The genesis of OH-stretching vibrational circular dichroism in chiral molecular crystals

Abstract

The stretching vibration of hydroxyl groups, ν(OH), appears with a strong absorption in the 3 µm region of the infrared (IR) spectrum. In chiral molecular crystals, also vibrational circular dichroism (VCD) can be observed for this band, which is demonstrated by the example of two chiral alcohols crystallising with space groups P21 and P3121, respectively. Measurements demonstrate that the VCD bands of the ν(OH) mode show an increased fine structure in comparison to the broad infrared absorption bands. In a computational study, the chiroptical signal can entirely be traced back to non-local terms emerging from the supramolecular environment, determined by the hydrogen-bonded network involving the hydroxyl groups. In turn, the VCD of individual molecules in the crystal related to the ν(OH) mode is almost zero. It can thus be concluded that the entire VCD band in the 3 µm region is determined by the chirality of the crystal, but not by that of the molecules. Further analysis reveals that while vibrational coupling mainly arises from the hydrogen-bonded network, the VCD is strongly influenced by the weaker interactions and long-range order. This highlights the significance of the OH stretching mode as a sensitive probe of supramolecular chirality.