Understanding the Origin of the VCD Signals on the Basis of a Nonredundant Coordinate Definition

The relationships between the chiroptical activity and the vibrational normal modes of epichlorohydrin have been investigated on the basis of a nonredundant internal coordinate definition not reported until now. These coordinates were verified by comparing, for the lower energy conformers, the diagonal quadratic force constants and were found to display similar values among conformers and to be consistent with the molecular structure of epichlorohydrin and its vibrational circular dichroism (VCD) spectrum. Boltzmann population factors were used to calculate the weighted sum of the individual VCD and IR spectra of the three lower energy conformers, which accurately fitted the experimental spectra of (R)-epichlorohydrin. The electric and magnetic transition dipole moments of the 24 vibrational normal modes were calculated for the most stable conformers. The combined analysis of these vectors and the normal mode description, given in terms of the potential energy distribution, allowed us to investigate the role of the functional groups (methylene, chlorine) and the type of internal coordinates (stretching, bending, etc.) in the chiroptical activity of the vibrations.