Bis(melaminium) DL-malate tetrahydrate.

The crystal structure of the title melaminium salt, bis(2,4,6-triamino-1,3,5-triazin-1-ium) DL-malate tetrahydrate, 2C(3)H(7)N(6)(+).C(4)H(4)O(5)(2-).4H(2)O, consists of singly protonated melaminium residues, DL-malate dianions and water molecules. The melaminium residues are connected into chains by four N-H.N hydrogen bonds, and these chains form a stacking structure along the c axis. The DL-malate dianions form hydrogen-bonded chains and, together with hydrogen-bonded water molecules, form a layer parallel to the (100) plane. The conformation of the malate ion is compared with an ab initio molecular-orbital calculation. The oppositely charged moieties, i.e. the stacks of melaminium chains and hydrogen-bonded DL-malate anions and water molecules, form a three-dimensional polymeric structure, in which N-H.O hydrogen bonds stabilize the stacking.