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.

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Measurements of vapor-liquid equilibria in the systems water-n-propanol and water-n-propanol-n-butanol at atmospheric pressure are reported. The results for the ternary system have been compared wit!) those predicted from the binary mixtures using the equation of Renon and Prausnitz. Agreement is very satisfactory. Densities and refractive indices of the binary and ternary mixtures at 25°C are also reported and the excess volumes of binary mixing have been calculated.