Communication: Spectroscopic characterization of a strongly interacting C (2) H group on the EMIM + cation in the (EMIM + ) 2 X - (X = BF 4 , Cl, Br, and I) ternary building blocks of ionic liquids.

To unravel the intermolecular interactions at play in the assemblies of EMIM-based ionic liquids, we report the vibrational spectra of the cryogenically cooled, cationic ternary complexes with halide ions, (EMIM+ )2 X- , X = Cl, Br, and I. This series specifically addresses the spectral response of the acidic C(2) H group upon complexation with an atomic anion, which is isolated by selective deuteration at this position. Unlike the behavior of the related BF4 - complex, the halide systems display large (∼175 cm-1 ) red-shifts and dramatic (∼30X) intensity enhancements in the C(2) D stretch (relative to that of the bare d(2) -EMIM+ cation), which are largest for the chloride anion. Electronic structure calculations indicate that, while the spectroscopic signatures of the interaction follow those expected for a traditional hydrogen bond to the C(2) H group, the C-H-X docking arrangement deviates substantially from linearity due to a "double contact" motif involving a weaker interaction to the nearby methyl group [Hunt et al., Chem. Soc. Rev. 44(5), 1257-1288 (2015)].