Ionic liquids (ILs) are receiving increasing interest for their use in synthetic laboratories and industry. Being composed of charged entities, they show a complex and widely unexplored dynamic behavior. Chiral ionic liquids (CILs) have a high potential as solvents for use in asymmetric synthesis. Chiroptical methods, owing to their sensitivity towards molecular conformation, offer unique possibilities to study the structure of these chiral ionic liquids. Raman optical activity proved particularly useful to study ionic liquids composed of amino acids and the achiral 1-ethyl-3-methylimidazolium counterion. We could substantiate, supported by selected theoretical methods, that the achiral counterion adopts an overall chiral conformation in the presence of chiral amino acid ions. These findings suggest that in the design of chiral ionic liquids for asymmetric synthesis, the structure of the achiral counter ion also has to be carefully considered.

Backscattered Raman optical activity (ROA) spectra are measured for Δ- and Λ-tris-(ethylenediamine)rhodium(III) chloride in aqueous solution. In addition, the spectra of the four possible conformers in the Λ configuration are investigated by ab initio calculations. The Λ(δδδ) conformer is in best agreement with experimental spectra and examined in more details. The two most stable conformers according to the calculations are not compatible with the experimental ROA spectrum. Insights into the origin of observed band intensities are obtained by means of group coupling matrices. The influence of the first solvation shell is explored via an ab initio molecular dynamics simulation. Taking explicit solvent molecules into account further improves the agreement between calculation and experiment. Analysis of selected normal modes using group coupling matrices shows that solvent molecules lead to normal mode rotation and thus contribute to the ROA intensity, whereas the contribution of the Rh can be neglected.

The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm^–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au₂₅(2-PET)₁₈]^0/–, Au₃₈(2-PET)₂₄, Au₄₀(2-PET)₂₄, and Au₁₄₄(2-PET)₆₀ (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au₂₅(2-PET)_18–2x(S-/rac-BINAS)_x (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au₂₅(CamS)₁₈ (CamS = 1R,4S-camphorthiol), and Au_nBINAS_m were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au₃₈(SCH₃)₂₄ was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending around 200 cm^–1 shifts to slightly higher wavenumbers for the dimeric as compared to the monomeric staples. Radial Au–S modes (250–325 cm^–1) seem to be sensitive toward the staple composition and the bulkiness of the ligand, having higher intensities for long staples and shifting to higher wavenumbers for sterically more demanding ligands. The introduction of only one BINAS dithiol has a dramatic influence on the Au–S vibrations because the molecule bridges two staples which changes their vibrational properties completely.