Competing hydrogen-bonding patterns and phase transitions of 1,2-diaminoethane at varied temperature and pressure

 Armand Budzianowski Anna Olejniczak and Andrzej Katrusiak

Acta Crystallographica Section B
Structural Science
Volume 62, Part 6 (December 2006)
Acta Cryst. (2006). B62, 1078-1089 [doi:10.1107/S010876810602982X]

Abstract

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: aI[alpha]=5.078(5), bI[alpha]=7.204(8), cI[alpha]=5.528(20)Å, [beta]I[alpha]=115.2(2)° at 0.15GPa, and aIII=5.10(3), bIII=5.212 (2), cIII=7.262 (12)Å, [beta]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 N...N 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 NH...N 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.

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