Inter- and Intramolecular Relaxation in Molecular Liquids by Field Cycling 1H NMR Relaxometry

Field cycling ¹H nuclear magnetic resonance (NMR) relaxometry is applied to study rotational as well as translational dynamics in molecular liquids. The measured relaxation rates, $ T_{1}^{ - 1} \left( \omega \right) \equiv R_{1} \left( \omega \right) $ , contain intra- and intermolecular contributions, $ R_{{1,{\text{intra}}}}^{{}} \left( \omega \right) $ and $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ . The intramolecular part is mediated by rotational dynamics, the intermolecular part by translation as well as rotation. The rotational impact on the intermolecular relaxation (eccentricity effect) is due to the spins not located in the molecule’s center. The overall relaxation rate is decomposed into $ R_{{1,{\text{intra}}}}^{{}} \left( \omega \right) $ and $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ by isotope dilution experiments. It is shown that the eccentricity model (Ayant et al. in J. Phys. (Paris) 38:325, 1977) reproduces fairly well the bimodal shape of $ R_{{ 1 , {\text{inter}}}}^{{}} \left( \omega \right) $ for o-terphenyl and glycerol. As the relaxation contribution associated with translational dynamics dominates at lower frequencies, the overall relaxation rate shows a universal linear behavior when plotted versus square root of frequency. This allows determining the self-diffusion coefficient, D, in a model-independent way. It is demonstrated that the shape of NMR master curves comprising relaxation data for different temperatures, linked by frequency–temperature superposition, reflects the relative strength of translational and rotational contributions.