Competition of interactions and a new high-temperature phase of selenourea

The aggregation of molecules is usually associated with a specific type of interaction, which can be altered by thermodynamic conditions. Under normal conditions, the crystal structure of selenourea, SeC(NH₂)₂, phase α is trigonal, space group P3₁, Z = 27. Its large number of independent molecules (Z_α' = 9) can be associated with the formation of an NH...N hydrogen bond substituting one of 36 independent NH...Se hydrogen bonds, which prevail among intermolecular interactions. Phase α approximates the trigonal structure with a threefold smaller unit cell (Z = 9), which in turn approximates another still threefold smaller unit cell (Z = 3). The temperature-induced transformations of selenourea have been characterized by calorimetry and by performing 21 single-crystal X-ray diffraction structural determinations as a function of temperature. At 381.0 K, phase α undergoes a first-order displacive transition to phase γ, with space group P3₁21 and Z reduced to 9, when the NH...N bond is broken and an NH...Se bond is formed in its place. Previously, an analogous competition was observed between NH...N and NH...O hydrogen bonds in high-pressure phase III of urea. The lattice vectors along the (001) plane in low- and high-temperature phases of selenourea are related by a similarity rule, while the lattice dimensions along direction c are not affected. This similarity rule also applies to the structures of phase γ and hypothetical phase δ (Z = 3). The thermally controlled transition between enantiomorphic phases of selenourea contrasts with its high-pressure transition at 0.21 GPa to a centrosymmetric phase β, where both the NH...Se and NH...N bonds are present. The compression and heating reduce the number of independent molecules from Z' = 9 in phase α, to Z' = 2 in phase β and to Z' = 1.5 in phase γ.