The title compounds, C₁₁H₁₀Cl₂N₂O₄, are constitutional isomers which exhibit similar molecular geometry. The configuration about the N=C bond is E in both molecules. The same packing pattern was found in both structures. Hydrogen-bonded dimers, formed around the inversion centres via O-HO interactions of carboxyl groups are connected into infinite chains. The intramolecular hydrogen bond N—HCl observed only in the 2,5-dichlorophenyl derivative contributes to the planarity of the molecule.

Absolute measurements of the diffuse intensity in a volume in reciprocal space have been made at six temperatures ranging from 2°C above T_c to 930°C. The Warren short-range order parameters were obtained after correcting for the intensity due to atomic displacements in a more complete manner than in earlier studies. As a result the short-range order parameters are considerably smaller than in these earlier investigations. The (previously disputed) specific heat anomalies in Cu₃Au above T_c and the L1₂ phase have been shown to be associated with unusual changes in diffuse X-ray scattering vs temperature. From studies of the scattering distributions, computer simulation and pair potentials obtained with the Warren short-range order parameters, the anomaly at 600°C appears to be due to the disappearance of DO₂₂-like fluctuations, whereas above 850°C, CuPt-like fluctuations develop. There are premonitory effects just above T_c; there is a large increase in the Debye-Waller factor, in the total intensity due to quadratic terms in atomic displacements, and an apparent change in the sign of average first-neighbor displacement. The long-range oscillations in the interatomic potentials determined from the diffuse-scattering data fit the Friedel potential only approximately. The electron-to-atom ratio in Cu₃Au was found to be ~ 0.97, in agreement with results on Cu alloys by other methods.

A single-stage computer procedure to calculate an electron density map suitable to detect errors in a tentative macromolecular model has been developed. In this procedure, an atom of the tentative model does not contribute to the phases used to calculate electron density values at or near its current position, that is within the region containing it and a neighborhood surrounding that region. In this way, the phases used to calculate electron density values within a region are not biased by the model atoms contained within that region or its neighborhood. The number of atoms which are omitted for a given region is maintained at a small fraction of the total structure so that the phases used to calculate electron density values may still be a good approximation to the phases of the complete structure. The procedure was used to improve the model of the Fab portion of the mouse galactan-binding immunoglobulin J539 (IgA2, κ), which contains 431 residues.