Ionic Liquid–Solute Interactions Studied by 2D NOE NMR Spectroscopy

Abstract

Intermolecular interactions between a Ru(2+)(bpy)3 solute and the anions and cations of four different ionic liquids (ILs) are investigated by 2D NMR nuclear Overhauser effect (NOE) techniques, including {(1)H-(19)F} HOESY and {(1)H-(1)H} ROESY. Four ILs are studied, each having the same bis(trifluoromethylsulfonyl)amide anion in common. Two of the ILs have aliphatic 1-alkyl-1-methylpyrrolidinium cations, while the other two ILs have aromatic 1-alkyl-3-methylimidazolium cations. ILs with both shorter (butyl) and longer (octyl or decyl) cationic alkyl substituents are studied. NOE NMR results suggest that the local environment of IL anions and cations near the Ru(2+)(bpy)3 solute is rather different from the bulk IL structure. The solute-anion and solute-cation interactions are significantly different both for ILs with short vs long alkyl tails and for ILs with aliphatic vs aromatic cation polar head groups. In particular, the solute-anion interactions are observed to be about 3 times stronger for the cations with shorter alkyl tails relative to the ILs with longer alkyl tails. The Ru(2+)(bpy)3 solute interacts with both the polar head and the nonpolar tail groups of the 1-butyl-1-methylpyrrolidinium cation but only with the nonpolar tail groups of the 1-decyl-1-methylpyrrolidinium cation.

Design Principles of Ionic Liquids for Transdermal Drug Delivery

“…Given the relevance of the interactions between the anion and cation and given that the properties of the anion alone did not correlate with the measured permeation ranking, we sought to determine whether the intraion interactions correlate with the overall permeation ranking. To this end, we investigated the intraionic interactions using 2D nuclear Overhauser effect spectroscopy (NOESY) . This technique plots the NMR data on two frequency axes and uses the diagonal peaks and crosspeaks to reveal spatial interactions between the underlying chemical groups (in this case, anion and cation) by counting the number of crosspeaks ( Figure a,b).…”

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

“…The interaction of the solute with the TFSA anion is observed to be three times higher for the IL with shorter alkyl chain as compared with the longer chain in IL. The 2D NMR results show that the Ru 2+ (bpy) 3 solute interacts only with the nonpolar tail groups of the –decyl substituted imidazolium‐based IL; however, it interacts with both the polar head and nonpolar tail groups of the –butyl substituted pyrrolidinium‐based IL …”

“…The 2D NMR results show that the Ru 2+ (bpy) 3 solute interacts only with the nonpolar tail groups of the -decyl substituted imidazolium-based IL; however, it interacts with both the polar head and nonpolar tail groups of the -butyl substituted pyrrolidinium-based IL. [85] The information regarding the ion pairing in the triethylaminebased proton conducting ILs has been obtained from the study of heteronuclear HOESY 15 N-1 H and 13 C-1 H 2D NMR experiments. [59] The 1 H-19 F and 1 H-7 Li HOESY experiments have been performed in [N 1114 ][TFSA]-LiTFSA mixtures in order to understand the interionic interactions in the mixture.…”

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 first application of NOE to ILs dated back 1995 by the group of Osteyoung [5]. Since then, many applications of NOE for ILs have been published, either homonuclear (typically { 1 H-1 H}NOE) [6][7][8][9] or heteronuclear (most of the cases { 1 H- 19 F}NOE) [10][11][12]. The main strength of the NOE approach is that it is originated by the mutual relaxation (cross-relaxation) of NMR interacting nuclei close in space, thus providing precious information on the three dimensional structure of molecules and/or aggregate containing the interacting nuclei.…”

“…Different models have been developed in order to account for these factors [13][14], all leading to the conclusion that the intermolecular NOE extends beyond the 5 Å threshold, thus assuming a long-range, rather than short-range, character. In the context of ILs, these findings have a quite important fall-out, as many investigations on the local structure of ILs are based on the intermolecular homo- [6][7][8][9] and heteronuclear NOE [10,12]. As remarked, many ILs are composed of cations containing H atoms in an organic frame (e.g.…”