Multimodal Dynamics in Ionic Liquids Revealed by Molecular-Dynamics-Guided Multinuclear NMR Relaxation Analysis

Ionic liquids are complex liquids characterized by high viscosity and high polarity. Their dynamics are of interest due to their use in many fields that require unconventional solvent environments. Here, we develop an MD-based forward-prediction framework to calculate NMR relaxation rates in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF₄], and investigate their temperature and magnetic-field dependence. We show that the combination of intra- and intermolecular dipolar couplings and chemical shift anisotropy interactions allows accounting for the experimentally observed nuclear spin lifetimes when considering multimodal relaxation processes. A regularized inverse Laplace transformation of MD-derived interactions reveals generally bimodal motional processes active in both inter- and intramolecular effects. The presence of two main motional processes may be somewhat surprising for intramolecular effects, especially in the symmetric [BF₄]^- unit, but can be attributed to the restricted motion due to the local environment. We also highlight that the Bloembergen-Purcell-Pound (BPP) approach that is often used to characterize average motional correlation times is not reliable in most situations encountered with realistic systems. Consequently, we argue that achieving quantitative agreement between predicted and measured NMR relaxation rates offers a more robust route to extracting structural and dynamical information from complex liquids.