Observation of aliphatic C-H···X hydrogen bonds in imidazolium ionic liquids.

Measurement of H/D isotope effects on the 19F 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.