Deciphering the anion dynamics in the fully deuterated pyridinium-based ionic liquid [C₅Py][NTf₂] via¹⁹F fast field...

Fast field cycling (FFC) NMR relaxometry allows the investigation of molecular dynamics over a broad frequency range from 10 kHz up to 100 MHz. However, quantitative data interpretation is often limited by system-specific motional models and simplified assumptions for the analytical form of the relevant time correlation functions. Here, we propose a different "quasi-first-principles" based approach utilizing atomistic molecular dynamics (MD) simulations to predict frequency dependent relaxation rates based on the time evolution of the dipolar spin-spin interactions. The combination of MD simulations with the analytical Hwang-Freed (HF) theory allows us to accurately describe the low-frequency behavior of the intermolecular relaxation. We show that the theory asymptotically converges with the MD data at long times, providing a physically consistent description of intermolecular relaxation over many orders of magnitude in frequency. As a proof of concept for a complex ionic liquid (IL), we investigate the temperature dependent ¹⁹F NMR relaxation of a fully deuterated IL 1-(n-pentyl)-pyridinium bis(trifluoromethanesulfonyl)imide [C₅Py-d₁₆][NTf₂]. Deuteration suppresses heteronuclear ¹H-¹⁹F relaxation pathways, restricting the system to a single NMR-active nucleus. Using this framework, we determine both the intermolecular and intramolecular ¹⁹F relaxation rates and additionally disentangle internal CF₃ rotations from overall anion tumbling motions. Our simulations highlight the importance of the so-called "eccentricity" effects for the intermolecular relaxation rate caused by mutual reorientations of adjacent ions. Despite the limitations of molecular models, the predicted frequency dependent relaxation rates are in reasonable agreement with the experimentally measured FFC NMR data.