The vibrational circular dichroism spectra of 2-methylaziridine: dominance of the asymmetric centre at nitrogen

Abstract

The experimental VCD spectrum of (2R)-2-methylaziridine has been measured in the region 800–1500 cm−1. The ab initio implementation of the Vibronic Coupling Theory (VCT) of Nafie and Freedman, using the 6-31G*(0.3) basis set, in the common origin and distributed origin gauges and with uniformly and optimally scaled quantum mechanical force fields, is used to investigate the chiroptical properties. VCD spectra are computed for both cis and trans invertomers. The predicted VCD spectrum of 2-methylaziridine (the equilibrium mixture) is dominated by that of the trans diastereomer, not simply because of its greater abundance but because the rotatory strengths of many absorptions in the mid-IR are oppositely signed and of similar magnitude in the two invertomers which differ in absolute configuration at nitrogen. The VCD spectrum of 2-methylaziridine is compared in detail to that of 2-methyloxirane. In the region of the methyl group deformations and CH2 scissor, the theoretical (R)-2-methyloxirane VCD spectrum displays a much closer similarity to the cis-(2R)-2-methylaziridine than to the trans diastereomer.