Local and framework stereochemical markers in vibrational circular dichroism: 1,2- and 2,3-dimethylaziridines

The vibrational circular dichroism (VCD) spectra of 2,3-dimethylaziridine 1 and 1,2-dimethylaziridine 2 were measured experimentally and interpreted with the help of ab initio Vibronic Coupling Theory (VCT) both in the common origin (CO) and distributed origin (DO) gauge. The absolute magnitudes of the VCD intensities by the VCT-CO method in the mid-IR range are closer to the experimental measurement. Both VCT-DO and VCT-CO methods, however, predict the same signs for almost all VCD bands. In general, both VCT-DO and VCT-CO methods predict VCD and IR spectra that are both in good agreement with the experimental spectra. The VCT method together with the 6-31G*(0.3) basis set can be employed with confidence in determining the absolute configuration of a chiral molecule. The theoretical and experimental VCD spectra of 1 and 2 were compared with the VCD spectra of the isostructural 2,3-dimethyloxirane and -thiirane. One of the Me rocking motions, in the wave number range 1050–1150 cm−1, was found to serve as a possible marker for the stereochemistry of the molecular skeleton. Two other modes were identified whose Cotton effects (CEs) are characteristic of the local asymmetry, and for which chirality rules were proposed. The first is a methyne vibration, ||CH, which is expected to have a (−) CE at a three-ring centre with (R) absolute configuration. The second is an umbrella motion of a methyl group, which is expected to have a (+) CE when attached to a three-ring centre with (R) absolute configuration.