Jan BLASIUS1, Paul ZABY1, Oldamur HOLLÓCZKI2, Barbara KIRCHNER1
1Mulliken Center for Theoretical Chemistry, Clausius Institute for Physical and Theoretical Chemistry, University of Bonn, Bonn, Germany
2Department of Physical Chemistry, Faculty of Science and Technology, University of Debrecen, Debrecen, Hungary
Chiral ionic liquids (CILs) can be used as solvents in asymmetric synthesis or as selectors in enantiomeric recognition and separation processes. In order to adjust CILs for an efficient use in these applications, knowledge about dynamic effects and the molecular structure are of key importance. Due to their sensitivity towards molecular conformations and conformational changes, chiroptical spectroscopy methods such as vibrational circular dichroism (VCD) offer unique possibilities to study the structure of CILs and in particular the occurrence of chirality transfer effects.
Herein, we investigate the chiral ionic liquid 1-ethyl-3-methylimidazolium L-alaninate ([C2C1Im][L-ala]) by means of ab initio molecular dynamics simulations. From the analysis of VCD spectra we observe indications for chirality transfer from the chiral anion to the achiral cation. Structural analyses reveal that this transfer of chirality induces a symmetry breaking in the conformational distribution of the achiral cation by a perturbation of the energetic degeneration of two mirror-imaged conformers. The presence of the chiral anion thus induces a preference for distinct cation conformations which can be tracked by VCD.
Due to the possible application of CILs as chiral selectors, we further investigate whether the observed chirality transfer might affect chiral recognition mechanisms. To this end, we studied two model systems being composed of (R)- or (S)-butan-2-ol dissolved in [C2C1Im][L-ala]. VCD spectra and structural analyses reveal discriminative interactions between the CIL and the enantiomers of butan-2-ol. Although the recognition mechanism of butan-2-ol is strongly conducted by the chiral anion via formation of diastereomeric complexes, we observe that butan-2-ol transfers chiral information to the achiral cation. We observe that (R)-butan-2-ol and L-alaninate induce the same chirality in the cation, whereas (S)-butan-2-ol and L-alaninate induce opposite chirality. This effect creates a more favorable environment for (R)-butan-2-ol in the CIL compared to (S)-butan-2-ol. The presented results can help with the design of new CIL based selectors for small molecules.