Core Electron Transitions as a Probe for Molecular Chirality: Natural Circular Dichroism at the Carbon K-edge of Methyloxirane

The Natural Circular Dichroism (NCD) of electronic and vibrational transitions has been widely used as a probe of molecular chirality. The extension of NCD to core transitions is now possible at third generation synchrotrons equipped with insertion devices which produce radiation with a high rate of circular polarisation. The first NCD measurements in the X-ray region were reported for core to valence state transitions of transition metal and lanthanide complex ions in oriented molecular crystals. Here we report the first NCD measurements on a randomly oriented system: the carbon K-edge absorption spectrum of a simple organic compound (methyloxirane) in the vapour phase. The signs and the intensity of the dichroism agree with previous calculations and indicate that a considerable hybridisation occurs between the core 1s and the valence molecular states. These results allow the possibility of direct assignment of the local chirality of single functional groups in a molecule. The observed dissymmetry factors are sufficiently large to support the hypothesis of differential photo-decomposition by circularly polarised X-ray as the origin of terrestial homochirality