How 3D electron diffraction reveals all about crystal structure

Structure analysis using 3D electron diffraction (3D ED, aka ADT, cRED, MicroED) data has become increasingly popular among chemists and materials scientists for its ability to solve crystal structures from single nanocrystals. Despite substantial progress in this method, it is still generally considered to provide relatively low-accuracy structure models, unsuitable for all but basic crystallographic analysis. Recent advances in data acquisition, data processing in PETS2 [1], including determination of the exact experimental geometry, and dynamical refinement in Jana2020/Dyngo [2] with modeling of the apparent crystal thickness distribution, have enabled 3D ED to answer questions about the finest details of structure, including partially occupied hydrogen positions or charge density analysis. Figure 1 shows methyl disorder in acetaminophen and charge density analysis in L-tyrosine obtained from 3D ED data. It is also possible to determine the absolute structure [3]. For this purpose, the so-called z-score has been introduced for enantiopure materials [4]. However, if certain conditions are fulfilled, the classical Flack parameter determination can be applied in electron crystallography. These advances bring electron crystallography to the same level of accuracy as X-ray diffraction, with the ability to see light atoms alongside heavy ones due to a smaller increase in scattering potential with atomic number and greater sensitivity to the absolute structure of structures consisting of only light atoms.