Solving the puzzling absolute configuration determination of a flexible molecule by vibrational and electronic circular dichroism spectroscopies and DFT calculations: The case study of a chiral 2,2′-dinitro-2,2′- biaziridine

摘要

The absolute configuration of a recently synthesized racemate of 2,2′-dinitro-2,2′-biaziridine (2a), a possible catalyst for asymmetric synthesis, has been determined by vibrational circular dichroism (VCD) spectroscopy in the mid-IR region and DFT calculations. Electronic circular dichroism (ECD) spectra have been obtained and Time-Dependent DFT (TD-DFT) calculations have been performed and found to be in agreement with the conclusions from VCD and DFT. A detailed conformational analysis, for approximately 300 geometries, has been carried out, allowing us to find evidence for the most populated conformers chiefly contributing to VCD and ECD spectra. The proposed absolute configuration and prevalent conformers are in correspondence with an equal configuration of the nitrogen atoms of the two aziridine rings. A 2,2′-dinitro-2,2′-biaziridine containing four "stable" chiral centers at carbon atoms furnishes just one pair of enantiomers among the 2~4 possible stereoisomers. Vibrational circular dichroism spectra and aconformational search allowed us to determine the absolute configuration (see figure). Also the configuration of the nitrogen atoms of the aziridine rings has been established.