L-Stercobilin-HCl and d-Urobilin-HCl. Analysis of Their Chiroptical and Conformational Properties by VCD, ECD, and CPL Experiments and MD and DFT Calculations

Vibrational circular dichroism (CD) and IR spectra of dichloromethane solutions of l-stercobilin and d-urobilin hydrochlorides have been recorded in the mid-IR region. The spectra are best interpreted by combining molecular dynamics calculations and density functional theory (DFT) calculations within the quantum mechanics/molecular mechanics ONIOM-type framework, and the combined predicted results are better and more informative than the more standard analysis provided by DFT calculations. The same approach also sheds light on the Cotton effect sign inversion of room temperature versus low-temperature electronic CD spectra of the same compounds in methanol–glycerol solution. Finally, circularly polarized luminescence spectra for l-stercobilin in chloroform solution provide information on the excited-state geometry of this molecule.