If a perhalogenated benzene ring were to remain planar, then geometric constraints would force adjacent halogens closer than their associated nonbonding radius.

Perfluorobenzene is an exception: as shown in the following table, two fluorines are small enough to avoid collision, retaining planarity and full aromaticity.

HFB has a single narrow 19F NMR signal and the spin lattice relaxation rate is highly sensitive to changes in pO2, yet minimally responsive to temperature.

It has been extensively applied to examine changes in tumor oxygenation in response to interventions such as breathing hyperoxic gases or as a consequence of vascular disruption.

[19] The National Institute for Occupational Safety and Health (NIOSH) lists it in its Registry of Toxic Effects of Chemical Substances as neurotoxicant.