}}_3$CN, and in 1:6 and 1:10 solid solutions of C${\mathrm{H}}_3$CN in C${\mathrm{D}}_3$CN, has been measured by pulsed NMR techniques at a frequency of 30.0 MHz, for temperatures between 79.5 and 173°K. The relaxation is nonexponential at the higher temperatures, and exponential only at the lowest temperature. The experimental results for 1:6 solutions are compared with the relaxation previously predicted by a calculation based on the assumptions that (1) the relaxation is due to intramolecular dipole-dipole interactions between protons, which are time-dependent because of hindered rotations of the methyl groups, (2) the axes of hindered rotation are randomly oriented, and (3) the correlation time for reorientation is the same for all methyl groups at the same temperature. The distinctly nonexponential relaxation predicted by the theory is due to the inclusion of the effects of the cross correlations of the intramolecular dipole-dipole interactions. The agreement between the experimental results and the calculations is fairly good, the discrepancy being attributable to the effects of other relaxation mechanisms, such as intermolecular dipole-dipole interactions. The results provide the first experimental evidence of nonexponential spin-lattice relaxation produced by the effects of cross correlations of dipole-dipole interactions.