POLYMORPHIC PACKING ARRANGEMENTS IN A CLASS OF ENGINEERED ORGANIC-CRYSTALS

Polymorphism occurs in the family of engineered cocrystals that form f rom N,N'-bis(p-X-phenyl)melamines (X = H, F, Cl, Pr, I, CH3, and CF3) and barbital. X-ray powder diffraction indicates that only one crystal line phase formed for X = Cl, F, and CH3, that primarily one phase occ urred for X = H and I, but that a small contribution from an alternati ve phase or phases was possibly also present, and that at least two di fferent polymorphs exist for both X = Pr and CF3. Agreement of positio ns of peaks in experimental XPD traces and reflections calculated from single-crystal data is good, while agreement in intensities is good i n some cases but only fair in others. Reasons for this disagreement ar e likely to include the physical design of the diffractometer and pref erred orientation of crystallites in the sample. Two of the polymorphi c forms of X = Br are converted to a third on annealing at 140 degrees C. Steric effects seem the most probable origin of the polymorphism: the bromine-substituted diphenylmelamine crystallizes in several polym orphs, and the bromine substituent is intermediate in size for the fam ily of substituents examined. This cocrystal forms one polymorph that is formed by cocrystals having larger substituents (I and CH3) and one that packs similarly to a cocrystal having a smaller substituent (Cl) .