NMR Investigation of Molecular Motion in the Neat Solid and Plastic-Crystalline Phases of Cyclohexane¹

The molecular dynamics of neat solid and plastic phase cyclohexane are investigated using solid state carbon, deuteron, and proton nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on deciphering the dynamical origin of a line width/line shape transformation in the static proton NMR spectrum of plastic phase cyclohexane near 199 K, and it is examined in terms of rotational, translational, and conformational contributions. It is shown that translational diffusion within lattice defect sites is the probable dynamical mode associated with the static proton NMR line shape modulation. The rate of conformational isomerism by ring inversion in the plastic phase is measured for the first time at two temperatures, and the time scale of this process suggests a mechanistic correlation with translational diffusion within single crystals in the plastic phase. Finally, the time scale and symmetry associated with rotational diffusion in solid-phase cyclohexane are found (respectively) to be longer and more complex than previously reported.