Molecular photoelectron spectroscopy. Part VII. The vertical ionisation potentials of benzene and some of its monosubstituted and 1,4-disubstituted derivatives

Abstract

Photoelectron spectra excited by helium 584 Å radiation in the vapours of benzenoid derivatives have been recorded by use of a magnetic deflection analyser. Vertical ionisation potentials measured from peaks in these spectra have been interpreted in terms of electron removal from molecular orbitals having energies above –21·21 eV. The results are discussed in relation to the abilities of the various substituent groups to perturb the π and σ orbital levels of the benzene ring. In particular the effect of suitably orientated p electrons of the substituent on the degeneracy of the highest occupied π levels (π₂, π₃) is examined and compared with theoretical predictions. A value of 11·8 eV is assigned to the second π ionisation (a_2u) and a peak at 16·93 eV to a C–C ionisation on the basis of their behaviour on substitution.