Induced Chirality and Vibrational Optical Activity in an Ionic-Liquid Anion

Here, we show that the four conformers of the regularly used ionic liquid anion bis(trifluoromethylsulfonyl)imide ${\left[{\mathrm{NTf}}_2\right]}^{-}$ are true enantiomeric pairs by analyzing their calculated vibrational circular dichroism spectra. The significant modes involve those atoms of the anion that form specific hydrogen bond patterns with the chiral probe molecule propylene oxide. Adding this probe molecule to the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, the experimental and simulated spectra indicate intermolecular interaction between the chiral molecule and the liquid and thus the induction of chirality in the anion. This emergence of chirality in the ionic liquid takes place via the hydrogen bonding between the anion of the ionic liquid and the solute, which results in a redistribution of the symmetric occurrence of conformers to an asymmetric one. Altogether, this study reveals the mechanism of chiral induction from the propylene oxide molecule to the ionic liquid anion, while pointing out the importance of rational over a random choice of the ionic liquid building units.