Molecular arrangements have been determined at the lowest limits of pressure ranges of benzene phase I, at 0.15 GPa, and phase II at 0.91 and 0.97 GPa, all at 295 K. All intermolecular contacts both in phase I and phase II to about 1.0 GPa exceed the sums of van der Waals radii; however, the transition between phases I and II does not affect the pattern of CH 3 3 3 π(arene) hydrogen bonds. In phase I the molecules are CH 3 3 3 π bonded approximately perpendicular into sheets, and there are substantial voids between the molecules within the sheets. The mechanism of transition to phase II involves a collapse of the voids, simultaneous with a shift of the CH 3 3 3 π bonded sheets. The thickness of sheets increases, which partly compensates the volume reduction due to the voids collapse; hence, the transition exhibits a large hysteresis of two GPa and a sluggish character at 295 K. No other phases of benzene have been observed between 0.15 and 5.0 GPa.

Pressure-induced transformations between gauche-, gauche+ and transoid conformations have been evidenced by X-ray single-crystal diffraction for 1,1,2-trichloroethane, and the energies of intermolecular interactions, conformational conversion, and the latent heat have been determined.

Yes, Walter, there is a polymorph of sucrose! At 4.80 GPa, (+)‐sucrose, common table sugar, transforms into a new polymorph. In its structure the network of intermolecular hydrogen bonds is reformulated, with new types of H bonds being formed, and the molecular conformation changes. This structural variability is characteristic of all carbohydrates, hinders their crystallization, and is vital for organisms for which sugars are important building blocks.

No abstract

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.

Pressure affects the competition between C-H⋯X hydrogen bonds and X⋯X halogen⋯halogen interactions. In bromomethane, CH 3 Br, pressure changes the molecular arrangement of the two solidstate phases of this compound: low-pressure phase α is dominated by halogen⋯halogen interactions, whereas above 1.5 GPa the β phase is governed by C-H⋯halogen bonds. The CH 3 Br phase α is isostructural with solid CH 3 I of orthorhombic space group Pnma, while CH 3 Br phase β is polar, isostructural with CH 3 Cl and CH 3 CN crystals, of orthorhombic space group Cmc2 1. The crystal structures of CH 3 Cl (b.p. = 249.1 K) and CH 3 Br (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 CH 3 Br, 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-CH 3 X are similar to those of acetonitrile, H 2 O ice VII and solidified X 2 halogens. The phase diagrams of CH 3 Br and CH 3 Cl have been constructed.

In the crystalline pyrimidine, the most basic building block of biochemical systems, the changes of entropy and enthalpy combine into a series of discrete structural transitions that have defied detection until now. The counterintuitive fully isostructural polymorphs of pyrimidine differ mainly by entropy, while the varied space occupied by molecules in partly isostructural polymorphs can be connected to the molecular dynamics, too. The interplay of thermodynamic and structural features affects the molecular interactions and environment and is most relevant to the functions of all organic compounds in the living tissue. The single crystals of four pyrimidine polymorphs have been grown at isobaric, isothermal, and isochoric conditions, and their structures have been determined by X-ray diffraction.

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.