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

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 center dot center dot center dot halogen distances. Calculated electrostatic potential surfaces have been related to the different crystal packing of dihalomethanes investigated.