A Field Cycling NMR Relaxometry Study on Molecular Liquids and Plastically Crystalline Phases

Abstract

We apply field-cycling (FC) nuclear magnetic resonance (NMR) relaxometry to measure the spin-lattice relaxation rate R_1 in condensed matter as a function of Larmor frequency ω and temperature T. We present the results by means of five publications, three of which deal with the molecular liquid glycerol, while the latter two address the dynamics of the plastically crystalline phase of cyanoadamantane. In the case of protons (1H), the focus of the present work, R_1 reflects the fluctuations of the magnetic dipole-dipole interactions transmitted through intra- and intermolecular pathways. As a consequence, we gain insight to translational as well as rotational dynamics of the molecules. We describe a relaxation model for molecular liquids which includes three distinct relaxation phenomena. Following the results of dielectric and light-scattering experiments, the rotational susceptibility is described by a main relaxation process and a high frequency excess wing. At low frequencies ωτ<1 we consider a translational relaxation contribution which is given by the force-free hard sphere model describing diffusion in spin systems. In general, translation and rotation in molecular liquids are coupled and take place approximately on the same time scale τ. In glycerol however, they are separated well enough to result in a distinct bimodal relaxation behavior. It turns out that applying our model to master curves extending over 15 decades in frequency, which are constructed by assuming frequency-temperature superposition (FTS), facilitates the analysis. We include in the analysis the R_1 (T) data of Noack, for which reproduction of our data is demonstrated. In a next step, we analyze 1H relaxation data of mixtures of glycerol with either dimethyl sulfoxide (DMSO) or pyridine, with the help of our relaxation model. This allows us to investigate the translational and rotational dynamics in a component selective way. The change of the separation of the dynamics is addressed as a function of concentration and temperature. In case of DMSO/glycerol, no decoupling of the rotational dynamics of the components is observed. Moreover, the separation of time scales of translation and rotation is described as constant. This is in contrast to the mixture with pyridine, where a systematic change of the separation is observed with concentration. In all cases, we describe the extent of separation as temperature independent. Regarding the analysis of the dynamics in plastically crystalline phases we first focus on cyanoadamantane. Applying FTS once more, we characterize the non-Lorentzian character of the spectral density reflecting the 90-degree jumps of the molecule. At low temperatures, the NMR relaxation is determined by the rotation of the molecule around its three-fold axis, the dynamics of which is described by a broad distribution of activation energies. As FTS does not apply in this case, we suggest a scaling procedure which again provides master curves in the case of dynamics controlled by thermally activated processes. Following…