A comparison of the absorption spectra of the fluorobenzenes and benzene in the region 4.5-9.5 eV

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Abstract
The gas-phase extinction-coefficient spectra of ten fluorosubstituted benzenes are presented and compared with benzene in the region 4.5-9.5 eV. The three singlet-singlet transitions analogous to the B2u from A1g, B1u from A1g and E1u from A1g transitions of benzene are presented and discussed. The appearance of the elusive 1E2g state is reported in 1,3,5-TFBz and two tetrafluorobenzenes, while in monofluorobenzene and o-DFBz the 3s and 4s members of the 1E1g Rydberg state are detected. From this work the authors conclude that they have the following order for benzene: 1B2u<1B1u1E1g)<1E1u1E2g while the 1A2u( sigma , pi *) state seems to be located near 1E1u. They do not see any splitting of 1E1u in molecules of lower symmetry. The band system detected by variable-angle electron impact spectroscopy is also present in the optical absorption spectra of the heavily fluorosubstituted benzenes and its origin is attributed to a pi to sigma * transition. For hexafluorobenzene at least, they suggest that the first unoccupied orbital is sigma *.