Vibrational Circular Dichroism Detects Symmetry Breaking due to Conformational Mobility in C2-Symmetry Chiral Molecules and Provides Further Insight into Inter-Chromophoric Interactions

Bicyclo[3.3.1]nonane-2,6-dione (1) and bicyclo[3.3.1]nona-3,7-diene-2,6-dione (2) have been examined by vibrational circular dichroism (VCD), which, as for most C2-symmetric systems, exhibits strong VCD signals. In the case of 2, VCD signals are stronger and sharper with several bisignate doublets; for 1, signals are less intense and broader. The VCD and IR spectra are excellently predicted by DFT calculations: only one conformer is present for 2, while for 1, three main conformers, related through concerted skeleton torsional motions are present (two of them being interchanged by C2-rotation). The VCD spectrum shows specific features for the different conformers, such that correct population factors are crucial for reproducing experimental data. Also, the TD-DFT prediction of ECD (electronic circular dichroism) spectra is good. By comparing the spectroscopic signature of the two molecules (both VCD and ECD) and by careful analysis of the theoretical results, the role of the C=C double bond in compound (2) is evidenced. The double bond contributes toward enhancing the CD response both electronically and vibrationally.

CF3: an overlooked chromophore in VCD spectra. A review of recent applications in structural determination

The VCD spectra of several chiral compounds containing the CF3 group are reviewed and analyzed. The list of compounds contains pharmaceutically relevant molecules as well as simple model molecules, having the value of case studies. In particular we point out the importance of the sign of the VCD band relative to some stretching normal mode of CF3 in the region 1110-1150 cm-1, as diagnostic of the configuration of stereogenic carbons C* to which the CF3 group is bound: the correspondence (-) ↔ (R) and (+) ↔ (S) holds for 100% of 1-aryl-2,2,2-trifluoroethanols. DFT calculations confirm these conclusions, but for the rule established here they serve just as a check. This rule is tested on two new compounds, namely N-tert-butanesulfinyl-1-(quinoline-4-yl)-2,2,2-trifluoroethylamine, 8, and 4-[2-(2,2,2-trifluoro-1-hydroxyethyl)pyrrolidin-1-yl]-2-(trifluoromethyl)benzonitrile, 10, both containing two stereogenic elements, one of them being an asymmetric carbon C* of unknown configuration binding a CF3 group. Discussion of the general validity of the rule is provided and some further tests are run on compounds in well-established drugs.

Vibrational spectroscopy as a direct stereochemical probe for polyhydroxylated molecules

Relative and absolute configurations of polyhydroxylated molecules via acetonide derivatives: the power of IR and VCD.

VCD spectroscopy as an excellent probe of chiral metal complexes containing a carbon monoxide vibrational chromophore

η⁵-Menthyl-C₅H₄Ru(ii) carbonyl complexes show VCD of CO; the stretching is very sensitive and diagnostic to the (R)_Ru/(S)_Rumetal chirality.