Antibonding σ* valence MOs in the inner-shell and outer-shell spectra of the fluorobenzenes

Electron transmission, inner-shell electron energy loss and magnetic circular dichroism spectra have been analyzed in an effort to trace the positions of the σ* antibonding valence MOs in benzene and its fluorinated derivatives. The correlation of negative-ion resonances in these systems shows clearly that a σ* valence level descends with increasing fluorination so as to become the lowest virtual MO in hexafluorobenzene. This is understandable in terms of the perfluoro effect acting upon virtual MOs in a way parallel to that known to occur for occupied MOs. In addition to the low-lying σ* negative-ion shape resonances, several negative-ion Feshbach resonances are identified as involving 3s and 3p Rydberg orbitals. The search for low-lying σ* levels in heavily fluorinated benzenes is extended to their C 1s and F 1s inner-shell spectra. The carbon K-shell spectra of benzene and its fluorinated derivatives below the respective C 1s ionization potentials are dominated by excitations to 1π* and 2π* valence levels. In the C 1s spectra of pentafluoro- and hexafluorobenzene, additional low-lying bands are observed and assigned to C 1s(C-F) → σ* (C-F) transitions. Spectral stripping indicates the location of the corresponding (C 1s(C-F)^-1, σ*) states in the spectra of the other fluorobenzenes. A systematic shift of these σ* levels to lower energy with increasing fluorination is observed which is consistent with the perfluoro effect. Resonances terminating at σ*(C-C) are found to dominate the C Is near continuum, with dramatic enhancement of these transitions in the more highly fluorinated species. Investigation of hexafluoro- and 1,2,4,5-tetrafluorobenzene by vacuum-ultraviolet magnetic circular dichroism in the vapor phase confirms the presence of bands which are not π → π*. Once again, low-lying σ* MOs are invoked as terminating orbitals.