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 T1 and second moments M2 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 T1 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 T1 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 T1 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 T1 and M2 is given in terms of the tunneling splitting of the torsional ground state. In 1,2,3,4-tetramethylbenzene discontinuities in T1 and M2 at 257°K indicate a phase transition not previously reported.