Expanded View of NMR Spin-Lattice Relaxation in Fluorine-Containing Ionic Liquids

Fluorine-containing anions are widely used in ionic liquids due to their unique physicochemical properties. However, the local dynamics of both cations and anions and their associated relaxation mechanisms remain incompletely understood. Here, we present a ¹H and ¹⁹F spin-lattice relaxation rate (R₁) study as a function of frequency over a broad frequency range from 30 kHz to 800 MHz for ionic liquids containing BF₄^-, PF₆^-, TFSI^-, and FSI^- anions and EMIM⁺ cation. By combining experimental R₁^H and R₁^F NMR dispersion (NMRD) profiles with relaxation models for both dipolar spin interactions and chemical shift anisotropy (CSA) contributions, we demonstrate that CSA is needed to accurately describe the R₁^F relaxation behavior above ∼300 MHz, the extent of which depends on the anion structure. These findings challenge the long-standing assumption that dipolar contribution is the main source of ¹⁹F relaxation in these systems and highlight the importance of including CSA to accurately interpret ¹⁹F relaxation in ionic liquids, particularly at high frequencies. This work provides new insights into the molecular dynamics of fluorine-containing species.