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Abstract: The effect of pressure on the crystal structures of the two ambient-pressure polymorphs of the amino acid l-histidine has been investigated. Single-crystal diffraction measurements, up to 6.60 GPa for the orthorhombic form I (P212121) and 6.85 GPa for the monoclinic form II (P21), show their crystal structures undergo isosymmetric single-crystal-to-single-crystal first-order phase transitions at 4.5 and 3.1 GPa to forms I′ and II′, respectively. Although the similarity in crystal packing and intermolecular interaction energies between the polymorphs is remarkable at ambient conditions, the manner in which each polymorph responds to pressure is different. Form II is found to be more compressible than form I, with bulk moduli of 11.6(6) GPa and 14.0(5) GPa, respectively. The order of compressibility follows the densities of the polymorphs at ambient conditions (1.450 and 1.439 g cm–3 for phases I and II, respectively). The difference is also related to the space-group symmetry, the softer monoclinic form having more degrees of freedom available to accommodate the change in pressure. In the orthorhombic form, the imidazole-based hydrogen atom involved in the H-bond along the c-direction swaps the acceptor oxygen atom at the transition to phase I′; the same swap occurs just after the phase transition in the monoclinic form and is also preceded by a bifurcation. Concurrently, the H-bond and the long-range electrostatic interaction along the b-direction form a three-centered H-bond at the I to I′ transition, while they swap their character during the II to II′ transition. The structural data were interpreted using periodic-density-functional theory, symmetry-adapted perturbation theory, and semiempirical Pixel calculations, which indicate that the transition is driven by minimization of volume, the intermolecular interactions generally being destabilized by the phase transitions. Nevertheless, volume calculations are used to show that networks of intermolecular contacts in both phases are very much less compressible than the interstitial void spaces, having bulk moduli similar to moderately hard metals. The volumes of the networks actually expand over the course of both phase transitions, with the overall unit-cell-volume decrease occurring through larger compression of interstitial void space.

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Abstract: Heating of the K2ZrF6 room temperature monoclinic form-I reveals a complex polymorphism investigated by synchrotron powder temperature-dependent diffraction. The first thermal event at 240 °C corresponds to the quasi-simultaneous apparition of four different phases (noted II to V) of which only three could be indexed and two completely characterized structurally. Form-II (structure unrefined) is very probably hexagonal (a = 6.4473(1) Å, c = 3.8606(1) Å at 264 °C, Z = 1). Form-III is cubic (space group Fm3 ̅m, a = 9.0804(1) Å at 264 °C, Z = 4), related to K3ZrF7. Form-IV remains unindexed, presenting broad diffraction peaks. Form-V is present as a weak impurity at this stage, increasing in proportion after the second thermal event at 288 °C where a new form-VI is built. Form-V is tetragonal (space group P4/nmm, a = 6.3281(1) Å, c = 9.3671(2) Å at 295 °C, Z = 2) and form-VI is orthorhombic (space group Cmcm, a = 6.40840(5) Å, b = 20.7697(2) Å, c = 19.9888(2) Å, at 295 °C, Z = 16). At 317 °C, form-VI transforms into form-V. At 449 °C, a different orthorhombic form-VII is disclosed (space group Pnma, a = 10.3084(1) Å, b = 6.0291(1) Å, c = 23.8023(2) Å, at 495 °C, Z = 8), coexisting with the cubic form-III. On cooling, form VII of which an approach of the crystal structure is provided, returns to form V at 411 °C which looks stable down to 139 °C giving form I back. Isolated ZrF6 octahedra are present in forms-III, -V, -VI, -VII. Form-VI contains also dimeric entities Zr2F12 built from a ZrF7 monocapped trigonal prism sharing a corner with a ZrF6 octahedron. Form-VII structure is related to form-V by quadrupling the tetragonal a parameter, the reversible VVII transition is very probably topotactic. The coexistence of several phases at each temperature above the first transition at 240 °C was observed. Peak multi-splitting is observed at many temperatures that suggests the occurrence of coexisting micro-phases. The parallel existence of several phases was observed also by 19F MAS NMR spectroscopy measured from 250 oC → 410 oC → RT.

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Abstract: A new porous and flexible metal-organic framework (MOF) has been synthesised from the flexible asymmetric linker N-(4-Carboxyphenyl)succinamate (CSA) and heptanuclear zinc oxo-clusters of formula [Zn7O2(Carboxylate)10DMF2] involving two coordinated terminal DMF ligands. The structural response of this MOF to the removal or exchange of its guest molecules has been probed using a combination of experimental and computational approaches. The topology of the material, involving double linker connections in the a and b directions and single linker connections along the c axis, is shown to be key in the materials anisotropic response. The a and b directions remain locked during guest removal, while the c axis linker undergoes large changes significantly reducing the material’s void space. The changes to the c axis linker involve a combination of a hinge motion on the linker’s rigid side and conformational rearrangements on its flexible end, which were probed in detail during this process despite the presence of crystallographic disorder along this axis which prevented accurate characterisation by experimental methods alone. While inactive during guest removal, the flexible ends of the a and b axis linkers are observed to play a prominent role during DMF to DMSO solvent exchange, facilitating the exchange reaction arising in the cluster.

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Abstract: Porous organic cages have emerged over the last 10 years as a subclass of functional microporous materials. However, among all of the organic cages published, large multicomponent organic cages with 20 components or more are still rare. Here, we present an [8+12] porous organic imine cage, CC20, which has an apparent surface area up to 1752 m2 g-1, depending on the crystallization and activation conditions. The cage is solvatomorphic and displays distinct geometrical cage structures, caused by crystal packing ef-fects, in its crystal structures. This indicates that larger cages can display a certain range of shape flexibility in the solid state, while remaining shape persistent and porous.

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Abstract: Olanzapine is a polymorphic drug molecule that has been extensively studied, with over 60 structures reported in the Cambridge Structural Database. All anhydrous and solvated forms of olanzapine known to date contain the SC0 dimer packing motif. In this study, a new screening approach was adopted involving heat-induced forced crystallization from a polymer-based molecular dispersion of olanzapine. Simultaneous differential scanning calorimetry-powder X-ray diffraction (DSC-PXRD) was used to heat the amorphous dispersion and to identify a novel physical form from diffraction and heat flow data. Comparison of the diffraction data with those from a computed crystal energy landscape allowed the crystal structure to be determined. The result was the discovery of a new polymorph, form IV, which does not use the SC0 motif. Hence, while dimer formation is the dominant process that defines crystal packing for olanzapine formed from solution, it seems that molecularly dispersing the drug in a polymeric ma-trix permits crystallization of alternative motifs. Having identified form IV, it proved possible to scale up the synthesis and demonstrate its enhanced dissolution properties over form I.