Beyond isotropic reorientation: probing anisotropic and internal motions in ionic liquids with fast field cycling NMR relaxometry and MD simulations

We investigate the rotational and translational dynamics of ionic liquids (ILs) through a combined approach utilizing fast field cycling nuclear magnetic resonance (FFC NMR) relaxometry and molecular dynamics (MD) simulations. The ILs examined, [TEA][NTf₂] and [C₅Py][NTf₂], were selected to explore differences arising from variations in ion shape and rigidity. FFC NMR relaxometry provides detailed spin-lattice relaxation rate data for both ¹H on cations and ¹⁹F nuclei on anions across broad frequency and temperature ranges, enabling the characterization of ion-specific dynamics. To dissect the total relaxation rates into intramolecular and intermolecular contributions and to accurately interpret these data, advanced relaxation models were employed, accounting for isotropic, anisotropic, and internal rotational motions. The dynamics of the nearly spherical [TEA]⁺ cation were described using the Bloembergen-Purcell-Pound (BPP) model, while the elongated [C₅Py]⁺ cation required a symmetric top model to capture anisotropic rotational behavior. Additionally, the [NTf₂]^- anion's rotational dynamics were modeled to include fast internal rotations of the CF₃ groups. For both ILs, self-diffusion coefficients were also obtained in addition to rotational dynamics. Notably, the analysis explicitly considered heteronuclear intermolecular contributions, which were found to play a significant role in accurately capturing the relaxation behavior. Complementary MD simulations provided rotational correlation times and self-diffusion coefficients, which showed excellent agreement with experimental results, thereby validating the employed relaxation models. These findings contribute to a deeper understanding of IL dynamics, emphasizing the role of ion geometry and internal motions in data evaluation. Thereby, this work establishes a comprehensive framework for future studies on complex IL systems.