The pentabromocyclopentadienyl radical

Abstract Results of pulse radiolysis and photolysis at several temperatures on dilute solutions of C 5 Br 6 are reported and lead to the evidence for the formation of the symmetrical radical C 5 Br 5 . The spectroscopic consequences of halogen substitution in the cyclopentadienyl system can be summarized firstly as a strong increase of the isotropic g -value of the ESR signal which correlates almost linearly with the spin-orbit coupling constant of the substituent and secondly as a large shift of the absorption spectrum to longer wavelengths, while the over all shape is conserved. The formation of an atom-radical complex [C 5 Br 5 …Br] in rigid solution is suggested by the behaviour of the absorption spectrum as a function of temperature.