X-ray powder diffraction as a pre-screening tool for 3D electron diffraction of nanocrystalline organic compounds

Three-dimensional electron diffraction (3D ED) allows the determination of crystal structures of nanocrystalline organic compounds. However, even 3D ED requires a minimum level of crystallinity. For the pre-selection of samples for 3D ED, a practical tool was developed that uses routine laboratory X-ray powder diffraction (XRPD) data. The procedure was evaluated on 29 samples covering 17 industrial organic pigments (aromatic or heterocyclic compounds with 21–57 non-hydrogen atoms). The samples exhibited a broad range of crystallinity, from well-crystalline to nearly amorphous; most were nanocrystalline and at the borderline of suitability for 3D ED. All samples were investigated by 3D ED with extensive attempts at structure solution and refinement. Additionally, XRPD data were recorded. From the XRPD data several quality indicators were derived: the number of visible peaks, the presence of peaks at 2 θ above 30°, peak widths, domain sizes, the peak-height-to-background ratio, and the indexability of the powder data. The most reliable indicator for 3D ED success is the domain size calculated from widths of three XRPD peaks using the Scherrer formula. For all samples with domain sizes of more than 31 nm the crystal structure could be solved by 3D ED, whereas structure solution failed for all samples with less than 31 nm. Hence, the XRPD-derived domain size serves as an easy-to-use pre-selection tool for 3D ED. Other indicators were less decisive. The value of approximately 30 nm should be regarded as a practical lower limit for successful structure determination of similar organic compounds by current 3D ED methods.