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 gaucheconformation 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 intra­molecular 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 carb­oxy­lic acid/amide complimentary O—H⋯O + N—H⋯O hydrogen bonds, which generate R₂²(8) loops. These chains are linked by further hydrogen bonds mediated by the halide ions and water mol­ecules to give a layered structure with cation and anion layers parallel to the ab plane. Compound (III) was refined as an inversion twin.