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Journal of Chemical Physics



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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.



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