Unusual asymmetry in halobenzenes, a solid-state, gas-phase and theoretical investigation

The molecular structures of 1-Br-4-F-C₆H₄ and 1-Cl-4-F-C₆H₄ have been studied in the gas phase using gas electron diffraction (GED) and ab initio methods. The structure of 1-Cl-4-F-C₆H₄ in the crystalline phase has also been studied, but whilst the gaseous structures were found to possess C_2v symmetry, the solid-state structure was found to be quite distorted, with three molecules in the asymmetric unit. These fragments only possess C_s symmetry in the plane of the molecules, as opposed to the C_2v symmetry observed in the gas phase. The bonding motifs within the solid-state structure are very unusual and unexpected, with quite different C-F bond lengths for the three moieties, and are a result of weak hydrogen-halogen interactions within the structure.