Br···Br and Br···H Interactions in Action:  Polymorphism, Hopping, and Twinning in 1,2,4,5-Tetrabromobenzene

The nature and importance of nonbonded halogen−halogen and halogen−hydrogen interactions in molecular crystals continues to attract significant attention. Here, we report the manifestation of such interactions in the twinning and polymorphism of 1,2,4,5-tetrabromobenzene. A combination of molecular modeling and experimental studies using DSC, hot stage optical microscopy, and in situ powder X-ray diffraction is used to explore the mechanism of phase transition and understand the basis of twin formation in this material. The propensity to twin, the observed polymorphism, and the fact that crystals hop as they transform are all consequences of Br···Br and Br···H interactions.