Cation-Cation, Cation-Anion, and Anion-Anion Translation Diffusion in Ionic Liquids─Insight from NMR Relaxometry.

Abstract

¹H and ¹⁹F spin-lattice relaxation experiments have been performed for a series of ionic liquids: [HMIM][TFSI], [OMIM][TFSI], and [DMIM][TFSI] including the same anion and cations with progressively longer alkyl chains. The experiments were performed in a wide frequency range from 10 kHz to 10 MHz (referring to the ¹H resonance frequency) versus temperature. This extensive data set has been analyzed in terms of a theoretical model including all relevant homonuclear (¹H-¹H and ¹⁹F-¹⁹F) and heteronuclear (¹H-¹⁹F) relaxation pathways and linking the relaxation features to the relative translational diffusion between the ion pairs (cation-cation, cation-anion, and anion-anion). In addition to the comprehensive theoretical approach, closed-form expressions have been provided and applied to determine the diffusion coefficients from the slopes of the linear dependences of the relaxation rates on the square root of the resonance frequency. The combined experimental and theoretical studies have led to the determination of the complete set of diffusion coefficients, forming a consistent picture of the dynamical scenario. In addition to revealing the dynamical properties of the liquids and the influence of the subtle changes in the cation structure on the movement of both cations and anions, the theoretical means for exploiting Nuclear Magnetic Resonance relaxometry for ionic liquids have been provided.