Molecular symmetry and isostructural relations in crystal phases of trihalomethanes CHCl3, CHBrCl2, CHBr2Cl, and CHBr3.

Bromodichloromethane (CHBrCl(2)), dibromochloromethane (CHBr(2)Cl), and their parent trihalomethanes, chloroform (CHCl(3)) and bromoform (CHBr(3)), form an intriguing series of isostructural crystal phases, the sequence of which depends on the Br/Cl substitution and thermodynamic conditions. The phase behavior of these compounds has been studied by isobaric calorimetry and isothermal compression, and the crystal structure of CHBrCl(2) has been determined at 0.10 MPa/200 K, 0.73, 1.26, 2.53 GPa (all at 295 K), and that of CHBr(2)Cl at 0.43, 1.24 GPa (all at 295 K). CHBrCl(2) frozen by isobaric cooling at 0.10 MPa crystallizes in space group P1 with Z = 2, while its high-pressure polymorph in space group Pnma (Z = 4) is stable at 295 K from its freezing pressure at 0.48 to at least 2.53 GPa. At the freezing pressure of 0.29 GPa, CHBr(2)Cl crystallizes in space group P6(3), with Z = 2, and at 1.27 GPa, it transforms to the orthorhombic structure, space group Pnma (Z = 4); CHCl(3) has the identical symmetries, but their reverse sequence was observed. A subtle isostructural phase transition has been observed at 0.10 MPa and 214.9 K in CHBr(2)Cl. The relations between isostructural phases, their symmetry, and site occupation factors of halogen atoms observed in the low-temperature and high-pressure phases of trihalomethanes (CHCl(3), CHBrCl(2), CHBr(2)Cl, and CHBr(3)) have been explained by the directional character of electrostatic interactions between the molecules. A gradual ordering of the disordered Br and Cl atoms has been achieved in the compressed crystals, where the narrower volume of the atomic sites correlates with the increased occupancy of the smaller atom (chlorine). The molecular symmetry has been shown to control the molecular aggregation in the crystalline state, consistent with the crystal site-symmetry and the balance of electrostatic matching and dispersion forces between molecules.