Proton Magnetic Resonance of the CH3 Group. III. Reorientation Mechanism in Solids

Abstract

The proton magnetic resonance absorption has been reported to change with temperature in a manner suggesting hindred C3 rotational motions of the CH3 groups in several solid compounds. In this paper, the reorientation mechanism is considered more closely. The second moments of the absorption lines are calculated as a function of temperature for three types of motion. Similar predictions are obtained for two classical models, a Brownian type rotation by random angles and a random jumping between fixed positions 2π/3 apart. The results are different for the third model, a quantum-mechanical tunnel effect, for which approximate calculations are given. The experimental data for methyl chloroform, 2,2-dinitropropane, and 2-chloro-2-nitropropane can be fitted either to a classical model or to the tunnel effect by adjusting the potential barrier to the reorientations. However, the activation energies for classical rotation are low, about 2.2 kcal mole—1 and the observed frequency factors of 107 are unexplained. The tunnel effect gives larger barriers, about 5 kcal, and the predicted frequency factors agree well with experiment. In the case of 2,2-dichloropropane a classical motion is indicated by the large frequency factor of 1012.