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

Abstract

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 13 TFSI), the homologue with bis ( Li}NOE 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 { 1 H- 19F}NOEs spot on the polar/apolar domains within the ILs, whereas shortrange (e.g. regarding the first coordination shell) { 1 H-7 Li}NOEs describe the contacts between first neighbors, with interesting correlation with the distances' statistics derived by crystallographic data of related systems.

A review of NMR methods used in the study of the structure and dynamics of ionic liquids

“…In another report by the same author, the intermolecular interaction between the Li + cation with the ions of ILs has been studied through the heteronuclear NOEs experiments in the LiTFSI mixtures of two pyrrolidinium‐based ILs . A significant amount of cross correlation has been observed between the Li + cation with both the ions of ILs as observed from the 1 H‐ 19 F and 1 H‐ 7 Li NOEs experiments …”

Determination of inter‐ionic and intra‐ionic interactions in a monofluorinated imidazolium ionic liquid by a combination of X‐ray crystallography and NOE NMR spectroscopy

“…In addition, we have determined all relevant H─F distances also from our crystal structure, to be able to compare the values. This approach has successfully been followed earlier already by other groups …”

Investigating Intermolecular Interactions in a DME-Based Hybrid Ionic Liquid Electrolyte by HOESY NMR

“…Nuclear Overhauser Effect (NOE) based experiments are particularly useful to probe nanostructured arrangements in ILs through the transfer of nuclear spin polarization by cross-relaxation (Damodaran, 2016). In particular, the Heteronuclear Overhauser Effect SpectroscopY (HOESY) pulse sequence (Lingscheid et al, 2012; Bai et al, 2013; Chiappe et al, 2013; Castiglione et al, 2014, 2015; Giernoth et al, 2014; Tripathi and Saha, 2014) has been used to probe the various molecular interactions in ILs and their interactions with other solvents.…”