Determining the Relative Strengths of Aromatic and Aliphatic C-H⋅⋅⋅X Hydrogen Bonds in Imidazolium Ionic Liquids through Measurement of H/D Isotope Effects on 19F Nuclear Shielding

The relative strengths of aromatic and aliphatic C―H⋅⋅⋅X hydrogen bonds in imidazolium ionic liquids were investigated through measurement of H/D isotope effects on the 19F nuclear shielding of deuterated isotopologues of 1-n-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate ([C4mim]PF6 and [C4mim]BF4). Δ19F(H,D) values ranging from 9.7 to 49.7 ppb were observed for [C4mim]PF6 isotopologues, while for the [C4mim]BF4 series these went from 26.2 to 83.8 ppb. Our findings indicate that the interactions between the fluorinated anions and protons on the C-1′ and C-1″ position of the N-alkyl sidechains are comparable to, and in some cases stronger than, those involving protons on the aromatic ring, underscoring the role that these weak interionic forces have on the local ordering of imidazolium salts in the liquid state. Copyright © 2017 John Wiley & Sons, Ltd.