Structure of ammonium hydrogen succinate at 80 and 20 K

The crystal structure of ammonium hydrogen succinate has been determined at 80 and 20 K in order to clarify the structural change at the second-order phase transition at 170 K. The geometries of all the hydrogen bonds in the structure were analyzed by combining new geometric data with data obtained previously at 297, 190 and 150 K. The O ... O distances of two O-H ... O hydrogen bonds involving the hydrogen succinate ions increase as the temperature decreases below T-c and the N ... O distance of a weak bifurcated hydrogen bond involving the ammonium ion decreases below T-c. It is noted that the phase transition occurs so that the bifurcated hydrogen bond is retained below T-c. There was no evidence of a descent in space group from P (1) over bar to P1 below T-c and the formation of a superstructure below T-c was not detected.