Naturally abundant 13C satelhtes have been detected in the tsotrop~c EPR spectrum ofCcF2, die 13C couphng being 12.1 G The magnitude of this couphng does not support a nonplanar structure For C6Fi. Also. the value is more con&tent v.Mb a (I* than a ?r* ConfigUratlon m D,sb s) mmetry. 1. introduction The structure of the hexafiuorobenzene radrcal anion has been controversial. From EPR studies in an ad~ant~e matrix, Yun and Wood [l] have argued cogently that the large isotropic lqF hyperfine couphng (lqa(6) = 137 G) rules out a IT* structure analogous to that of C6 Hz _ Assum~g that the Dhh symmetry of the parent molecule is retained, these authors have proposed that the ground state of the amon IS a cr* radical with the unpaired electron in either alg or eZE orbitals, the latter assignment being preferred to estabhsh greater consistency with the EPR results for the penta-and 1,2,4,5-tetra-fluorobenzene radical amens [I] _ On the other hand, Symons and his co-workers [2] have argued that the 19F anisotropy in the powder spectrum of the C6Fi radxcal in rigid solution is inconsistent with a planar I& structure and postulate both u and pseudo n delocahzation of the unpaved electron in a puckered ring structure [2], the local orbital at each carbon atom resembhng that of the pyramidal CFi radical [3] _ Studies of the negative Ion states of hexafluorobenzene in the gas phase have also led to contradictory fmdmgs [4,5] _ Gant and ~~stop~orou [4] observed resonances at aO.0 and 0.73 eV by the electron-swarm technique and attnbuted these processes to electron capture into the degenerate ezu (7~ + x5) and bzg (7~) orbitals, respectively. However, subsequent studies by * Research supported by the Dlvlston oFChemuzat Snences, Office of basic Energy Sciences, l_J S. Department of Energy (document No.