Pamoic acid determined from powder diffraction data

The title compound [ systematic name: 4,4'-methylenebis(3-hydroxy-2-naphthoic acid)], C23H16O6, has one half-molecule in the asymmetric unit. The molecular twofold rotational axis about the central C atom is preserved on crystallization. A chain formed by R-2(2)(8) hydrogen bonds runs along the c axis and an intramolecular O-H center dot center dot center dot O=C-OH hydrogen bond is also formed. The crystal structure was solved by simulated annealing from laboratory X-ray powder diffraction data, with data collected at room temperature. Rietveld refinement of this model led to a final R-wp value of 0.0391 at 1.39 angstrom resolution.