3D electron diffraction for structure determination of small‐molecule nanocrystals: A possible breakthrough for the pharmaceutical industry

TAAM refinement on high-resolution experimental and simulated 3D ED/MicroED data for organic molecules

3D electron diffraction (3D ED), or microcrystal electron diffraction (MicroED), has become an alternative technique for determining the high-resolution crystal structures of compounds from sub-micron-sized crystals. Here, we considered L-alanine, α-glycine and urea, which are known to form good-quality crystals, and collected high-resolution 3D ED data on our in-house TEM instrument. In this study, we present a comparison of independent atom model (IAM) and transferable aspherical atom model (TAAM) kinematical refinement against experimental and simulated data. TAAM refinement on both experimental and simulated data clearly improves the model fitting statistics (R factors and residual electrostatic potential) compared to IAM refinement. This shows that TAAM better represents the experimental electrostatic potential of organic crystals than IAM. Furthermore, we compared the geometrical parameters and atomic displacement parameters (ADPs) resulting from the experimental refinements with the simulated refinements, with the periodic density functional theory (DFT) calculations and with published X-ray and neutron crystal structures. The TAAM refinements on the 3D ED data did not improve the accuracy of the bond lengths between the non-H atoms. The experimental 3D ED data provided more accurate H-atom positions than the IAM refinements on the X-ray diffraction data. The IAM refinements against 3D ED data had a tendency to lead to slightly longer X-H bond lengths than TAAM, but the difference was statistically insignificant. Atomic displacement parameters were too large by tens of percent for L-alanine and α-glycine. Most probably, other unmodelled effects were causing this behaviour, such as radiation damage or dynamical scattering.

High Temperature Electron Diffraction on Organic Crystals: In Situ Crystal Structure Determination of Pigment Orange 34

Small molecule structures and their applications rely on good knowledge of their atomic arrangements. However, the crystal structures of these compounds and materials, which are often composed of fine crystalline domains, cannot be determined with single-crystal X-ray diffraction. Three-dimensional electron diffraction (3D ED) is already becoming a reliable method for the structure analysis of submicrometer-sized organic materials. The reduction of electron beam damage is essential for successful structure determination and often prevents the analysis of organic materials at room temperature, not to mention high temperature studies. In this work, we apply advanced 3D ED methods at different temperatures enabling the accurate structure determination of two phases of Pigment Orange 34 (C34H28N8O2Cl2), a biphenyl pyrazolone pigment that has been industrially produced for more than 80 years and used for plastics application. The crystal structure of the high-temperature phase, which can be formed during plastic coloration, was determined at 220 °C. For the first time, we were able to observe a reversible phase transition in an industrial organic pigment in the solid state, even with atomic resolution, despite crystallites being submicrometer in size. By localizing hydrogen atoms, we were even able to detect the tautomeric state of the molecules at different temperatures. This demonstrates that precise, fast, and low-dose 3D ED measurements enable high-temperature studies the door for general in situ studies of nanocrystalline materials at the atomic level.

3D electron diffraction analysis of a novel, mechanochemically synthesized supramolecular organic framework based on tetrakis-4-(4-pyridyl)phenylmethane

Tetrakis-4-(4-pyridyl)phenylmethane (TPPM) is a tetrahedral rigid molecule that crystallizes forming a dynamically responsive supramolecular organic framework (SOF). When exposed to different stimuli, this supramolecular network can reversibly switch from an empty to a filled solvated solid phase. This article describes a novel expanded form of a TPPM-based SOF that has been mechanochemically synthesized and whose crystal structure has been determined by 3D electron diffraction analysis using a novel electron diffractometer.

Stable Sn-Based Hybrid Perovskite-Related Structures with Tunable Color Coordinates via Organic Cations in Low-Temperature Synthesis

Organic-inorganic Pb-free layered perovskites are efficient broadband emitters and thus are promising materials for lighting applications. However, their synthetic protocols require a controlled atmosphere, high temperature, and long preparation time. This hinders the potential tunability of their emission through organic cations, as is instead common practice in Pb-based structures. Here, we present a set of Sn-Br layered perovskite-related structures that display different chromaticity coordinates and photoluminescence quantum yield (PLQY) up to 80%, depending on the choice of the organic monocation. We first develop a synthetic protocol that is performed under air and at 4 °C, requiring only a few steps. X-ray and 3D electron diffraction analyses show that the structures exhibit diverse octahedra connectivity (disconnected and face-sharing) and thus optical properties, while preserving the organic-inorganic layer intercalation. These results provide key insight into a previously underexplored strategy to tune the color coordinates of Pb-free layered perovskites through organic cations with complex molecular configurations.

Electron crystallography and dedicated electron-diffraction instrumentation

Electron diffraction (known also as ED, 3D ED or microED) is gaining momentum in science and industry. The application of electron diffraction in performing nano-crystallography on crystals smaller than 1 µm is a disruptive technology that is opening up fascinating new perspectives for a wide variety of compounds required in the fields of chemical, pharmaceutical and advanced materials research. Electron diffraction enables the characterization of solid compounds complementary to neutron, powder X-ray and single-crystal X-ray diffraction, as it has the unique capability to measure nanometre-sized crystals. The recent introduction of dedicated instrumentation to perform ED experiments is a key aspect of the continued growth and success of this technology. In addition to the ultra-high-speed hybrid-pixel detectors enabling ED data collection in continuous rotation mode, a high-precision goniometer and horizontal layout have been determined as essential features of an electron diffractometer, both of which are embodied in the Eldico ED-1. Four examples of data collected on an Eldico ED-1 are showcased to demonstrate the potential and advantages of a dedicated electron diffractometer, covering selected applications and challenges of electron diffraction: (i) multiple reciprocal lattices, (ii) absolute structure of a chiral compound, and (iii) R-values achieved by kinematic refinement comparable to X-ray data.

Ultrafast magic angle spinning NMR characterization of pharmaceutical solid polymorphism: A posaconazole example

Protons represent the most NMR-sensitive nucleus in pharmaceutical compounds. Therefore, proton-detected solid-state NMR techniques under fast magic angle spinning are among the few solutions to overcome the challenge of low sensitivity to analyze natural abundant drug substances and products. In this study, we report the structural characterization of crystal polymorphs of a commercial drug molecule, posaconazole, with a relatively large molecular weight of 700.8 g·mol^-1 and at the natural abundance. The enhanced sensitivity and resolution at 100 kHz MAS enables the exploration of the distinct intermolecular packing in posaconazole forms I, III, and γ. These results demonstrate that proton-detected homo- and heteronuclear correlation methods can probe the structural details of pharmaceutical polymorphism.