New Polar Phases of 1,4-Diazabicyclo[2.2.2]octane Perchlorate, An NH+···N Hydrogen-Bonded Ferroelectric

1,2-Diaminoethane has been in-situ pressure- and temperature-frozen; apart from two known low-temperature phases, I [alpha] and II, three new phases, I [beta] , I [gamma] and III, have been observed and their structures determined by X-ray diffraction. The measurements at 0.1MPa were carried out at 274, 243 and 224K, and 296K measurements were made at 0.15GPa (phase I [alpha] ), at 0.3 and 1.1GPa (phase I [beta] ), at 1.5GPa (phase I [gamma] ), 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 I [alpha] and III are very different at 296 K: a_I=5.078(5), b_I=7.204(8), c_I=5.528(20)Å, _I=115.2(2)° at 0.15GPa, and a_III=5.10(3), b_III=5.212 (2), c_III=7.262 (12)Å, _III = 111.6(4)° at 0.2GPa, respectively; in both phases Z=2. An ambient-pressure low-temperature phase II has been observed below 189K. Discontinuities in the unit-cell dimensions and in the NN distance mark the isostructural transition between phases I [alpha] and I [beta] at 0.2GPa, which can be attributed to a damping process of the NH₂ group rotations. In phase I [gamma] 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 NHN 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.

Download:

Association CH···π and No van der Waals Contacts at the Lowest Limits of Crystalline Benzene I and II Stability Regions

Andrzej Katrusiak, Marcin Podsiadło and Armand Budzianowski

Cryst. Growth Des., 2010, 10 (8), pp 3461–3465 DOI: 10.1021/cg1002594

Pressure phase transitions in beznene has been confirmed by crystal structures

REPRINTED WITH PERMISSION FROM CRYST. GROWTH DES., 2010, 10 (8), PP 3461–3465 DOI: 10.1021/CG1002594. COPYRIGHT (2010) AMERICAN CHEMICAL SOCIETY.

Continue reading →

Pressure-frozen benzene I revisited

Armand Budzianowski and Andrzej Katrusiak

Acta Crystallographica Section B
Structural Science
Volume 62, Part 1 (February 2006)
Acta Cryst. (2006). B62, 94-101 [doi:10.1107/S010876810503747X]
Authorised by IUCr electronic reprint of the article

Abstract

The crystal structure of benzene, C₆H₆, in situ pressure-frozen in phase I, has been determined by X-ray diffraction at 0.30, 0.70 and 1.10GPa, and 296K. The molecular aggregation within phase I is consistent with van der Waals contacts and electrostatic attraction of the positive net atomic charges at the H atoms with the negative net charges of the C atoms. The C-Haromatic ring centre contacts are the most prominent feature of the two experimentaly determined benzene crystal structures in phases I and III, whereas no stacking of the molecules has been observed. This specific crystal packing is a likely reason for the exceptionally high polymerization pressure of benzene. The changes of molecular arrangement within phase I on elevating the pressure and lowering the temperature are analogous.

Download: