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.