We report a quantitative measure of the nonexponential 1 H spin-lattice relaxation resulting from methyl group (CH 3 ) rotation in six polycrystalline van der Waals solids. We briefly review the subject in general to put the report in context. We then summarize several significant issues to consider when reporting 1 H or 19 F spin-lattice relaxation measurements when the relaxation is resulting from the rotation of a CH 3 or CF 3 group in a molecular solid.
IntroductionIn 1964, Runnells [1] and, independently, Hilt and Hubbard [2] showed that the nuclear spinlattice relaxation resulting from the modulation of the spin-spin interactions among the three spin- This phenomenon is often neglected, which means that, knowingly or unknowingly, an average relaxation rate is reported. Here, following a brief review of this subject, we pull together results from six polycrystalline van der Waals organic solids with quite different methyl group environments and quite different motions of the methyl group rotation axes, to show that this phenomenon is ubiquitous. We also show that the relaxation at lower temperatures, though always reported as being exponential within experimental uncertainty, can, in a statistical sense when many experiments are considered, be seen also to be slightly nonexponential.Background