Crystal structure of a new ordered form of ammonium hydrogen succinate NH4HC4H4O4

International audienceTriclinic ammonium hydrogen succinate compound (space group P1−) is known to present very short O–H⋅⋅⋅O hydrogen bonds in association with a second-order phase transition around 170 K. Whatever the temperature, its crystal structure is characterized by a statistical distribution of the hydrogen atom belonging to the hydrogen succinate molecule on two crystallographic sites, leading to a disordered structure. In this study, we report on the crystal structure of a new ordered structural form of the title compound NH4HC4H4O4, determined by single-crystal X-ray diffraction at room temperature. This new ordered form crystallizes in the triclinic space group P1− with refined cell parameters: a = 6.4868(16) Å, b = 7.4313(17) Å, c = 7.4339(17) Å, α = 112.058(8)°, β = 96.477(9)°, γ = 109.026(8)° and V = 302.45(12) Å3. Hydrogen succinate molecules are in trans conformation in the ordered form and in gauche conformation in the disordered one. It leads to different molecular assemblies of the ammonium cations and hydrogen succinate anions, and consequently, to different hydrogen bond networks. The crystal structure stability of this new ordered structural form is discussed and compared to that of the disordered form