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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]

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]

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]