Search

Two phase transitions in compressed benzimidazole polymorphsreveala remarkable interplay of the H-site in NH···N hydrogen-bondedaggregates and the crystal structure. The ambient-pressure polar polymorphα, space group Pna21, at p1= 0.26 GPa transforms into centrosymmetricphase β, space group Pccn, and above p2= 2.26 GPa into another centrosymmetric polymorphγ, space group Pbca. Single crystals of formsα, β, and γ have been in situgrownin isothermal and isochoric conditions in a diamond-anvil cell, andtheir structures were determined by X-ray diffraction. Both at p1and p2the H-bonddistance N···N increases in the higher-pressure phase.However, the H-atom always assumes the site for which the H···Ndistance in the homoconjugated NH···N bond is shorter. [ABSTRACT FROM AUTHOR]

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]

Exceptionallyrich diagram of ten phases, I–X, has beenrevealed for ferroelectric 1,4-diazabicyclo[2.2.2]octane perchlorate,[C6H13N2]+ClO4–, dabcoHClO4. Pressure- and temperature-induced transformations of thisNH+···N hydrogen bonded structure have beencharacterized by single-crystal X-ray diffraction, calorimetric, anddielectric measurements. At high pressure the single crystals werein situ grown in a diamond-anvil cell at isochoric conditions. Thereare nine phases of dabcoHClO4between 400 and 11 K at ambientpressure, whereas phase X is formed at 1.5 GPa and 296 K. Tetragonalphase I space group P4/mmmtransformsto orthorhombic phases II (Pm21n), III (Pc21n), IV (Pc2a), and V (orthorhombic-modulated)on decreasing temperature. Phase II is stable up to 0.22 GPa, at 0.6GPa phase III is formed, and above 1.50 GPa a new phase X (Pc21a) has been obtained. Atnormal conditions dabcoHClO4is isostructural with theferroelectric crystal of dabcoHBF4; however, their phasediagrams are very different. Above 1.8 GPa dabcoHClO4cocrystallizeswith solvate molecules. [ABSTRACT FROM AUTHOR]