The Same at a First Glance: The Diffractogram of a New Polymorph of Anhydrous Sodium Luminolate Almost Perfectly Resembles the Diffraction Trace of an Already Known Polymorph

A so far unknown third polymorph (form III) of 5‐amino‐2, 3‐dihydro‐1, 4‐phthalazinedione (luminolate) monosodium salt could be isolated phase pure and its structure was determined by powder X‐ray diffraction and Rietveld refinement [cell parameters: a = 8.0364(2) Å, b = 3.69765(5) Å, c = 26.4246(7) Å, β = 96.346(2)°, V = 780.42(3) Å3, space group P21/c]. The new structure shows many similarities to the structure of the known form II and the consequently slim differences in the diffraction traces could have easily been overlooked. Slurry experiments prove the new form to be more stable than form II. Both form II and form III crystallize in a lamellar structure. The layers are in turn built by shifted π‐stacked strands of hydrogen bonded luminolate dimers. The strands are canted by ± 25° in respect to the layers. While in form II the tilt direction alternates from strand to strand within a single layer, in form III all strands within a layer are tilted in the same direction and the orientation rather alternates between adjacent layers.