Methyl reorientation in methylphenanthrenes. I: Solid state proton spin-lattice relaxation in the 3-methyl, 9-methyl, and 3,9-dimethyl systems

• Published in: The Journal of chemical physics 1987-07, Vol.87 (1), p.20-27
• Main Authors: CONN, K. G, BECKMANN, P. A, MALLORY, C. W, MALLORY, F. B
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; Exact sciences and technology; Physics; Structure of solids and liquids; crystallography
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.453617

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.