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.自然圆二色性（Natural circular dichroism）是表征手性化合物的核心手段。正因如此，手性（chirality）与光学活性（optical activity）这两个术语常被混用。然而二者的内涵并不等同，这一点可通过对比紫外-可见光范围与X射线范围的光学活性得到最清晰的体现。理论上，存在部分兼具对映异构性（enantiomorphic）与紫外活性的晶体空间群（crystal space groups），此类空间群中X射线范围的光学活性是禁阻的；同时也存在非手性空间群，其X射线光学活性却是允许的。但凡未被禁阻的现象必然可被观测到，本文首次提出针对属于XNCD活性、但为非手性且非极性的晶体学点群-42m的晶体，开展X射线自然圆二色性（X-ray Natural Circular Dichroism，XNCD）测量的研究构想。