H/D Isotope Effects on Nuclear Shielding as a Tool for the Study of Interionic Hydrogen Bonds in Ionic Liquids

Abstract

The use of H/D isotope effects on nuclear shielding (IENS) as a tool to investigate interionic hydrogen bonds (H-bonds) in ionic liquids (ILs) is showcased. Results obtained with deuterated isotopologues of 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl), hexafluorofosphate ([C4mim]PF6), and tetrafluroborate ([C4mim]BF4) indicate that the H/D IENS on the 35/37Cl and 19F resonances of the anions, or Δ35/37Cl(H,D) and Δ19F(H,D), allow to identify and differentiate H-bonds between the anion and the different aromatic protons on the cation. H/D IENS for sequentially deuterated [C4mim]BF4 and [C4mim]PF6 isotopologues also reveal the formation of aliphatic C-H⋅⋅⋅X hydrogen bonds between the fluorinated anions and protons along the sidechains, and indicate that the interactions between the anions and hydrogens on the N-alkyl C-1’ and C-1” positions are, in some cases, stronger than those involving protons on the aromatic ring. Preliminary studies aimed at determining the effects of sidechain length and structure on intrerionic H-bonds in ILs are also discussed.