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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]

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]

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···π(arene) hydrogen bonds. In phase I the molecules are CH···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···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. High pressure transition between benzene phases I and II, involving a collapse of intermolecular voids, exhibits a huge pressure hysteresis of 2 GPa at 295 K and an intriguingly tiny volume change. It has been explained by determining the crystal structures of benzene I and II at their lowest stability limits. [ABSTRACT FROM AUTHOR]