Implementation, Calculation and Interpretation of Vibrational Circular Dichroism Spectra

Baerends, E.J. [Promotor

An Artificial Intelligence Approach for Tackling Conformational Energy Uncertainties in Chiroptical Spectroscopies

Determination of the absolute configuration of chiral molecules is a prerequisite for obtaining a fundamental understanding in any chirality-related field. The interaction with polarised light has proven to be a powerful means to determine this absolute configuration, but its application rests on the comparison between experimental and computed spectra for which the inherent uncertainty in conformational Boltzmann factors has proven to be extremely hard to tackle. Here we present a novel approach that overcomes this issue by combining a genetic algorithm that identifies the relevant conformers by accounting for the uncertainties in DFT relative energies, and a hierarchical clustering algorithm that analyses the trends in the spectra of the considered conformers and identifies on-the-fly when a given chiroptical technique is not able to make reliable predictions. The effectiveness of this approach is demonstrated by considering the challenging cases of papuamine and haliclonadiamine, two bis-indane natural products with eight chiral centres and considerable conformational heterogeneity that could not be assigned unambiguously with current approaches.</p

Robust normal modes in vibrational circular dichroism spectra

The use of calculations of the rotational strengths of normal modes in order to determine the absolute configuration (AC) of a molecule, by comparing a calculated vibrational circular dichroism (VCD) spectrum to an experimental one, can be made much more reliable when the vibrational modes are classified as either robust or non-robust. The robust modes are the ones with a robust sign of the rotational strength in the sense that it will not change by small perturbations in either experiment or calculation. The signs of non-robust modes may change. Clearly only robust modes should be used to establish the AC. We recommend that programs which calculate VCD spectra should provide, per normal mode, information that indicates the robustness of a mode, and therefore its usefulness for the AC determination. Such information consists of the angle ξ between the electric and magnetic dipole transition moments, and the magnitudes of these dipole transition moments. © 2009 the Owner Societies