ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.453617
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