Probing relaxation models by means of Fast Field-Cycling relaxometry, NMR spectroscopy and molecular dynamics simulations: Detailed insight into the translational and rotational dynamics of a protic ionic liquid

A combination of temperature dependent Fast Field-Cycling (FFC) relaxometry and high-resolution NMR spectroscopy provides more than five orders of magnitude in frequency range for studying translational and rotational dynamics of ionic liquids. However, to make use of this broad frequency range, certain requirements have to be met: The viscosity of the liquid has to be of the right order of magnitude, and the material has to exhibit a sufficiently broad liquid range. In addition, NMR sensitive nuclei have to be present (ideally) on both, cation and anion, and appropriate relaxation models properly describing the molecular dynamics have to be available. For the latter, Molecular Dynamics (MD) simulations are ideally suited to suggest meaningful relaxation models. In this study we employ the protic ionic liquid triethylammonium bis(trifluoromethylsulfonyl)-imide [TEA][NTf2] as model compound to gain insight into details of the molecular dynamical processes. By addressing different NMR sensitive nuclei on both ions, 1H nuclei on the triethylammonium cation, and 19F on the NTf2 anion, we are able to obtain translational dynamics as well as rotational dynamics for both species at the same time. The obtained temperature dependent translatoric diffusion coefficients are consistent with our MD simulations, and are found to be in agreement with data reported in the literature. In addition, two types of NMR relaxation processes are employed to investigate the intramolecular relaxation: (1) dipolar relaxation at low frequencies (employing FFC) addressing 1H and 19F nuclei, and (2) quadrupolar relaxation at high frequencies addressing 2H nuclei. In near quantitative agreement with MD simulations, we show that both, dipolar relaxation obtained via FFC and quadrupolar relaxation obtained via high-field NMR are leading to consistent rotational correlation times, suggesting that the rotational motion of the cation is dominantly isotropic. Hence, the intramolecular 1H relaxation rate of the cation determined via FFC can be appropriately expressed by a single rotational correlation time, compatible with the Bloembergen Purcell Pound (BPP) approach. MD simulations of the NTf2 anion, however, suggest that its reorientational dynamics is strongly anisotropic. Consequently, the proper description of the intramolecular 19F relaxation rate of the anion requires a more complex model beyond the BPP approach employing at least two correlation times: By taking internal rotation of the CF3 groups into account, we are able to determine correlation times compatible with our MD simulations.