Abstract

Fast field cycling (FFC) NMR relaxometry provides unique insights into molecular dynamics across broad frequency and temperature ranges, making it particularly powerful for the investigation of ionic liquids (ILs). This class of materials exhibits a complex interplay of rotational and translational motions that governs their distinctive macroscopic properties. However, capturing these coupled dynamics across multiple timescales remains challenging. In this contribution, we present a framework that combines FFC NMR relaxometry with molecular dynamics (MD) simulations to disentangle rotational and translational motions in ILs.

In our previous work on translational dynamics [1], we demonstrated that intermolecular heteronuclear 1H-19F interactions play a crucial role in the quantitative analysis of NMR dispersion (NMRD) profiles in ILs where both cations and anions carry NMR-active nuclei. In such systems, intermolecular relaxation is not solely governed by homonuclear 1H-1H and 19F-19F interactions, but also includes significant 1H-19F heteronuclear terms. Neglecting these contributions leads to systematic errors in the evaluation of relaxation data, particularly in the low-frequency regime commonly used to extract translational self-diffusion coefficients. In this regime, application of the low-frequency approach (LFA) to total relaxation rates results in a substantial underestimation of self-diffusion coefficients. To overcome this limitation, we employ an isotopic substitution strategy in which either the cation, the anion, or neither component is selectively deuterated, enabling a controlled comparison between “single-spin” and “two-spin” systems. This approach allows for a reliable determination of translational self-diffusion coefficients, either directly from total relaxation rates in the single-spin case or from properly separated intermolecular contributions in two-spin systems.

With respect to rotational dynamics, our recent studies [2,3] introduces advanced motional models that go beyond the commonly assumed isotropic reorientation. Using the model systems [TEA][NTf₂] and [C₅Py][NTf₂], we demonstrate how ion-specific geometry and internal flexibility govern the relaxation behavior. While nearly spherical cations can be described by classical Bloembergen-Purcell-Pound (BPP) models, elongated ions require anisotropic descriptions, and anions containing CF3 groups necessitate the inclusion of fast internal rotations. The simultaneous analysis of 1H and 19F relaxation data over a wide temperature range enables a consistent decomposition of relaxation rates, including both homo- and heteronuclear intermolecular contributions. The resulting rotational correlation times and self-diffusion coefficients show excellent agreement with MD simulations, providing strong validation of the proposed framework.

References

[1] L. Kruse, A. M.C. Tony, D. Paschek, P. Stange, R. Ludwig, A. Strate, J. Phys. Chem. Lett., 15, 1, (2024).

[2] L. Kruse, T. van Alphen, J. Busch, D. Paschek, R. Ludwig, A. Strate, Phys. Chem. Chem. Phys., 27, 10927-10938, (2025).

[3] L. Kruse, A. M. Chiramel Tony, D. Rauber, R. Ludwig, D. Paschek, A. Strate, Magn. Reson. Chem., 64, 3, 265–272, (2026).