Structures of disodium hydrogen citrate mono-hydrate, Na 2 HC 6 H 5 O 7 (H 2 O), and di-ammonium sodium citrate, (NH 4 ) 2 NaC 6 H 5 O 7 , from powder diffraction data

The crystal structures of disodium hydrogen citrate monohydrate, Na HC H O (H O), and di-ammonium sodium citrate, (NH ) NaC H O , have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. In NaHC H O (H O), the NaO coordination polyhedra share edges, forming zigzag layers lying parallel to the plane. The hydro-phobic methyl-ene groups occupy the inter-layer spaces. The carb-oxy-lic acid group makes a strong charge-assisted hydrogen bond to the central carboxyl-ate group. The hydroxyl group makes an intra-molecular hydrogen bond to an ionized terminal carboxyl-ate oxygen atom. Each hydrogen atom of the water mol-ecule acts as a donor, to a terminal carboxyl-ate and the hydroxyl group. Both the Na substructure and the hydrogen bonding differ from those of the known phase Na HC H O (H O) . In (NH ) NaC H O , the NaO coordination octa-hedra share corners, making double zigzag chains propagating along the -axis direction. Each hydrogen atom of the ammonium ions acts as a donor in a discrete N-H⋯O hydrogen bond. The hydroxyl group forms an intra-molecular O-H⋯O hydrogen bond to a terminal carboxyl-ate oxygen atom.