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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]