Solid state proton spin-lattice relaxation in polycrystalline methylphenanthrenes. IV. 1,4-dimethylphenanthrene

We present and model the NMR frequency (8.50, 22.5, and 53.0 MHz) and temperature (97-300 K) dependence of the solid state ¹H spin-lattice relaxation process in polycrystalline 1,4-dimethylphenanthrene. The solid state gives rise to a situation where methyl group rotation is the only motion on the NMR time scale and the relaxation rates due to the rotations of the 1- and 4-methyl groups are conveniently well-separated in temperature. At these low NMR frequencies, both the slow- and fast-motion limits are observed for the rotation of both methyl groups which allows for a more stringent test of the models. The relaxation is nonexponential as expected when it is caused by methyl group rotation in which case the initial relaxation rate is modeled. Parameters characterizing stretched-exponential fits of the relaxation process are also used both to quantify the degree of nonexponential relaxation and indicate that the observed relaxation is indeed due to the rotation of the two methyl groups. The results are compared with several other polycrystalline methylphenanthrenes and dimethylphenanthrenes. These systems allow for an investigation into how intramolecular and intermolecular interactions between methyl groups and neighboring atoms on the same and neighboring molecules determine the barriers to methyl group rotation.