Vibrational circular dichroism study of optically pure cryptophane-A

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of optically pure cryptophane-A molecule. This large molecule (120 atoms) that possess a globular shape, but no chiral centers, exceeds the molecular size of published structures for which VCD has been used to determine the absolute configuration. VCD spectra recorded in CDCl3 solution for the two resolved enantiomers are near mirror images, and very good agreement between the observed IR and VCD spectra and intensity calculations performed at the DFT (B3PW91/6-31G*) level establish, besides the absolute configuration, the preferential anti conformation of the aliphatic linkers of the chloroform-cryptophane-A complex. Experiments performed in CD2Cl2 and C2D2Cl4 solutions show no significant modifications in the IR and VCD spectra, indicating that the conformation of the aliphatic linkers is similar for empty (C2D2Cl4 solution) and encaged (CDCl3 and CD2Cl2 solutions) cryptophane-A molecules.