Two solid phases of pyrimidin-1-ium hydrogen chloranilate monohydrate determined at 225 and 120 K

The crystal structures of two solid phases of the title compound, C4H5N2 ⁺·C6HCl2O4 ^-·H2O, have been determined at 225 and 120 K. In the high-temperature phase, stable above 198 K, the transition temperature of which has been determined by ³⁵Cl nuclear quadrupole resonance and differential thermal analysis measurements, the three components are held together by O-H⋯O, N⋯H⋯O, C-H⋯O and C-H⋯Cl hydrogen bonds, forming a centrosymmetric 2+2+2 aggregate. In the N⋯H⋯O hydrogen bond formed between the pyrimidin-1-ium cation and the water molecule, the H atom is disordered over two positions, resulting in two states, viz. pyrimidin-1-ium-water and pyrimidine-oxonium. In the low-temperature phase, the title compound crystallizes in the same monoclinic space group and has a similar mol-ecular packing, but the 2+2+2 aggregate loses the centrosymmetry, resulting in a doubling of the unit cell and two crystallographically independent molecules for each component in the asymmetric unit. The H atom in one N⋯H⋯O hydrogen bond between the pyrimidin-1-ium cation and the water molecule is disordered, while the H atom in the other hydrogen bond is found to be ordered at the N-atom site with a long N-H distance [1.10 (3) Å].