C-H center dot center dot center dot F-C hydrogen bonding in 1,2,3,5-tetrafluorobenzene and other fluoroaromatic compounds and the crystal structure of alloxan revisited

The crystal structure of 1,2,3,5-tetrafluorobenzene, 1, has been determined and is compared with those of other polyfluoro-substituted benzenes. Compound 1 has a layered monoclinic structure in which the layers are held by bifurcated C-H center dot center dot center dot F-C interactions. Short F center dot center dot center dot F separations are also observed. The layers are stacked at van der Waals separation to give a 4 angstrom packing. This structure is adopted instead of an alternative tetragonal structure adopted by fluorobenzene and a number of related compounds such as benzonitrile, pyridine N-oxide and alloxan. Compound 1 does not take the tetragonal structure mostly because this structure would demand the formation of a C-F center dot center dot center dot pi interaction, which appears improbable for the molecule. The role of weak intermolecular interactions in the crystal packing of predominantly non-polar compounds is highlighted.