Molecular Mobility in a Set of Imidazolium-Based Ionic Liquids [bmim]⁺A^- by the NMR-Relaxation Method

The detailed investigation of the local mobility in a set of dried imidazolium-based ionic liquids (1-butyl-3-methylimidazolium) in a wide temperature range and varying anions (BF₄^-, I^-, Cl^-, Br^-, NO₃^-, TfO^-) is presented. The measurements of temperature dependencies of the spin-lattice relaxation times of ¹H and ¹³C nuclei are motivated by the need to obtain a fundamental characterization of molecular mobility of the substances under study, namely, to estimate the correlation times, τ_c, for the motion of individual molecular groups. In particular, it follows from obtained results that the mobility of the hydrocarbon "tail" is higher (smaller τ_c) than that of the imidazole ring, and this expected tendency is quantified. The effect of the influence of an anion type on the cation mobility is also analyzed.