Photoelectron spectra of benzene and some fluorobenzenes

Abstract

Photoelectron spectra of all the fluorobenzenes (except 1,2,3-trifluorobenzene) have been obtained with photon energies up to 40.8 eV. The changes in spectral features arising from the progressive substitution of fluorine, as well as those associated with the various stereoisomers of these molecules, illustrate clearly many important features of the benzene orbitals and assist in their correct assignment. The nature of the Jahn–Teller effect in states arising from the ionization of the doubly degenerate π and σ orbitals is shown to be markedly different, and this in turn differs from the effect of substitutional splitting in these degenerate orbitals. The analysis leads to the following reassignment of adiabatic ionization potentials for C₆H₆; (1a_2u)^–1, 11.490 eV; (3e_2g)^–1, 11.570 eV; (3a_1g)^–1, 15.446 eV and (2b_1u)^–1, 16.848 eV. Other changes relative to previous band assignments arise from the recognition of large electronic Jahn–Teller splittings in the σ systems. In the π systems, because of their much weaker binding properties, the correspondingly weak Jahn–Teller forces are found to be just strong enough to cause excitation of e-type vibrations without giving rise to any electronic splitting. A study of the isotopic blue shifts of the band origins observed for benzene-d₆ reflects the bonding character and hydrogen dependence of the orbitals and supports the analysis. Selective enhancement of (3e_2g)^–1 relative to (1a_2u)^–1 in the C₆D₆ spectrum obtained with Ne I radiation was used to identify these states.