Photoelectron spectroscopy of carbonyls. 1,4-benzoquinones

He I photoelectron spectra have been obtained for 1,4-benzoquinone (PBQ) and some selected derivatives. A correlative study, which is based on the vibronic characteristics of the various ionization events and on the differential sensitivities of these events to chemical substitution (usually methylation, fluorination, or chlorination) on the benzenoid ring, leads to an MO assignment for the low-energy ionization regime. The assignment for PBQ, in order of decreasing MO energy, is n/sub -/ > n/sub +/ > ..pi../sub circled +/ > ..pi../sub -/; that for tetrafluoro-1,4-benzoquinone (TFPBQ) is n/sub +/ > n/sub -/ > ..pi../sub -/ > ..pi../sub circled +/. The perfluoro effect on PBQ induces shifts of the n orbitals which are approx.1 eV. The perfluoro shifts of the ..pi.. orbitals are somewhat smaller than those of the n orbitals. CNDO/2 computations provide uniformly poor agreement with the experimental data and retain no assignment utility unless used in a perturbative sense.