Natural dircular dichroism beyond chirality
Résumé
While natural optical activity in crystals belonging to specific achiral point groups has been predicted by symmetry arguments, birefringence hampers its measurement in the UV–vis energy ranges. Here we use X-rays to reveal natural circular dichroism at the K-edges of copper and iron coordination salts crystallizing in the achiral point groups 4̅2m and 4̅ respectively. The experimental signals follow the angular dependence exactly as predicted by the symmetry of the point groups. In addition, an angular phase shift was observed for 4̅, which is explained by the crystalline a and b axes not being determined by the crystal point group but by the translation operations of the space group.
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