Resolving a Nearly Two‐Centuries‐Old Mystery: On the Structural Chemistry and Physicochemical Properties of Compounds Associated With the Term Ammelide

Ammelide, also known as melanuric acid, is a simple molecular compound, which can be regarded as the double hydrolysis product of the industrially relevant compound melamine. Within this work, the first structural description of ammelide was provided almost 200 years after its discovery. This not only gave the first unambiguous evidence that ammelide's preferred tautomeric form in the solid is 6-amino-1,3,5-triazine-2,4(1H,3H)-dione, but also showed that ammelide adopts a layered structure similar to the closely related ammeline. Moreover, the crystal structures of three modifications of the 1:1 adduct between ammeline and ammelide were elucidated, which form layers with a honeycomb motif and hexagonal voids analogous to melamine cyanurate. Differential thermal analysis and thermogravimetric analysis demonstrated that both pure ammelide and its adduct with ammeline have exceptionally high thermal stabilities, resulting from their dense hydrogen bonding networks. Finally, the synthesis and structural characterization of the ammelide's nitrate and perchlorate salts were carried out to examine the potential of ammelide as part of insensitive high-energy-density materials through sensitivity measurements and theoretical calculations.