Structures of disodium hydrogen citrate monohydrate, Na2HC6H5O7(H2O), and diammonium sodium citrate, (NH4)2NaC6H5O7, from powder diffraction data

The crystal structures of disodium hydrogen citrate monohydrate, Na2HC6H5O7(H2O), and di­ammonium sodium citrate, (NH4)2NaC6H5O7, have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. In NaHC6H5O7(H2O), the NaO6 coordination polyhedra share edges, forming zigzag layers lying parallel to the bc plane. The hydro­phobic methyl­ene groups occupy the inter­layer spaces. The carb­oxy­lic acid group makes a strong charge-assisted hydrogen bond to the central carboxyl­ate group. The hydroxyl group makes an intra­molecular hydrogen bond to an ionized terminal carboxyl­ate oxygen atom. Each hydrogen atom of the water mol­ecule acts as a donor, to a terminal carboxyl­ate and the hydroxyl group. Both the Na substructure and the hydrogen bonding differ from those of the known phase Na2HC6H5O7(H2O)1.5. In (NH4)2NaC6H5O7, the NaO6 coordination octa­hedra share corners, making double zigzag chains propagating along the b-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 intra­molecular O—H⋯O hydrogen bond to a terminal carboxyl­ate oxygen atom.