NMR study of molecular motions in cyclohexanol, a glass-forming rotor crystal

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Abstract
The motions in the rotor and glassy crystal phases of solid cyclohexanol are studied with proton NMR from the melt down to 5 K. Particular attention is paid to the variation of the linewidth with temperature and to the temperature and frequency dependences of T1. We find there are two distinct motions that cause minima in T1 as a function of temperature. These two motions are also responsible for the two dielectric loss peaks observed previously. From the proton line narrowing and C13 high-resolution solid state spectra the high temperature α motion is identified as overall molecular rotation. The low temperature β motion is identified as a uniaxial internal rotation of the cyclohexyl ring about the CO bond, with the COH group remaining stationary. This explains both the strong spin relaxation and the weak dielectric relaxation associated with the β motion. Both motions have distributions of correlation times, as seen from the shallow T1 minima and the weak temperature and frequency dependences of T1. From 100 K down to 5 K the temperature dependence continues to be weak and the frequency dependence remains less than ω20. These surprising results indicate that some components of the motion remain faster than NMR frequencies even at 5 K. The behavior of cyclohexanol is compared to that of other disordered solids.