Acta Crystallographica Section B

Structural Science

Volume 63, Part 6 (December 2007)

research papers

 

Isobaric and isochoric freezing of CH₂BrCl and isostructural relations between CH₂Cl₂, CH₂Br₂ and CH₂BrCl

M. Podsiadlo and A. Katrusiak

Abstract: Bromochloromethane, CH₂BrCl, has been temperature-frozen and in situ pressure-frozen and the structure determined by X-ray diffraction at low temperatures of 170 and 100 K at ambient pressure (0.10 MPa), and at high pressures of 1.04 and 1.72 GPa at room temperature (295 K). CH₂BrCl exhibits a remarkable polymorphism: at low temperature it crystallizes in the monoclinic space group C2/c (phase I), isostructural to the crystals of CH₂Br₂. The pressure-frozen crystal of CH₂BrCl is orthorhombic, space group Pbcn, and is isostructural to the crystal of CH₂Cl₂. In both phases I and II the Br and Cl atoms are substitutionally disordered. The freezing temperatures and pressures of simple dihalomethanes have been correlated to their molecular weight and halogen halogen distances. Calculated electrostatic potential surfaces have been related to the different crystal packing of dihalomethanes investigated.

Keywords: dihalomethanes; halogenhalogen interactions; high-pressure pressure freezing; anisotropic van der Waals radii.

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