Robust normal modes in vibrational circular dichroism spectra

Abstract

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 xi between the electric and magnetic dipole transition moments, and the magnitudes of these dipole transition moments.