Proton spin-lattice relaxation, methyl group rotation and states of an organic solid

We have measured the temperature-dependence of the proton spin-lattice relaxation rate in the solid state of 1,3-di-isopropyl-benzene at nuclear magnetic resonance Larmor frequencies of 8.50, 22.5 and 53.0 MHz. We are able to determine, as predicted, that only the four methyl groups are reorienting on the nuclear magnetic resonance Larmor frequency time scale. We can also conclude that the spectral density is well characterized by a Davidson-Cole distribution of activation energies (with a cut-off of E(dc) = 12 +/- 1 kJ mol(-1) and a width parameter of epsilon(dc) = 0.7 +/- 0.1) even though we are unable to observe the high-temperature frequency-independent regime. This distribution of barriers arises from the non-crystalline macro-state of the solid. There is considerable scattering in the observed relaxation rate in the solid state due to slightly different thermal preparations of the room-temperature liquid sample. We speculate on the state of the solid at the 'several-molecule' level. Finally, there are additional motions, probably translation or whole-molecule rotation, present for about 50 degrees C below the melting point, as indicated by the narrowing of the nuclear magnetic resonance line. However, these motions do not have a significant effect on the spin-lattice relaxation rate. Previous work from our laboratory is reviewed and the present study put into context.