Halogen⋯halogen contra C–H⋯halogen interactions

Abstract

Pressure affects the competition between C–H⋯X hydrogen bonds and X⋯X halogen⋯halogen interactions. In bromomethane, CH3Br, pressure changes the molecular arrangement of the two solid-state phases of this compound: low-pressure phase α is dominated by halogen⋯halogen interactions, whereas above 1.5 GPa the β phase is governed by C–H⋯halogen bonds. The CH3Br phase α is isostructural with solid CH3I of orthorhombic space group Pnma, while CH3Br phase β is polar, isostructural with CH3Cl and CH3CN crystals, of orthorhombic space group Cmc21. The crystal structures of CH3Cl (b.p. = 249.1 K) and CH3Br (b.p. = 276.7 K) have been determined by high pressure single-crystal X-ray diffraction up to 4.38 GPa and 2.85 GPa, respectively. In CH3Br, pressure of 1.5 GPa enforces the close packing and opposite electrostatic-potential matching between molecular surfaces in contact. The interweaved C–H⋯X bonded diamondoid networks of β-CH3X are similar to those of acetonitrile, H2O ice VII and solidified X2 halogens. The phase diagrams of CH3Br and CH3Cl have been constructed.