Nuclear Magnetic Resonance Study of Molecular Motions in Hexamethylbenzene

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Abstract
An investigation has been made of the molecular motions occurring in solid hexamethylbenzene by measuring the proton spin—lattice relaxation time and second moment over the temperature range from helium temperatures to 450°K. The reorientation of the methyl groups is found to persist down to 2°K, presumably due to a tunneling mechanism; while the rotation of the whole molecule about its sixfold axis proceeds at a rate greater than a few kilocycles per second above 150°K. No other type of motion seems to occur at such rates, or greater, in the solid. The temperature dependence of T 1 is used to estimate the activation energies and motional rates for the respective motions in each of the three solid phases.