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 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