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Intermolecular contacts between organic molecules in crystals have been categorized according to the shortest distances between the van der Waals spheres. The distributions of the first, second and next shortest contacts reveal systematic changes in the role of different types of interactions in the aggregation of molecules. These distributions of the shortest contacts can be related to the specific types of cohesion forces as well as to the effect of close packing of molecules. The sequence of close contacts reflects the role of cohesion forces in a given crystal, and their contribution to the stability of crystals in general can be evaluated. Such an analysis has been applied to measure the changes in the types of cohesion forces in polymorphs and in structures undergoing phase transitions. This information is essential to understand the molecular aggregation as well as to validate the crystallographic data. It has been established for the chemical compounds investigated that their phase transitions and polymorphic modifications do not change the closest intermolecular contacts of the first order, and only higher orders of contacts are modified. [ABSTRACT FROM AUTHOR]

Two phase transitions in compressed benzimidazole polymorphsreveala remarkable interplay of the H-site in NH···N hydrogen-bondedaggregates and the crystal structure. The ambient-pressure polar polymorphα, space group Pna21, at p1= 0.26 GPa transforms into centrosymmetricphase β, space group Pccn, and above p2= 2.26 GPa into another centrosymmetric polymorphγ, space group Pbca. Single crystals of formsα, β, and γ have been in situgrownin isothermal and isochoric conditions in a diamond-anvil cell, andtheir structures were determined by X-ray diffraction. Both at p1and p2the H-bonddistance N···N increases in the higher-pressure phase.However, the H-atom always assumes the site for which the H···Ndistance in the homoconjugated NH···N bond is shorter. [ABSTRACT FROM AUTHOR]

Several procedures have been employed for validating structural models refined on poor quality single-crystal diffraction data. Analysis of intra- and intermolecular distances in the structures of 2,2-aziridinedicarboxamide polymorphs proved to be a robust means, and a means independent of the chosen unit cell and symmetry, of detecting several incorrect atom-type assignments in the reported structure of the triclinic polymorph of 2,2-aziridinedicarboxamide [Brückner (1982). Acta Cryst. B 38, 2405–2408]. The corrected model, refined in the space group , rules out the existence of any conformational polymorphism in this compound. Small differences in the powder-diffraction patterns calculated for the original and corrected structures of the triclinic polymorph illustrate the sensitivity of the above method for polymorph validation. [ABSTRACT FROM AUTHOR]

The reaction of chlorination of maleic-hydrazide (1) with POCl3 is specific to the substrate polymorph. Chlorination of either the triclinic polymorph of maleic-hydrazide (1), denoted MH1, or the monoclinic polymorph MH2 leads to a single product of 3,6-dichloropyridazine (2), while chlorination of the monoclinic polymorph MH3 under the same conditions results in two products: 3,6-dichloropyridazine (2) and 6-chloro-3-pyridazinone (3). The structural differences between polymorphs and their topochemical chlorination in phosphorus oxychloride are discussed. The crystal structure of 6-chloro-3-pyridazinone (3) monohydrate, determined in this study by X-ray diffraction, is built of ribbons of hydrogen-bonded 3 and water molecules. It has been established that the chlorination of ground crystals of polymorph MH3 yields exclusively the dichloropyridazine, which suggests that the crystal habit of MH3, the size of crystallites and the lower solubility of MH3 than MH1 and MH2 are the main reasons for the different course of the reaction. [Copyright &y& Elsevier]

Directional ClCl type I and II interactions govern the low-density aggregation of 1,1,2,2-tetrachloroethane molecules in synclinalconformation in the crystalline state at low temperature, whereas the dense molecular packing in high-pressure is achieved for the antiperiplanarconformers and electrostatically less favored ClCl contacts. The mechanism of transformation between loose and dense associations involves the collapse of ClCl contacts and conformational conversion. [ABSTRACT FROM AUTHOR]

The main directions and achievements in the research on transformations of hydrogen-bonds during the last decade have been briefly outlined. [ABSTRACT FROM AUTHOR]

The structures of a series of the simplest aliphatic amines determined at high pressure by single crystal X-ray diffraction reveal systematic structure property relations involving intermolecular interactions and molecular symmetries. Five new polymorphs have been found: one of methylamine (phase II, space group Fdd2), two of dimethylamine (phase I, space group C2/c and phase II, P21/c), and two of trimethylamine (phase II, space group P21/n and phase III, P21/m). The methylamine and dimethylamine molecules are arranged according to the NH...N hydrogen bonds. The phase diagrams of methylamines have been outlined up to 6 GPa. [ABSTRACT FROM AUTHOR]

A new high-pressure crystal form of ethane has been revealed above 2.46 GPa/295 K. This phase IV is tetragonal, of space group P42/mnm, with the H atom disordered due to the molecular rotations about the C-C bond. The crystal structure has been determined by single-crystal X-ray diffraction up to 5.90 GPa. The structure of phase IV resembles the previously determined low-temperature plastic phase I (stable below 90.32 K), of cubic space group Im3m, metastable phase II observed between 89.78 and 89.68 K, as well as monoclinic phase III (space group P21/n) stable below 89.68 K. In phases III and IV, the molecules are arranged according to the weak electrostatic complementarity between neighbors. Liquid-ethane convection erodes the crystal plate, hence colorful interference pattern. At 2.46 GPa, ethane crystallizes in a new tetragonal phase IV with rotating molecules. Spectacular convection of liquid ethane above the single-crystal surface has been observed. [ABSTRACT FROM AUTHOR]

High pressure destabilizes the high Z' polymorph of 3-hydroxy-4,5-dimethyl-1-phenylpyridazin-6-one (1α); however, recrystallization is needed for obtaining a low Z', more dense polymorph. Three polymorphs α, β, and γ can be monotonically compressed to 2.0 GPa at least. At ambient pressure 1 crystallizes in space group C2/c, Z' = 4 as polymorph 1α, or as lower-density polymorph 1β, of space group P21/c and Z' = 1. Polymorph β is metastable, and after about one year, it transforms to phase α. The isochoric recrystallization above 0.40 GPa yields a new polymorph γ of space group P21/a and Z' reduced to 1. The γ polymorph retrieved to ambient conditions for months has showed no signs of transformations. The main motif of OH…O bonded chains is retained in all three phases, but high pressure enforced identical conformation of closely packed molecules and their identical crystal environment in phase γ. [ABSTRACT FROM AUTHOR]

Three polymorphs of 2,4,5-triiodo-1H-imidazole (tIIm), all of exceptionally low symmetry (with three and four independent molecules), differ in the contributions of halogen bonds to their cohesion forces. The crystals of phase α, of monoclinic space group P21/c and Z' = 3, were obtained after the first synthesis of the compound and several of its crystallizations. Above 1.9 GPa phase α isostructurally transformed to phase β. The transition, marked by a huge strain visibly changing the crystal shape, could be reversibly repeated on increasing and releasing pressure. Repeated syntheses of tIIm, by using the same recipe as well as the same and varied conditions, resulted exclusively in a new γ phase, of monoclinic space group P21/c and Z' = 4, while phase α disappeared. Each of phases α, β, γ is built of Z' independent molecules and linked by Z' independent NH…N bonds into chains. In phases β and γ new I…I bonds are formed when one of the NH…N bonds is bent to about 130°. [ABSTRACT FROM AUTHOR]

High pressure strongly favors the highest-density polymorph Z of active pharmaceutical ingredient 2-(2,6-xylidino)-5,6-dihydro-4H-1,3-thiazine hydrochloride (xylazine hydrochloride, XylHCl) up to about 0.1 GPa only, but still higher pressure destabilizes this structure. Above 0.1 GPa, XylHCl preferentially crystallizes as solvates with CH2Cl2, CHCl3, or (CH3)2CHOH depending on the solvent used. However, when XylHCl·H2O is dissolved in any of these solvents, the high-pressure crystallizations yield the hydrate XylHCl·H2O only. The single crystals of the CH2Cl2, CHCl3, and (CH3)2CHOH solvates could be grown in situ in a diamond anvil cell, which allowed their structure determination from the single-crystal diffraction data. At 0.4 GPa the XylHCl·H2O hydrate undergoes a pressure-induced phase transition doubling the unit cell dimensions. [ABSTRACT FROM AUTHOR]

Pyrazine molecules aggregate by remarkably similar CH···N hydrogen bonds into four polymorphs. Of eight CH···N bonds to each molecule, six H-bonds are very similar for all pyrazine structures. A diagram of four crystalline phases has been outlined and their low-temperature and high-pressure structures determined. Phase I is metastable and occurs only within the ambient-pressure region of phase II. [ABSTRACT FROM AUTHOR]

The formation of solvates in relation to the neat-crystal close-packing efficiency and the symmetry, involving the number of independent molecules (Z'), has been investigated. In the Cambridge Structural Database solvates are most frequent when the corresponding neat compounds have several independent molecules Z' > 1. High pressure changes the energy difference between inequivalent molecules and their interactions, which can trigger phase transitions, solvation, multicomponent aggregation, and other transformations. When 2,4,5-triiodoimidazole (tIIm) polymorph α (Z' = 3) was recrystallized above 0.2 GPa in methanol, a hemisolvate 2tIIm·CH3OH was obtained. Later, the hemisolvate could be obtained at normal conditions and then polymorphs α and β disappeared, and new syntheses and recrystallizations at 0.1 MPa yielded exclusively a new polymorph γ (Z' = 4); when dissolved in methanol it precipitates as the methanol hemisolvate only. In its structure, chains of NH…N and NH…OH…N bonded molecules are formed. These chains interact through halogen bonds, almost absent in much looser neat α-tIIm, but numerous in considerably more compact phases β and γ. The statistical distribution of the solvation effect for the crystal volume has been compared to the molecular volume calculated and measured in several ways for the most frequent solvates. [ABSTRACT FROM AUTHOR]

Undisturbed by stronger interactions, CH···π hydrogen bonded motifs have been determined in four polymorphs of toluene. Two new toluene phases, monoclinic phase III of space group symmetry P21 crystallized between 1.1 and 1.5 GPa and orthorhombic phase IV of space group Pbca obtained between 1.27 and 2.1 GPa, reveal 3-dimensional CH···π aggregation competing with CH···HC interactions of molecules, while the 2-dimensional aggregation is characteristic of low-temperature toluene phases I and II. The most frequent CH···π bonding motif involves the methyl H-donor and the acceptor atoms β and γ. The motifs of H-bonds and their frequency depend on the stability regions of toluene phases outlined in the pressure-temperature phase diagram. [ABSTRACT FROM AUTHOR]