Distributions of methyl group rotational barriers in polycrystalline organic solids

We bring together solid state (1)H spin-lattice relaxation rate measurements, scanning electron microscopy, single crystal X-ray diffraction, and electronic structure calculations for two methyl substituted organic compounds to investigate methyl group (CH3) rotational dynamics in the solid state. Methyl group rotational barrier heights are computed using electronic structure calculations, both in isolated molecules and in molecular clusters mimicking a perfect single crystal environment. The calculations are performed on suitable clusters built from the X-ray diffraction studies. These calculations allow for an estimate of the intramolecular and the intermolecular contributions to the barrier heights. The (1)H relaxation measurements, on the other hand, are performed with polycrystalline samples which have been investigated with scanning electron microscopy. The (1)H relaxation measurements are best fitted with a distribution of activation energies for methyl group rotation and we propose, based on the scanning electron microscopy images, that this distribution arises from molecules near crystallite surfaces or near other crystal imperfections (vacancies, dislocations, etc.). An activation energy characterizing this distribution is compared with a barrier height determined from the electronic structure calculations and a consistent model for methyl group rotation is developed. The compounds are 1,6-dimethylphenanthrene and 1,8-dimethylphenanthrene and the methyl group barriers being discussed and compared are in the 2-12 kJ mol(-1) range.