Least-squares analyses of diffuse scattering from substitutionally disordered molecular crystals: application to 2,3-dichloro-6,7-dimethylanthracene

Diffuse intensities of X-rays scattered from substitutionally disordered molecular crystals have been measured from Weissenberg photographs and analysed using linear least-squares methods to determine the short-range correlations between molecular sites. The measured diffuse scattering was represented as a linear combination of calculated random and correlation distributions. This procedure is completely general and makes no assumptions about correlations between non-nearest neighbours. Of the 74 correlation coefficients describing the short-range order in 2,3- dichloro-6,7-dimethylanthracene, 35 were found to be significant, 13 at the 3σ level. The largest coefficients within each correlation neighbourhood were +0.46 (7), +0.37 (4) and -0.24 (5) involving six of the nearest-neighbour sites.