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

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 0.1 MPa were carried out at 274, 243 and 224 K, and 296 K measurements were made at 0.15 GPa (phase I alpha), at 0.3 and 1.1 GPa (phase I beta), at 1.5 GPa (phase I gamma), and at 0.2, 0.3 and 0.5 GPa (phase III). All these phases are monoclinic, space group P2(1)/c, but the unit-cell dimension of phases I gamma and III are very different at 296 K: a(I alpha) = 5.078 (5), b(I alpha) = 7.204 (8), c(I alpha) = 5.528 (20) A, beta I alpha = 115.2 (2)degrees at 0.15 GPa, and a(III) = 5.10 (3), b(III) = 5.212 (2), c(III) = 7.262 (12) A , beta(III) = 111.6 (4)degrees at 0.2 GPa, respectively; in both phases Z = 2. An ambient-pressure low-temperature phase II has been observed below 189 K. Discontinuities in the unitcell dimensions and in the (NN)-N-... distance mark the isostructural transition between phases I alpha and I gamma at 0.2 GPa, 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)-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.