Nuclear Magnetic Resonance Study of Hindered Rotations in Some Methylbenzenes

Abstract

The hindered rotations of methyl groups have been investigated in the solid state of a series of methyl‐benzenes. The materials studied were the three tetramethylbenzenes and pentamethylbenzene. Their proton spin‐lattice relaxation times T 1 and second moments M 2 were measured from helium temperatures to the melting points of the respective samples. The reorientation of the methyl groups is found to persist down to 2°K, and in the three tetramethylbenzenes this is the only motion which is well established in the solid state at a rate greater than about 30 kc/sec. However, the T 1 results from the low‐temperature phase of each hydrocarbon do not always exhibit a single minimum. Furthermore, there exists a correlation between the number of nonequivalent methyl sites in these compounds and the number of minima displayed in their T 1 data. The identification of a particular site with a particular minimum is not unambiguous and only tentative suggestions can be made of the relative magnitude of the constraints on the various sites. Activation energies, calculated from the T 1 results, for the methyl group motion around each of the minima are shown in Table II. As the temperature is reduced to ∼1°K the NMR data can no longer be accounted for in terms of the theory which was applicable at high temperatures, and a qualitative explanation of T 1 and M 2 is given in terms of the tunneling splitting of the torsional ground state. In 1,2,3,4‐tetramethylbenzene discontinuities in T 1 and M 2 at 257°K indicate a phase transition not previously reported.