The modulated crystal structure of the molecular adduct of 2,4,6-trinitrobenzoic acid with 2,6-diaminopyridine

The 1 : 1 adduct of 2,4,6-trinitrobenzoic acid (tnba) with 2,6-diaminopyridine (2,6-dap), [(2,6-dap)(+)(tnba)(-)], has been prepared and the low-temperature crystal structure has been determined by X-ray crystallography. A modulated structure has been identified and refined by using a stacking fault model that requires reflection data to be put on three scales depending on an index condition. Crystals are triclinic, space group P (1) over bar, with Z 8 in a cell of dimensions a 13.538(3), b 14.516(4), c 16.480(4) Angstrom; alpha 97.17(2), beta 105.69(2), gamma 106.09(2)degrees. The structure involves proton transfer from the tnba molecule to the 2,6-dap molecule, with the resulting pyridinium proton and an amine proton interacting with the carboxyl oxygens of the tnba molecule in a primary cyclic hydrogen-bonding association. Additional peripheral hydrogen bonding completes a two-dimensional sheet structure.