Crystal structure of nedocromil sodium heptahemihydrate and its comparison with that of nedocromil sodium trihydrate

The crystal structure of one of the hydrated forms of nedocromil sodium, the heptahemihydrate, has been determined. Crystal data: a = 26.687(5), b = 29.479(8), c = 6.394(3) Angstrom, orthorhombic, space group Aba2, Z = 8. This structure is compared with the previously determined structure of the trihydrate [Freer, A. A., Payling, D. W., and Suschitzky, J. L., Acta Crystallogr. 1987, C43, 1900-1905.]. The two structures differ in the molecular conformation of the nedocromil anion, the intensity of color, the coordination of the sodium ions, and the environment of the water molecules. In the heptahemihydrate two of the water molecules from dimers, whereas the remaining water molecules form a two-dimensional network consisting of a pentameric chain in one direction and an infinite chain in the other. From another viewpoint, 5 1/2 water molecules in the heptahemihydrate are present in the first coordination sphere of the sodium ions, whereas the remaining two water molecules are present as ''lattice'' water in the crystal.