On the Equivalence of Conformational and Enantiomeric Changes of Atomic Configuration for Vibrational Circular Dichroism Signs

We study systematically the vibrational circular dichroism (VCD)
spectra of the conformers of a simple chiral molecule, with one chiral
carbon and an “achiral” alkyl substituent of varying
length. The vibrational modes can be divided into a group involving
the chiral center and its direct neighbors and the modes of the achiral
substituent. Conformational changes that consist of rotations around
the bond from the next-nearest neighbor to the following carbon, and
bond rotations further in the chain, do not affect the modes around
the chiral center. However, conformational changes within the chiral
fragment have dramatic effects, often reversing the sign of the rotational
strength. The equivalence of the effect of enantiomeric change of
the atomic configuration and conformational change on the VCD sign
(rotational strength) is studied. It is explained as an effect of
atomic characteristics, such as the nuclear amplitudes in some vibrational
modes as well as the atomic polar and axial tensors, being to a high
degree determined by the local topology of the atomic configuration.
They reflect the local physics of the electron motions that generate
the chemical bonds rather than the overall shape of the molecule.