Molecular motion in solid hexamethylethane studied by nuclear magnetic resonance

Abstract

The nuclear magnetic resonance line width and second moment for the molecular crystal hexamethylethane have been measured from 85°k to the melting point, 374°k. The spin-lattice relaxation time T ₁ and the spinlattice relaxation time in the rotating frame T _1σ have also been measured over the same temperature range. The results have been interpreted in terms of the various molecular motions occurring in the crystal. Below the thermal transition point at 152·5°k methyl group reorientation and anisotropic molecular reorientation appear to occur simultaneously and with the same correlation time. The activation enthalpy for this motion is 13·4±0·5 k_J mole^-1. Above the phase transition, the second moment measurements indicate isotropic molecular reorientation, and relaxation time measurements give an activation enthalpy of 9·2±0·5 k_J mole^-1 for this process. Above 250°k molecular self-diffusion becomes the dominant controlling mechanism for line width and T _1p . The activation enthalpy for this process is found to be 82±2 k_J mole^-1. The T _1p results allow the associated correlation frequency for self-diffusion to be followed from 20 hz to 6 Mhz. The results are consistent with a vacancy diffusion mechanism.