Chiroptical Properties of 1-Azabicyclo[3.1.0]hexane in the Vacuum-UV and IR Regions

The structure and force field of a bridgehead aziridine, l-azabicyclo(3.l.0)hexane (ABH), have been determined by ab initio computational methods at three levels, RHF/6-31G*(0.3), MP2/6-31G*, and Becke3LYP/6- 31G*. All three methods are shown to give very similar geometries and force fields. The chiroptical properties of ABH have been investigated experimentally and theoretically in the UV region by the ab initio PCI method and in the IR region by the vibronic coupling theory as implemented in the VCT90 program. The W/CD investigation represents the fist such study on an aziridine. The lower electronic transitions of ABH, all of which occur in the vacuum-UV region, are to Rydberg states. No simple chiral rule emerges from the UV/CD study. The experimental vibrational circular dichroism spectrum displays a prominent feature which is due to wagging of the methine hydrogen and obeys a previously identified I ICH methine wagging rule. Comparison of the CEs attributable to local modes of the three-ring methylene group of ABH with analogous published data on related aziridines suggests that some of these modes may serve as indicators of N atom and skeletal absolute configurations.