Primary and secondary relaxation process in plastically crystalline cyanocyclohexane studied by 2H nuclear magnetic resonance. II. Quantitative analysis

We analyze the results of our previously reported H-2 nuclear magnetic resonance (NMR) experiments in the plastically crystalline (PC) phase of cyanocyclohexane (Part I of this work) to study the fast secondary relaxation (or beta-process) in detail. Both, the occurrence of an additional minimum in the spin-lattice relaxation T-1 and the pronounced effects arising in the solid-echo spectrum above the glass transition temperature T-g = 134 K, allow for a direct determination of the restricting geometry of the beta-process in terms of the wobbling-in-a-cone model. Whereas at temperatures below T-g the reorientation is confined to rather small solid angles (below 10 degrees), the spatial restriction decreases strongly with temperature above T-g, i.e., the distribution of cone angles shifts continuously towards higher values. The beta-process in the PC phase of cyanocyclohexane proceeds via the same mechanism as found in structural glass formers. This is substantiated by demonstrating the very similar behavior (for T < T-g) of spin-lattice relaxation, stimulated echo decays, and spectral parameters when plotted as a function of < log tau(beta)> (taken from dielectric spectroscopy). We do, however, not observe a clear-cut relation between the relaxation strength of the beta-process observed by NMR (calculated within the wobbling-in-a-cone model) and dielectric spectroscopy. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790398]