Methyl group rotation and H-1 and H-2 Zeeman relaxation in organic solids

We have measured the solid state H-1 and H-2 Zeeman relaxation rates R at r oom temperature in two methyl-deuterated samples of 1,9-dimethylphenanthren e. The H-1 dipolar rate Rd (d for 1H dipolar and 9 for 9-methyl group) and the H-2 quadrupolar rate R-q(1) (q for H-2 quadrupolar and 1 for 1-methyl g roup) were measured in 1-(trideuteriomethyl)-9-methylphenanthrene. The H-1 dipolar rate R-d(1) and the H-2 quadrupolar rate R-q(9) were measured in 1- methvl-9-(trideuteriomethyl)phenanthrene. Models are developed for both R-d (m) and R-q(m) (m = 1 or 9) due to methyl group rotation. In a large class of simple dynamical models for spin relaxation in the solid state, the rati os R-q(m)/R-d(m) are independent of the dynamics (except for the mass diffe rence between the H-1 and H-2 nuclei). In the present case. these ratios ar e also independent of in. In addition, the ratios R-k(9)/R-k(1) (k = 1d and q) are dependent only on the activation energy for methyl group rotation a nd another related dynamical parameter, but they are independent of the int eraction being modulated. Because many parameters factor out, these ratios can be predicted with few or no adjustable parameters, depending on the sop histication of the theoretical model. The agreement between theory and expe riment is good, even for the simplest theoretical models. These agreements give one a greater confidence in the models for quadrupolar and dipolar rel axation, but particularly the latter, which, because of spin diffusion, are more difficult to test.