Development of new nano-electron diffraction method for identification of crystal chirality and polarity

点击翻译按钮获取中文摘要

Enantiomorphism is usually referred to and used to describe objects that are lacking of improper rotation(rotoinversion and rotoreflections). Because of the absence of a center of symmetry (1^^-), a mirror plane (m=2^^-) and a 4^^-axis, such enantiomorphic (chiral) crystals or molecules can occur in two different forms that are related as a right hand and a left hand and these enantiomorphically-related crystals belong to either of the 11 crystal classes (point groups). These crystals are mirror related and are not superimposable with each other and one of the two enantiomorphic crystals exhibit optical bioactivities different from the other in most cases Therefore, distinction of chirality of enantiomorphic crystals is sometimes very important, as the well-known example of Thalidmide accident, which occurred as a tragedy due to the harmful side effect of one of the two members of the enantiomorphic crystals, has indicated. We have developed a new CBED method for identification of chirality and polarity of enantiomorphic crystals, in which asymmetry in the intensity of reflections of Bijvoet pairs in an experimental symmetrical zone-axis CBED pattern is compared with that of an computer simulated CBED pattern. The intensity difference for reflections of these Bijvoet pairs results from multiple scattering (dynamical nature of electron diffraction) among relevant Bijvoet pairs of reflections, each pair of which have identical amplitude and different phase angles. Therefore, the crystal thickness where chiral identification is made with the present method is limited by the extinction distance of Bijvoet pairs of reflections relevant to multiple scattering to produce the intensity asymmetry, which is usually of the order of a few tens nm. With the present method, only a single CBED pattern is sufficient and chiral identification can be made for all possible enantiomorphic crystals that are allowed to exist in crystallography.