Isostructural relations in dihalomethanes and disproportionation of bromoiodomethane

Abstract

The directional character of halogen⋯halogen forces has been evidenced by specific structural features of bromoiodomethane crystals, CH₂BrI. The crystals were in-situ grown in isobaric conditions in a glass capillary and in isothermal and isochoric conditions in a diamond-anvil cell, and the structures have been determined by single-crystal X-ray diffraction at 0.10 MPa/220 K, 0.10 MPa/100 K, 0.54 GPa/295 K. The freezing point for isothermal compression at 295 K is 0.47 GPa. At 0.80 GPa/490 K CH₂BrI disproportionates to CH₂I₂ and CH₂Br₂. The ambient-pressure/low-temperature and 0.54 GPa/ambient-temperature CH₂BrI crystals are monoclinic, space-groupC2/c, isostructural with CH₂Br₂, CH₂I₂ and CH₂BrCl phases. High-pressure structure of CH₂I₂, determined at 0.80 GPa/295 K, is orthorhombic, space-group Fmm2. Crystals of CH₂Br₂, CH₂BrI and CH₂I₂ are 2-dimensionally isostructural. The polar arrangement of molecular sheets in CH₂I₂ is favoured by CH⋯I hydrogen bonds, which are absent in the centrosymmetric CH₂BrI structure. The temperature at which CH₂BrI disproportionates into CH₂Br₂ and CH₂I₂ is raised by pressure, as this process requires that CH₂BrI be liquid.