Dynamics in the plastic crystalline phase of cyanocyclohexane and isocyanocyclohexane probed by 1H field cycling NMR relaxometry

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over a wide frequency and temperature range to get insight into the dynamic processes occurring in the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) and isocyanocyclohexane. The spin-lattice relaxation rate, R1(?), is measured in the 0.01-30 MHz frequency range and transformed into the susceptibility representation ? N M R ? ? = ? R 1 ?. Three relaxation processes are identified, namely, a main (?-) relaxation, a fast secondary (?-) relaxation, and a slow relaxation; they are very similar for the two isomers. Exploiting frequency-temperature superposition, master curves of ? N M R ? ? ? are constructed and analyzed for different processes. The ?-relaxation displays a pronounced non-Lorentzian susceptibility with a temperature independent width parameter, and the correlation times display a non-Arrhenius temperature dependence - features indicating cooperative dynamics of the overall reorientation of the molecules. The ?-relaxation shows high similarity with secondary relaxations in structural glasses. The extracted correlation times well agree with those reported by other techniques. A direct comparison of FC NMR and dielectric master curves for CNCH yields pronounced difference regarding the non-Lorentzian spectral shape as well as the relative relaxation strength of ?- and ?-relaxation. The correlation times of the slow relaxation follow an Arrhenius temperature dependence with a comparatively high activation energy. As the ?-process involves liquid-like isotropic molecular reorientation, the slow process has to be attributed to vacancy diffusion, which modulates intermolecular dipole-dipole interactions, possibly accompanied by chair-chair interconversion of the cyclohexane ring. However, the low frequency relaxation features characteristic of vacancy diffusion cannot be detected due to experimental limitations.