Translational versus rotational molecular dynamics in plastic crystals studied by NMR relaxometry and diffusometry

Abstract

Cyclohexane, succinonitrile, cyclohexanol and pivalic acid were investigated in the plastic state by means of nuclear magnetic resonance (NMR) field-cycling relaxometry and NMR fieldgradient diffusometry. Measurements of the field-dependent relaxation times T ₁ (υ) of cyclohexane and succinonitrile are described assuming anisotropic molecular rotation on short timescales and translational diffusion on long timescales leading to a modulation of the intermolecular dipole–dipole coupling. Correlation times and anisotropy parameters are evaluated from the experimental results. The self-diffusion coefficients calculated from the field-cycling data are found to coincide with the values determined directly employing fieldgradient diffusometry. In the cases of pivalic acid and cyclohexanol, a more complicated relaxation dispersion is observed which indicates the presence of correlated motions between neighbouring molecules. For the cyclohexanol, the presence of a liquid component well below the freezing point is described quantitatively.