Multiple points of view of heteronuclear NOE: Long range vs short range contacts in pyrrolidinium based ionic liquids in the presence of Li salts

The nuclear Overhauser enhancement (NOE) is a powerful tool of NMR spectroscopy extensively used to gain structural information in ionic liquids (ILs). A general model for the distance dependence of intermolecular NOE in ILs was recently proposed showing that NOE spots beyond the first solvation shell and accounts for long-range effects. This conclusion prompted for a deep rethinking of the NOE data interpretation in ILs. In this paper we present an extensive and quantitative study of N-propyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR<inf>13</inf>TFSI), the homologue with bis(fluorosulfonyl)imide (PYR<inf>13</inf>FSI), and their mixtures with LiTFSI based on ¹H-¹⁹F and ¹H-⁷LiNOE correlation experiments (HOESY). The former is mainly tuned on long-range interactions, the latter on short-range ones, due to the small and large Larmor frequency differences of the involved nuclei. The collected data are discussed in two different way: long-range ¹H-¹⁹FNOEs spot on the polar/apolar domains within the ILs, whereas short-range (e.g. regarding the first coordination shell) ¹H-⁷LiNOEs describe the contacts between first neighbors, with interesting correlation with the distances' statistics derived by crystallographic data of related systems.