Glycine zinc sulfate pentahydrate: Redetermination at 10 K from time-of-flight neutron Laue diffraction

Abstract

Single crystals of glycine zinc sulfate pentahydrate [systematic name: hexaaquazinc tetraaquadiglycinezinc bis(sulfate)], [Zn(H2O)6][Zn(C2H5NO2)2 (H2O)4](SO4)2, have been grown by isothermal evaporation from aqueous solution at room temperature and characterized by single-crystal neutron diffraction. The unit cell contains two unique ZnO6 octahedra on sites of symmetry -1 and two SO4 tetrahedra with site symmetry 1; the octahedra comprise one [tetraaqua-diglycine zinc]2+ ion (centred on one Zn atom) and one [hexaaquazinc]2+ ion (centred on the other Zn atom); the glycine zwitterion, NH3+CH2COO-, adopts a monodentate coordination to the first Zn atom. All other atoms sit on general positions of site symmetry 1. Glycine forms centrosymmetric closed cyclic dimers due to N - H⋯O hydrogen bonds between the amine and carboxylate groups of adjacent zwitterions and exhibits torsion angles varying from ideal planarity by no more than 1.2°, the smallest values for any known glycine zwitterion not otherwise constrained by a mirror plane. This work confirms the H-atom locations estimated in three earlier single-crystal X-ray diffraction studies with the addition of independently refined fractional coordinates and Uij parameters, which provide accurate internuclear X - H (X = N, O) bond lengths and consequently a more accurate and precise depiction of the hydrogen-bond framework.