The knowledge of the absolute structure, i.e. the knowledge which of both enantiomers is present, is of high importance especially for pharmaceuticals. In most cases it is very difficult or at least impossible to gain single crystals of substances for X-ray structure analysis the only method to derive the absolute structure directly. Here VCD spectroscopy reveals as an alternative to determine the absolute structure. By nature enantiomers possess the same physical properties and therefore the same absorption spectra but their VCD spectra show opposite signs, q.v. purity of enantiomers.
Besides other spectroscopic characteristics modern ab initio software enables to calculate VCD spectra of enantiomers. The comparison of recorded and calculated VCD spectra yields an unambiguous determination of the absolute structure for a present enantiomer. A combination of VCD measurements and ab initio calculations of VCD spectra allows to determine the absolute structure of enantiomers.