organic compounds
The structures of hydrated 2-(phosphonomethyl)acrylic acid, C4H7O5P.0.5H2O, (I) and its ammonium salt, [NH4]+.(C4H6O5P)-, (II), are described. In (I), the two crystallographically independent 2-(phosphonomethyl)acrylic acid residues differ slightly in the orientation of the phosphonic group relative to the rest of the molecule. In both compounds, the phosphonic groups projects out the plane of the 2-methyleneacrylate system. The carboxylic hydroxyl is trans planar to the terminal =CH2 group in (II) but is syn planar in (I). In both crystals, all OH and NH groups are involved in intermolecular hydrogen bonds.