We have investigated the dynamics of methyl group reorientation in solid methyl-substituted phenanthrenes. The temperature dependence of the proton spin–lattice relaxation rates has been measured in polycrystalline 3-methylphenanthrene (3-MP), 9-methylphenanthrene (9-MP), and 3,9-dimethylphenanthrene (3,9-DMP) at Larmor frequencies of 8.50, 22.5, and 53.0 MHz. The data are interpreted using a Davidson–Cole spectral density which implies either that the correlation functions for intramolecular reorientation are nonexponential or that there is a distribution of exponential correlation times. Comparing the fitted parameters that characterize the relaxation data for the three molecules shows that the individual contributions to the relaxation rate from the 3- and 9-methyls in 3,9-DMP can be separated and that the parameters specifying each are similar to the equivalent group in the two single methylphenanthrenes. The 9-methyl group is characterized by effective activation energies of 10.6±0.6 and 12.5±0.9 kJ/mol in 9-MP and 3,9-DMP, respectively, whereas the 3-methyl group is characterized by effective activation energies of 5.2±0.8 and 5±1 kJ/mol in 3-MP and 3,9-DMP, respectively. The agreement between the fitted and calculated values of the spin–lattice interaction strength, assuming only intramethyl proton dipole–dipole interactions need be considered, is excellent. A comparison between experimentally determined correlation times and those calculated from a variety of very simple dynamical models is given, and the results suggest, as have several previous studies, that at high temperatures where tunneling plays no role, methyl reorientation is a simple, thermally activated, hopping process. We have also analyzed many published data in methyl-substituted phenanthrenes, anthracenes, and naphthalenes (14 molecules) in the same way as we did for the phenanthrene data presented here, and a consistent picture for the dynamics of methyl reorientation emerges.

We bring together solid state 1H 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 1H relaxation measurements, on the other hand, are performed with polycrystalline samples which have been investigated with scanning electron microscopy. The 1H 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.

We report proton Zeeman relaxation rates ii as a function of temperature Tat 8.5 and 53 MHz in polycrystalline 1,9-dimethyIphenanthrene (1,9-DMP) and l-trideuteriomethyl-9-methylphenanthrene (1, 9-DMP[l-d3]). The data are interpreted using a Davidson-Cole spectral density for intramolecular reorientation and the implications of this are discussed. R vs T"' data for !,9-DMP[I-d3] are used to determine the parameters that characterize the reorientation of the 9-methyl group. By assuming that the parameters characterizing the dynamics of the 9-methyl group are the same in 1,9-DMP and l,9-DMP[l-d3], we subtract out the R vs 7""' contribution of the 9-methyl group in 1,9-DMP to determine the parameters that characterize the dynamics of the l-methyl group. We find that the barrier for reorientation of the 9-methyl group is larger than the barrier for the t-methyl group and this is discussed in terms of the various contributions to the barrier.

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