First observationof X-ray Natural Circular Dichroism in achiral, non-polar crystals

Natural circular dichroism is the principal tool used to characterize chiral compounds. For this reason, the terms chirality and optical activity are often used interchangeably. However, these terms are distinct, which can be most clearly appreciated by comparing optical activity in the UV-visible range and in the X-ray range. Theoretically speaking, there are some enantiomorphic and UV-active crystal space groups where optical activity in the X-ray range is forbidden, while there also exists non-chiral space groups for which X-ray optical activity is allowed. As whatever is not forbidden must be observable, we here propose to record, for the first time, X-ray Natural Circular Dichroism (XNCD) for crystals belonging to the XNCD-active, but achiral and nonpolar, crystal class -42m.