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

The molecular structures of 1-Br-4-F-C 6 H 4 and 1-Cl-4-F-C 6 H 4 have been studied in the gas phase using gas electron diffraction (GED) and ab initio methods. The structure of 1-Cl,4-F-C 6 H 4 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.