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Ambient‐pressure trigonal phase α of selenourea SeC(NH2)2 is noncentrosymmetric, with high Z′ = 9. Under high pressure it undergoes several intriguing transformations, depending on the pressure‐transmitting medium and the compression or recrystallization process. In glycerine or oil, α‐SeC(NH2)2 transforms into phase β at 0.21 GPa; however in water, phase α initially increases its volume and can be compressed to 0.30 GPa due to the formation of α‐SeC(NH2)2·xH2O. The single crystals of α‐SeC(NH2)2 and of its partial hydrate α‐SeC(NH2)2·xH2O are shattered by pressure‐induced transitions. Single crystals of phase β‐SeC(NH2)2 were in situ grown in a diamond‐anvil cell and studied by X‐ray diffraction. The monoclinic phase β is centrosymmetric, with Z′ = 2. It is stable to 3.20 GPa at least, but it cannot be recovered at ambient conditions due to strongly strained NH...Se hydrogen bonds. No hydrogen‐bonding motifs present in the urea structures have been found in selenourea phases α and β. [ABSTRACT FROM AUTHOR]

New polymorphs β and γ of bis‐3‐nitrophenyl disulphide, crystallized above 0.3 GPa, are less dense than the ambient‐pressure polymorph α. This counterintuitive density relation results from the high‐entropy nucleation and subsequent kinetic crystallization. The work performed by pressure compensates the high entropy and temperature product, substantiated in varied conformers and increased chemical potential. Pressure‐increased viscosity promotes the kinetic polymorphs, in accordance with empirical Ostwald's rule of stages. It contrasts to mechanochemical techniques, favouring high‐density polymorphs. [ABSTRACT FROM AUTHOR]

The zinc aspartate (ZnAsp2) complex, a common dietary supplement, preferentially crystallizes as the dihydrate (ZnAsp2·2H2O) from aqueous solution. Under normal conditions the dihydrate easily transforms into the sesquihydrate (ZnAsp2·1.5H2O). The dihydrate crystal structure is triclinic, space group P1, and the sesquihydrate is monoclinic, space group C2/c. However, their structures are closely related and similarly consist of zinc aspartate ribbons parallel to pores accommodating water molecules. These porous structures can breathe water molecules in and out depending on the temperature and air humidity. High pressure above 50 MPa favours the sesquihydrate, as shown by recrystallizations under pressure and compressibility measured by single‐crystal X‐ray diffraction up to 4 GPa. This preference is explained by the reduced volume of the sesquihydrate and water compressed separately, compared with the dihydrate. The sesquihydrate undergoes an isostructural phase transition when the voids collapse at 0.8 GPa, whereas no phase transitions occur in the dihydrate, because its pores are supported by increased water content. [ABSTRACT FROM AUTHOR]

Pressure efficiently changes the crystallization preferences of three polymorphs and three solvates of 4,4'-bipyridinium perchlorate, [C10H9N2]+ClO4-. At ambient pressure a triclinic dihydrate [C10H9N2]+ClO4-·2H2O is formed of the solutions in water and in the water/methanol mixture, while the methanol solution in open vials yields concomitant monoclinic quarterhydrate 4([C10H9N2]+ClO4-)·H2O and triclinic anhydrate [C10H9N2]+ClO4-. At 0.30 GPa an orthorhombic anhydrate is formed, stable to 0.60 GPa, when it collapses into a monoclinic phase. The transition changes the conformation of cations from planar to twisted, differently than the analogue phase transition in [C10H9N2]+Br·H2O. Recrystallizations from the methanol solution above 0.55 GPa leads to methanol solvate [C10H9N2]+ClO4-·CH3OH. In the solvates, chains of the cations are aggregated through either NH+···N bonds (like in the neat polymorphs) or NH+···OH···N bonds, or both these bonds in one crystal. The pyridinium protons are disordered in all these structures, which implies disproportionation between neutral molecules, C10H8N2, monocations [C10H9N2]+ and dications [C10H10N2]2+. In quarterhydrate 4([C10H9N2]+ClO4-)·H2O the disorder of protons propagates through the hydrogen bonds mediated by disordered water molecules, also involved in weak H-bonds to perchlorate anions. [ABSTRACT FROM AUTHOR]

The cation of 3-(trimethylammonium)-benzoic acid exhibits a considerable conformational flexibility connected to the orientation of carboxyl group, coupled to the proton position, and rotations of the trimethylammonium group. This conformational flexibility considerably affects the aggregation of ions and molecules of 3-(trimethylammonium)-benzoic acid iodide hydrate, which crystallizes in a low-symmetry structure with four symmetry-independent formula units (monoclinic space group Pc, Z' = 4, model I). However this structure approximates the symmetry of space group P2/ c in the same unit cell ( Z' = 2, model II), and also even a still more highly symmetric structure of space group P2/ c in a half-smaller unit cell ( Z' = 1, model III). The independent formula units are differentiated by the conformations of trimethylammonium groups (ordered in model I, but disordered in models II and III), conformations of the carboxyl groups and positions of OH···O bonded water molecules (ordered in molecules I and II, and disordered in model III), while the bulks of the cations and iodide anions are consistent for all models. While the calorimetric study and varied-temperature structural determinations exclude phase transitions of the highly pseudosymmetric model I. This pseudosymmetry is independent of temperature between 100 K and the m.p. at 463 K, and illustrates subtle structural effects leading complex molecular aggregation in the crystal. Theoretical computations confirm the low energy differences between the conformers. [ABSTRACT FROM AUTHOR]

ABSTRACT Recalcitrant crystallization and syrup formation are frequent features of natural sugars. This is the case of d-ribose, yielding low-quality crystals of mixed α- and β-pyranose anomers. However, large crystals of dl-ribose can be grown easily. The crystal structures of stable d-ribose forms I and II as well as dl-form II have been analyzed in terms of their compatibility with the molecular aggregation. The comparison of the potential energy of all conformers and their OH···O hydrogen-bonding patterns is consistent with the preferential racemate crystallization in terms of departures from the optimized isolated ribose molecule and its directional interactions. This analysis is aimed at rationalizing the interplay between the molecular structure and spontaneous crystallization of chiral compounds. Chirality 26: 806-810, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Highly hygroscopic pyridine N-oxide, C5H5NO, dissolves in water absorbed from atmospheric air, but it crystallizes in the neat form of the aqueous solution under high pressure. The crystals grown at high-pressure isochoric conditions are of the same phase as that obtained from anhydrous crystallization at ambient pressure. This feature can be employed for retrieving compounds highly soluble in water from their aqueous solutions. The crystal structure is strongly stabilized by CH...O contacts. The crystal compression and thermal expansion as well as three shortest H...O distances comply with the inverse-relationship rule of pressure and temperature changes. [ABSTRACT FROM AUTHOR]

A series of 1,2-dihalotetrafluoroethanes X(CF2)2X (X = Br, I) has been in-situpressure crystallized in a diamond-anvil cell (DAC), their structures determined by single crystal X-ray diffraction and rationalized by weak intermolecular interactions. For isoelectronic and isostructural 1,2-dibromotetrafluoroethane (BrCF2CF2Br), 1,2-diiodotetrafluoroethane (ICF2CF2I) and 1-bromo-2-iodotetrafluoroethane (BrCF2CF2I), the crystals are monoclinic, space group P21/n, with the midpoint of the C–C bond located at the centre of inversion. The freezing pressures of these compounds have been determined to be 0.80 GPa, 0.30(5) GPa and 0.10(5) GPa for BrCF2CF2Br, BrCF2CF2I and CF2ICF2I, respectively. In the structure of ICF2CF2I, the –CF2–CF2– moiety is orientationally disordered at 0.16 GPa, but it becomes ordered at 0.86 GPa; in the BrCF2CF2I crystal the Br and I atoms are substitutionally disordered; and the BrCF2CF2Br structure is completely ordered. The formation of isostructural crystals by these compounds and different types of molecular disorder can be rationalized by the intermolecular interactions at varied thermodynamical conditions. The phenomenon of “self-healing” has been observed, in which crystal defects, formed during initial crystallization in the DAC, repaired themselves during the annealing process, which greatly facilitates the formation of single crystals. Pressure considerably affects the crystal morphology of ICF2CF2I, where the elimination of –CF2–CF2– disorder coincides with the appearance of dendritic forms of the crystal phases. [ABSTRACT FROM AUTHOR]

Abstract: Intermolecular interactions and the role of fluorine substitution have been investigated for a halogenated-ethane anesthetic. 2-Bromo-2-chloro-1,1,1-trifluoroethane, BrClCHCF3 (Halothane), has been in situ pressure frozen in a diamond anvil cell and its structure determined by single-crystal X-ray diffraction at 1.85(5)GPa/296K. Crystal is triclinic, space group . In this racemic structure the enantiomorphic molecules are substitutionally disordered at the same general positions in that way that bromine and chlorine atoms occupy the same site at the 50:50 ratio. Despite the fact that only the Br and Cl atoms are disordered, the crystal packing is dominated by halogen⋯halogen and halogen⋯hydrogen interactions. This X-ray diffraction study provides structural explanation of considerably increased vapor pressure of Halothane compared to its hydrogenated analogue. [Copyright &y& Elsevier]

Abstract: Pentafluoropyridine has been pressure frozen in situ in a diamond-anvil cell (DAC) and its structure determined at 0.30(5) and 1.10(5)GPa and at room temperature by single-crystal X-ray diffraction. The freezing pressure of pentafluoropyridine has been determined to be 0.10(5)GPa. The crystals are monoclinic, space group P21/c. The crystal packing is governed by F⋯F and C/N⋯F van der Waals contacts, but no ring stacking is observed. The intermolecular interactions are non-directional, and the crystal compresses nearly isotropically between 0.3 and 1.1GPa. [Copyright &y& Elsevier]

The structure of 1,2,3-trichloropropane, ClCH2CHClCH2Cl, in-situ crystallized in a diamond–anvil cell, has been determined by single-crystal X-ray diffraction at 0.28 and 0.35 GPa. A melting point at 295 K and 0.22 GPa has been determined. The molecular conformation of aliphatic chain and terminal Cl atoms is approximately C2 symmetric. Of the intermolecular contacts, the tightest are the Cl⋯Cl and then the Cl⋯H contacts, whereas the H⋯H distances are considerably longer than the sum of the van der Waals radii and leave narrow voids in the structure. The elevated pressure reduces the H⋯H separations but hardly affects the Cl⋯Cl distances. The crystal growth rates, compression and types of intermolecular interactions in the structures can be correlated with the directions of the Cl⋯Cl contacts, which can be considered the main motif responsible for the molecular arrangement in the structure. [ABSTRACT FROM AUTHOR]

Dibromomethane, CH2Br2, and diiodomethane, CH2I2, have been in situ pressure-crystallized in a diamond–anvil cell and their structures determined by single-crystal X-ray diffraction at 0.61 and 0.16 GPa, respectively. The pressure-frozen CH2Br2 crystal is isostructural with its C2/ c phase obtained by cooling. CH2I2 is known to form several phases at low temperature, one of which is isostructural with CH2Br2. However, pressure freezing leads to the polar Fmm2 phase. The formation of the polar CH2I2 structure at 0.16 GPa has been rationalized by the electrostatic and anisotropic van der Waals interactions of the I atoms. No ferroelectric behaviour of the Fmm2 polar phase II of CH2I2 has been determined. The diffraction, calorimetric and dielectric constant studies reveal considerable temperature hysteresis of transformations between the CH2I2 phases, as well as metastable regions strongly dependent on the sample shape and history. [ABSTRACT FROM AUTHOR]

1,2-Diaminoethane has been in-situ pressure- and temperature-frozen; apart from two known low-temperature phases, Iα and II, three new phases, Iβ, Iγ and III, have been observed and their structures determined by X-ray diffraction. The measurements at 0.1 MPa were carried out at 274, 243 and 224 K, and 296 K measurements were made at 0.15 GPa (phase Iα), at 0.3 and 1.1 GPa (phase Iβ), at 1.5 GPa (phase Iγ), and at 0.2, 0.3 and 0.5 GPa (phase III). All these phases are monoclinic, space group P21/ c, but the unit-cell dimension of phases Iα and III are very different at 296 K: aIα = 5.078 (5), bIα = 7.204 (8), cIα = 5.528 (20) Å, βIα = 115.2 (2)° at 0.15 GPa, and aIII = 5.10 (3), bIII = 5.212 (2), cIII = 7.262 (12) Å, βIII = 111.6 (4)° at 0.2 GPa, respectively; in both phases Z = 2. An ambient-pressure low-temperature phase II has been observed below 189 K. Discontinuities in the unit-cell dimensions and in the N⋯N distance mark the isostructural transition between phases Iα and Iβ at 0.2 GPa, which can be attributed to a damping process of the NH2 group rotations. In phase Iγ the unit-cell parameter a doubles and Z increases to 4. The molecule has inversion symmetry in all the structures determined. 1,2-Diaminoethane can be considered as a simple structural ice analogue, but with NH⋯N hydrogen bonds and with the H-atom donors (four in one molecule) in excess over H-atom acceptors (two per molecule). Thus, the transformations of 1,2-diaminoethane phases involving the conformational dynamics affect the hydrogen-bonding geometry and molecular association in the crystal. The 1,2-diaminoethane:1,2-dihydroxyethane mixture has been separated by pressure-freezing, and a solid 1,2-diaminoethane crystal in liquid 1,2-dihyroxyethane has been obtained. [ABSTRACT FROM AUTHOR]

The structure of dichloromethane, CH2Cl2, crystallized in situ in a diamond–anvil cell, has been determined by single-crystal X-ray diffraction at 1.33 and 1.63 GPa. The pressure-frozen crystal was determined to be orthorhombic, with the space group Pbcn, and isostructural with the low-temperature phase at 0.1 MPa. The CH2Cl2 molecules are located on one set of crystallographic twofold axes. The characteristics determined for the CH2Cl2 crystal (compression of the close intermolecular contacts, molecular association and the crystal habit of dichloromethane) suggest that the crystal cohesion forces are dominated by H⋯Cl interactions rather than by Cl⋯Cl attractions. [ABSTRACT FROM AUTHOR]

1,4-Diazabicyclo[2.2.2]octane perchlorate,[C6H13N2]+ClO4–, dabcoHClO4, the first NH+···Nhydrogen-bonded ferroelectric, exhibits at least ten unsolvated polymorphs,depending on temperature and pressure. When crystallized from themethanol solution above 2 GPa, dabcoHClO4crystallizeswith two molecules of methanol to form solvate dabcoHClO4·2CH3OH. This solvate structure has been determinedby single-crystal X-ray diffraction up to 3 GPa. High pressure transformsthe H-bonding pattern and the hierarchy of different cohesion forces.The solvate formation is additionally favored by its molecular volumelower than the sum of the molar equivalents of unsolvated components. [ABSTRACT FROM AUTHOR]

Similar halobenzene derivatives 1,3-dichlorobenzene (m-C6H4Br2) and 1,3-dibromobenzene (m-C6H4Cl2) form crystals remarkably similar in the unit-cell dimensions, in two symmetry-independent molecules (Z' = 2) and in their atomic positions along plane yz. However, the atomic x-coordinates are different and the space-group symmetry of m-C6H4Cl2 is P21/c and that of m-C6H4Br2 is P212121. These symmetries are not changed by isochoric recrystallizations at least to 2.7 and 1.6 GPa, respectively. The one-dimensional structural differences originate from one additional Br...Br halogen bond in otherwise very similar halogen...halogen ...bonds patterns. Of the m-C6H4Br2/m-C6H4Cl2 1:1 mixture a solid solution (symmetry of space-group P21/c) is formed at 0.5 GPa. However, an addition of methanol to the mixture counteracts the formation of the solid solution, and at 0.45 GPa the pristine m-C6H4Cl2 crystals precipitate. For the 1:1:6 mixture with methanol, a pressure of 1.0 GPa is required for the formation of solid solution with the major contents of m-C6H4Cl2 and minor contents of m-C6H4Br2. It is characteristic for the obtained solid solution that intermolecular interactions discriminate the occupation of two symmetry-independent sites in the crystal by molecules dClB and dBrB. [ABSTRACT FROM AUTHOR]