The π‐Orbital Energies of the Acenes

The ‘observed’ π‐orbital energies ϵ v,j = −I v,j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A(1), naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with π‐orbital energies calculated by three different approximations: (a) the standard Hückel HMO model; (b) a first‐order perturbation treatment, based on (a), that takes into account bond length changes which follow the ionization process; (c) a SCF π‐electron model of the type proposed by Pople and by Pariser & Parr . In agreement with previous experience it is found that model (b) yields the most satisfactory parametrization of the experimental data.