The melaminium dihydrogencitrate, (C3H7N6)(HOOC-CH2-C(OH)(COOH)-CH2-COO), crystallizes from water solution at room temperature in the P21/c space group of the monoclinic system with the lattice parameters of a = 5.531(1), b = 20.869(4), c = 11.282(2) A and = 99.96(3)_ and Z = 4. The crystals are built up from singly protonated at the one N-ring atom melaminium cations that interact in a near linear fashion through a pair of N-HN hydrogen bonds to form the centrosymmetric dimeric structure. The dihydrogencitrate(-) anions interact in the head-to-tail fashion via the terminal dissociated (COO-) and non-dissociated (COOH) carboxyl groups to form O-H _ O hydrogen bonded zigzag infinite chains. The hydroxyl group of dihydrogencitrate(-) ions is involved into O-HOhydrogen bonds that linked together the dihydrogencitrate(-) chains into two-dimensional network. The centrosymmetric dimers of melaminium moieties interact with the sheets of dihydrogencitrate(-) to form the three-dimensional hydrogen bonded network. The conformation of the dihydrogencitrate(-) ion in the crystal is compared with the conformation in the gas-phase obtained by the ab-initio molecular orbital calculation.
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