Vibrational Circular Dichroism enhancement in conformationally flexible transition metal complexes

Abstract

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 population distribution over the $Λ$ and $Δ$ conformers. Finally, we numerically confirm the symmetry-selective nature of the enhancement and formulate the corresponding selection rules within the framework of the pertaining Sum-Over-States expressions.