Measurement of H/D isotope effects on the (19)F resonances of sequentially deuterated 1-n-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate isotopologues reveals the formation of aliphatic C-H···X hydrogen bonds between the fluorinated anions and protons along the N-alkyl sidechains of the cations.

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 F nuclear shielding of deuterated isotopologues of 1-n-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate ([C mim]PF and [C mim]BF ). Δ F(H,D) values ranging from 9.7 to 49.7 ppb were observed for [C mim]PF isotopologues, while for the [C mim]BF 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.

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