Convolutional Neural Networks to Assist the Assessment of Lattice Parameters from X-ray Powder Diffraction

This article presents the development of convolutional neural networks (CNNs) for the estimation of lattice parameters in organic compounds across various crystal systems. A comprehensive collection of 92,085 organic compounds was utilized to train the CNNs, encompassing crystals with unit cells containing up to 512 atoms and a maximum unit cell volume of 8000 Å3. Simulated diffraction patterns were generated for each compound, comprising four diffraction patterns with different crystal sizes. These diffraction patterns were generated within a 2θ window of 3-60°, employing a step size of 0.02051°. Two distinct CNN architectures were developed with differing input data. The first CNN, referred to as XRD-CNN, was trained solely on diffraction patterns. In the test set, XRD-CNN demonstrated a mean absolute percentage error (MAPE) of 11.04% for unit cell vectors, 7.40% for angles, and 26.83% for unit cell volume. The second CNN, XRDElem-CNN, incorporated a binary representation of atoms within the unit cell as an additional input. XRDElem-CNN achieved improved performance, yielding MAPE values of 4.73% for unit vectors, 6.49% for angles, and 6.05% for the unit cell volume. To validate the performance of XRDElem-CNN, real diffraction patterns obtained from a conventional laboratory diffractometer (Cu Kα wavelength) were employed. Various representations of atoms within the unit cell were proposed, which were computationally efficient for evaluation with the CNNs. The assessed lattice parameters by XRDElem-CNN were validated using the Lp-search method, showing agreement with the reported values.

Single-crystal-to-single-crystal phase transition of 18β-glycyrrhetinic acid isopropyl ester

Due to the destruction of the integrity of the parent crystal, single-crystal-to-single-crystal phase transition in organic compounds is still a relatively rare phenomenon. The phase transition in glycyrrhetinic acid isopropyl ester is triggered by temperature change. The increasing volume of the isopropyl substituent as a result of increasing temperature forces a remodelling of the structural motifs. These changes cause a single-crystal-to-single-crystal phase transition. The low-temperature form is isostructural with glycyrrhetinic acid methanol solvate, while the high-temperature phase is isostructural with the ethyl ester of this acid.

The influence of structure on the methyl group dynamics of polymorphic complexes: 6,6′-dimethyl-2,2′-dipyridyl with halo derivatives of benzoquinone acids

The structural analysis, neutron scattering, ¹H NMR and computational methods combined to investigate new molecular complexes.

Reorientational dynamics of organic cations in perovskite-like coordination polymers

Here we report the dynamics of organic cations as guest molecules in a perovskite host-framework. The molecular motion of CH₃NH₃⁺ (MAFe), (CH₃)₂NH₂⁺ (DMAFe) and (CH₃)₃NH⁺ (TrMAFe) in the cage formed by KFe(CN)₆^3- units was studied using a combination of experimental methods: (i) thermal analysis, (ii) dielectric and electric studies, (iii) optical observations, (iv) EPR and ¹H NMR spectroscopy and (v) quasielastic neutron scattering (QENS). In the case of MAFe and TrMAFe, the thermal analysis reveals one solid-to-solid phase transition (PT) and two PTs for the DMAFe crystal. A markedly temperature-dependent dielectric constant indicates the tunable and switchable properties of the complexes. Also, their semiconducting properties are confirmed by a dc conductivity measurement. The broadband dielectric relaxation is analyzed for the TrMAFe sample in the frequency range of 100 Hz-1 GHz. QENS shows that we deal rather with the localized motion of the cation than a diffusive one. Three models, which concern the simultaneous rotation of the CH₃ and/or NH₃ group, π-flips and free rotations of the organic cation, are used to fit the elastic incoherent structure factor. The ¹H NMR spin-lattice relaxation time for all compounds under study, as well as the second moments, has been measured in a wide temperature range. In all studied samples, the temperature dependence of the second moment of the proton NMR line indicated the gradual evolution of the molecular movements from the rigid state up to a highly disordered one.

Crystal structural analysis of methyl-substituted pyrazines with anilic acids: a combined diffraction, inelastic neutron scattering,1H-NMR study and theoretical approach

The molecular complexes of the pyrazine derivatives with anilic acids were analyzed in terms of the structure of molecules, the vibrational spectra, INS,¹HNMR and theoretical approach.

Structures and phase transitions in molecular complexes containing tetrafluoroboric acid and tetramethylpyrazine

Crystals of tetrafluoroboric acid with tetramethylpyrazine, TMP + HBF₄ + H₂O (1 : 1 : 2, A), TMP + HBF₄ (1 : 1, B) and TMP + HBF₄ (1 : 2, C) were synthesized and characterized by physicochemical experimental methods.