Revealing the Nature of Non-Covalent Interactions in Ionic Liquids by Combined Pulse EPR and ¹⁹F NMR Spectroscopy

Ionic liquids (ILs) are a unique class of compounds that have attracted interest for numerous and diverse applications, ranging from solvents for sustainable synthesis to sustainable electrolytes. Understanding their nanostructure and solute-solvent interactions is a prerequisite to harnessing the full potential of ILs. It has been proposed that ILs solvate non-polar solutes via their alkyl chains through the formation of nanoscale structures, such as micelles. Here, we determine the non-covalent interactions responsible for such nanostructuring in ILs. We use pulse electron paramagnetic resonance (EPR), paramagnetic relaxation enhancement (PRE) NMR, molecular dynamics (MD), and density functional theory (DFT) calculations in combination with ILs tailored to probe specific interactions through spin and isotopic labelling. Inter- and intramolecular cation-anion interactions are probed by electron-nuclear double resonance (ENDOR) and ¹⁹F PRE experiments and show that nitroxide solutes associate with the polar domains of the imidazolium cation through weak hydrogen bonding with the imidazolium ring protons, as supported by MD simulations. Thus, this study reveals a less structured nanostructure than a micellar picture might suggest, but with clear IL cation-solute interactions. Our methodology to reveal nanostructure not only has implications for ILs but is also applicable to other soft matter systems.