Vibrational circular dichroism enhancement in conformationally flexible transition metal complexes

We extend our previously developed approach for calculating enhanced vibrational circular dichroism (VCD) spectra of transition-metal complexes to systems for which conformational flexibility is key to reproduce and elucidate experimental spectra. Treating both the Gibbs free energies and electronic excitation energies as fitting parameters, we show for Co(II)bis[N-(1-arylethyl)-salicylaldiminato] Schiff base complexes that these calculations can excellently reproduce the experiment. An important conclusion is that the DFT-optimized conformer set is spectrally redundant and that the model can be reduced from 18 conformers to only two conformers with opposite chirality at the metal center (Λ/Δ) without affecting the agreement between theory and experiment, both with respect to the VCD spectrum as well as population distribution over the Λ and Δ conformers. Finally, we provide a rigorous demonstration of the symmetry-selective enhancement mechanism proposed by Pescitelli et al. by deriving the corresponding selection rules starting from Sum-Over-State expressions. We show that our approximate C₂ symmetry analysis numerically confirms that low-lying electronic states of B symmetry predominantly enhance B-like vibrational modes and find that with this computational model the nearly monosignate experimental VCD spectrum is well reproduced.