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Monotonic compression of two iso-structural hybrid metal–organic perovskites [DMA]M(HCOO)3 (M = Co, Ni) has been connected to their structural distortions. The strongest effects of pressure occur in the tilts of the MO6 octahedra, M–O bond compression, angular distortions within the MO6 octahedra and small folding between the octahedra and formate units. The anisotropic stronger compression along the trigonal axis is consistent with NH…O bonding nearly perpendicular to this direction. Raman spectra indicate a first order solid-state phase transition between 3–5 GPa in both formates, which can be associated with the symmetry breaking induced by the ordering of the Dma cation. [ABSTRACT FROM AUTHOR]

Distortions of the directional requirements in halogen…halogen contacts between hexachloroethane (HCE), C2Cl6, molecules lead to a loose crystal under ambient conditions. Single-crystal X-ray diffraction shows that the orthorhombic HCE phase of space group Pnma, with the molecules in the staggered conformation, extends at least from 85 to 305 K and from 0.1 MPa to 5.42 GPa. At ambient pressure, all intermolecular distances are longer than the sum of van der Waals radii, reached only at the pressure of ca. 1.2 GPa. [ABSTRACT FROM AUTHOR]

In pentachloroethane, C2HCl5, high pressure initially strongly compresses the C–H…Cl bonds in phase I; however, in phase II which is stable above 0.62 GPa the role of hydrogen bonds is diminished and molecular aggregation is dominated by halogen bonds Cl…Cl. Both phases have been determined by X-ray diffraction and the phase diagram of C2HCl5 has been outlined. The transition between phases I and II retains some relation between their structures and reduces the symmetry from class mmm (space group Pnma) to 2/m (space group P21/n11). The discontinuous transition, with the shear strain exceeding 21°, is so strong that its progress can be visually observed even for powdered samples. The single-crystal growth suggests a surface-roughening transition, as phase I grows in the form of dendrites combining into oval plates, while phase II forms prisms with sharp edges. [ABSTRACT FROM AUTHOR]

5,6-Dimethylbenzimidazole (C9H9N2) crystallizes exclusively in the unsolvated form from an aqueous solution up to 0.25 GPa and from methanol, ethanol or their mixed solutions up to 0.6 GPa. At still higher pressures, solvated crystals with water, methanol and ethanol are formed depending on the solvent used for recrystallization. The NH…N bonds of the neat crystals are replaced by NH…OH…N bonds in the solvates. This preference for high-pressure solvation has been rationalized in terms of changed hydrogen bonds as well as volume balance between neat and solvated compounds. The methanol solvate C9H9N2·MeOH undergoes a pressure-induced phase transition at 1.40 GPa, lowering its symmetry from monoclinic space group P21/c (phase α) to triclinic space group P1̅ (high-pressure phase β) and ordering the H-atoms in hydrogen bonds NH…OH…N, disordered in phase α. [ABSTRACT FROM AUTHOR]

High pressure and temperature trigger symmetric chemical reactions of 4,4′-bipyridine hydrobromide monohydrate (44′biPyHBr·H2O) in methanol solution. Above 0.1 GPa and 423 K, the 4,4′-bipyridinium dibromide salt (44′biPy2HBr) precipitates, while the 4,4′-bipyridine free base remains dissolved in the methanol–water mixture. At 0.35 GPa and 473 K, both pyridine moieties are N-methylated and N,N-dimethylbipyridinium dibromide (44′biPy2CH3Br) is formed. None of the high-pressure compounds of 44′biPy2HBr and 44′bPy2CH3Br are solvated, which contrasts with the strong preference of analogous 1,4-diazabicyclo[2.2.2]octane (dabco) monosalts and disalts to form solvates at high-pressure. The high-pressure reactivity of 44′biPyHBr is analogous to that of 1,4-diazabicyclo[2.2.2]octane hydrobromide (dabcoHBr); however, dabcoHBr is asymmetrically N-methylated at one amine site only. This asymmetric mono N-methylation of dabcoHBr has been associated with strong electrostatic interactions between the transannular N-atoms. [ABSTRACT FROM AUTHOR]

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]

Anhydrous 5,6-dimethylbenzimidazole (dMBzIm) crystals are obtained from an aqueous solution at ambient pressure, but above 0.25 GPa the hemihydrate dMBzIm·½H2O is formed. It can be stored for over a year in open vials. Counter-intuitively, the volume of dMBzIm·½H2O is smaller than that of the anhydrate. Ambient and high-pressure structures of dMBzIm and dMBzIm·½H2O crystals grown in situ in a diamond-anvil cell have been determined by X-ray diffraction methods. The isostructural phase transition observed in dMBzIm at 0.25 GPa coincides with the changed preference to hemihydrate crystallization. The significantly different ambient and high-pressure hydration preferences of imidazole and benzimidazole derivatives have been explained by the interplay of interactions and the efficiency of crystal packing measured by the size and volume of voids. [ABSTRACT FROM AUTHOR]

The first betaine–mineral acid complex of 3 : 2 stoichiometry, as well as a 1 : 1 complex common for this class of compounds, has been obtained. Crystalline [di-(3-trimethylammonium-benzoic acid)–(3-trimethylammonium-benzoate)]diiodide (1) is the only betaine–mineral acid 3 : 2 complex obtained so far. Two concomitant polymorphs of this 3 : 2 complex, 1-I and 1-II, have been isolated and characterized by X-ray diffraction and FTIR spectroscopy. Polymorphs 1-I, of monoclinic space group P21/c, and 1-II, of orthorhombic space group Pccn, have very similar lattices and aggregates of one zwitterionic molecule and two cations O–H…O bonded into a 3-membered catemeric carboxyl–carboxylate–carboxyl interval, but polymorph 1-II dissolves better in methanol than 1-I. Alternative hypothetical aggregates of 3-trimethylammonium-benzoate hydroiodide 1 : 1, 2 : 1 and 3 : 2 complexes optimized at the B3LYP/6-31G(d,p) level of theory suggest that the betaine-to-acid 3 : 2 stoichiometry is favoured at the stage of small cluster aggregation and not due to the complex crystal packing preferences. [ABSTRACT FROM AUTHOR]

The compression and thermal expansion of crystalline 1,2,4-trichlorobenzene, C6H3Cl3, 124TCB, investigated under isobaric and isothermal conditions, are in reverse relation, as for most of crystals, however, the isochoric strain along direction c is clearly different from those along a and b. Single crystals of 124TCB have been in situ grown under isochoric and isobaric conditions, at 270 K/0.1 MPa and 295 K/0.16 GPa, and also at 100 K/0.1 MPa and 295 K/0.64 GPa, when the unit-cell volume is similar. All crystallizations yielded the same phase, of monoclinic space group P21/n, with two symmetry-independent molecules (Z′ = 2). The structure is governed by Cl…Cl and Cl…H interactions and the molecules are all nearly parallel to the [001] direction. Systematic differences of these specific contacts afford rationalizing the isochoric strain of the crystal. The phase diagram of 124TCB has been outlined. [ABSTRACT FROM AUTHOR]

Pressure affects the competition between C-HX hydrogen bonds and X⋯X halogenhalogen interactions. In bromomethane, CH3Br, pressure changes the molecular arrangement of the two solid-state phases of this compound: low-pressure phase α is dominated by halogenhalogen interactions, whereas above 1.5 GPa the β phase is governed by C-H⋯halogen bonds. The CH3Br phase α is isostructural with solid CH3I of orthorhombic space group Pnma, while CH3Br phase β is polar, isostructural with CH3Cl and CH3CN crystals, of orthorhombic space group Cmc21. The crystal structures of CH3Cl (b.p. = 249.1 K) and CH3Br (b.p. = 276.7 K) have been determined by high pressure single-crystal X-ray diffraction up to 4.38 GPa and 2.85 GPa, respectively. In CH3Br, pressure of 1.5 GPa enforces the close packing and opposite electrostatic-potential matching between molecular surfaces in contact. The interweaved C-H⋯X bonded diamondoid networks of β-CH3X are similar to those of acetonitrile, H2O ice VII and solidified X2 halogens. The phase diagrams of CH3Br and CH3Cl have been constructed. [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]

Isobaric freezing of 1,1,1-trichloroethane yields crystals where all the intermolecular contacts are much longer than the sums of the van der Waals radii and only in the structure compressed to ca.1.2 GPa do the first ClCl contacts become commensurate with this sum. This sheds new light on the range of intermolecular interactions that are capable of controlling molecular re-orientation and arrangement. [ABSTRACT FROM AUTHOR]