Polymorphism of maleic hydrazide. I

The third polymorph (denoted MH3) of maleic hydrazide (3,6-dihydroxypyridazine in the monolactim form, 6-hydroxy-3-pyridazinone, C4H4N2O2) has been studied by X-ray diffraction and shown to be monoclinic, space group P2(1)/n. Polymorph MH3 was found as the prevailing form along with the rare triclinic polymorph MH1, space group P (1) over bar, but they were obtained separately from monoclinic MH2, space group P2(1)/c. The structure of MH1, previously studied by photographic methods, has been redetermined. Polymorph MH3 exhibits the same scheme of molecular association into hydrogen-bonded ribbons as in MH1 and MH2, but the arrangements of the aggregates and details of their supramolecular conformations are different. The accommodation of the supramolecular conformations to the requirements of close packing of the aggregates in crystal lattices, as well as the symmetries of the polymorphs, are analyzed.