Translational Dynamics of Cations and Anions in Ionic Liquids from NMR Field Cycling Relaxometry: Highlighting the Importance of Heteronuclear Contributions

NMR field cycling relaxometry is a powerful method for determining the rotational and translational dynamics of ions, molecules, and dissolved particles. This is in particular true for ionic liquids (ILs) in which both ions carry NMR sensitive nuclei. In the IL triethylammonium bis(trifluoromethanesulfonyl)imide ([TEA][NTf₂]), there are ¹H nuclei at the [TEA]⁺ cations and ¹⁹F nuclei at the [NTf₂]⁻ anions. Moreover, the high viscosity of this IL leads to frequency-dependent relaxation rates, leaving the so-called extreme narrowing regime. Both the rotational and the translational dynamics of the constituents of ILs can be obtained by separating the contributions of intra- and intermolecular relaxation rates. In particular, the translational dynamics can be obtained separately by applying the so-called “low-frequency approach” (LFA), utilizing the fact that the change in the total relaxation rates at low frequencies results solely from translational motions. However, for systems containing multiple NMR active nuclei, heteronuclear interactions can also affect their relaxation rates. For [TEA][NTf₂], the intermolecular relaxation rate is either the sum of ¹H–¹H cation–cation and ¹H–¹⁹F cation–anion interactions or the sum of ¹⁹F–¹⁹F anion–anion and ¹⁹F–¹H anion–cation interactions. Due to the lack of available experimental information, the ¹H–¹⁹F heteronuclear intermolecular contribution has often been neglected in the past, assuming it to be negligible. Employing a suitable set of ILs and by making use of isotopic H/D substitution, we show that the ¹H–¹⁹F heteronuclear intermolecular contribution in fact cannot be neglected and that the LFA cannot be applied to the total ¹H and total ¹⁹F relaxation rates.