# How to Disentangle Cation and Anion Dynamics of Fully Protonated Ionic Liquids: A Fast Field Cycling NMR Case Study

## TL;DR

This paper introduces a new method to study the movement of ions in fully protonated ionic liquids using NMR relaxometry.

## Contribution

The study presents the first use of selective deuteration to disentangle cation and anion dynamics in fully protonated ionic liquids.

## Key findings

- Selective deuteration allows quantification of cation-anion relaxation contributions in fully protonated ionic liquids.
- The total relaxation rate of the fully protonated IL can be reconstructed from partial deuteration data.
- This method extends the applicability of FFC NMR to more complex ionic liquid systems.

## Abstract

The molecular dynamics of ionic liquids (ILs) can be probed using fast field cycling (FFC) NMR relaxometry. Conventionally, such studies focus on ILs where only one ionic species carries NMR‐active nuclei or on systems combining
H nuclei on the cations with
F nuclei on the anions. This way, the dynamics of cations and anions can be resolved individually. However, the situation becomes considerably more complex in fully protonated systems where both ions contain protons, because the various relaxation pathways can no longer be disentangled. Here we report the first FFC NMR investigation of such a case, using the IL triethylammonium methanesulfonate ([TEA][OMs]). Our strategy exploits selective partial deuteration of the ionic species, which enables the separate evaluation of cation and anion dynamics. We demonstrate for the first time that, from the known partial relaxation rates together with the determined interionic distances and self‐diffusion coefficients, the relaxation contribution arising from cation–anion interactions can be quantified. Remarkably, this approach even allows reconstruction of the total relaxation rate observed experimentally for the fully protonated IL. This methodology provides a fundamentally new route to overcoming the limited spectral resolution of FFC NMR relaxometry at low fields. More broadly, it establishes a framework for disentangling relaxation processes in complex multicomponent systems, thereby extending the applicability of FFC NMR to more challenging classes of ILs and related materials.

This work presents the first FFC NMR relaxometry study of a fully protonated ionic liquid, [TEA][OMs]. Using selective partial deuteration, cation and anion dynamics were disentangled, enabling the hidden cation–anion relaxation contributions to be quantified and the total relaxation rate to be reconstructed.