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

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over awide frequency and temperature range to get insight into the dynamic processes occurringin the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) andisocyanocyclohexane. The spin-lattice relaxation rate,R1(ω), is measured in the 0.01–30 MHzfrequency range and transformed into the susceptibility representationχNMR″ω=ωR1ω. Three relaxation processes are identified, namely, a main(α-) relaxation, a fast secondary (β-) relaxation, and a slow relaxation; they are verysimilar for the two isomers. Exploiting frequency–temperature superposition, master curvesof χNMR″ωτ are constructed and analyzed for different processes. Theα-relaxation displays a pronounced non-Lorentzian susceptibility with a temperatureindependent width parameter, and the correlation times display a non-Arrhenius temperaturedependence—features indicating cooperative dynamics of the overall reorientation of themolecules. The β-relaxation shows high similarity with secondary relaxations in structuralglasses. The extracted correlation times well agree with those reported by othertechniques. A direct comparison of FC NMR and dielectric master curves for CNCH yieldspronounced difference regarding the non-Lorentzian spectral shape as well as the relativerelaxation strength of α- and β-relaxation. The correlation times of the slow relaxationfollow an Arrhenius temperature dependence with a comparatively high activation energy. Asthe α-process involves liquid-like isotropic molecular reorientation, the slow process hasto be attributed to vacancy diffusion, which modulates intermolecular dipole–dipoleinteractions, possibly accompanied by chair–chair interconversion of the cyclohexane ring.However, the low frequency relaxation features characteristic of vacancy diffusion cannotbe detected due to experimental limitations.