Photoelectron spectroscopy of 9,10-dihaloanthracenes

The photoelectron spectra of anthracene and its 9,10-dichloro- and dibromo-derivatives have been obtained. Analysis of the spectra is based mainly on the shifts in the anthracene molecular orbital ionization energies observed upon substitution. These shifts are interpreted on the basis of a perturbation model comprising short-range and long-range inductive terms and a mesomeric term. The electronic perturbation caused by the halogen atom is observed to depend largely on the unperturbed energy of the halogen np electrons and the charge distribution in the anthracene molecular orbitals. It has been possible to separate out Coulombic interactions and mesomeric interactions which depend on the energy gap between hydrocarbon and substituent orbitals. Although the observations are not conclusive, owing to the fact that only two orbitals have measurable mesomeric shifts, results indicate that α_Cl′, the Coulombic perturbation parameter, may well be negative, in contrast to previous work.