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 C2v symmetry, the solid-state structure was found to be quite distorted, with three molecules in the asymmetric unit. These fragments only possess Cs symmetry in the plane of the molecules, as opposed to the C2v 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.