Armand Budzianowski Anna Olejniczak and Andrzej Katrusiak
1,2-Diaminoethane has been in-situ pressure- and temperature-frozen; apart from two known low-temperature phases, I and II, three new phases, I, I 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), at 0.3 and 1.1GPa (phase I), at 1.5GPa (phase I), 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 and III are very different at 296 K: aI=5.078(5), bI=7.204(8), cI=5.528(20)Å, I=115.2(2)° at 0.15GPa, and aIII=5.10(3), bIII=5.212 (2), cIII=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 and I at 0.2GPa, which can be attributed to a damping process of the NH2 group rotations. In phase I 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.