Crystal structures of three halide salts of l-asparagine: an isostructural series

The structures of three monohydrated halide salt forms of l-asparagine are presented, viz. l-asparaginium chloride monohydrate, C₄H₉N₂O₃ ⁺·Cl^-·H₂O, (I), l-asparaginium bromide monohydrate, C₄H₉N₂O₃ ⁺·Br^-·H₂O, (II), and l-asparaginium iodide monohydrate, C₄H₉N₂O₃ ⁺·I^-·H₂O, (III). These form an isomorphous and isostructural series. The C-C-C-C backbone of the amino acid adopts a gauche conformation in each case [torsion angles for (I), (II) and (III) = -55.4 (2), -55.6 (5) and -58.3 (7)°, respectively]. Each cation features an intra-molecular N-H⋯O hydrogen bond, which closes an S(6) ring. The extended structures feature chains of cations that propagate parallel to the b-axis direction. These are formed by carb-oxy-lic acid/amide complimentary O-H⋯O + N-H⋯O hydrogen bonds, which generate R ₂ ²(8) loops. These chains are linked by further hydrogen bonds mediated by the halide ions and water mol-ecules to give a layered structure with cation and anion layers parallel to the ab plane. Compound (III) was refined as an inversion twin.