Natural circular dichroism in XANES of chiral transition metal systems

Abstract
The possibility of natural circular dichroism measurements in XANES and EXAFS is promised by new helical undulators at high brilliance synchrotrons. The study of chiral transition metal sites can provide important structural information on enzyme active sites and synthetic asymmetric catalysts. In the present work the mechanism for the manifestation of rotational strength in core-valence transitions of naturally chiral transition metal systems is considered. The approach taken is to evaluate the effect of pseudoscalar and pseudotensor perturbations in cubic achiral parent geometries. The expected magnitudes of the rotational strengths of K- and L-edge transitions are discussed for the O_h-D₃ system tris(1,2-diaminoethane) Co^(III) in the context of recent rotation measurements. The key problem of the source of core-valence magnetic dipole transition moments is examined. A treatment is outlined incorporating the effects of valence state mixing and core level mixing brought about by the chiral molecular potential. Sum rules and restricted sum rules for rotational strengths are discussed.