On the Correlation between Electronic Spectra of Alternant Aromatic Hydrocarbons and Their Radical Cations

On the basis of the spectroscopic data for some 40 alternant aromatic hydrocarbons, a simple correlation is given between the excited singlet state (p band) of such molecules and the excited lowest-energy non-Koopmans state (A band) for their radical cations. It is shown that, with the use of this relation, the energy of the doublet state for a radical cation can be reasonably estimated if the energy of the p band for its neutral precursor is known. We have also computed the doublet state energies of the aromatic hydrocarbons using the “SDT” relations, which involve three spectroscopic quantities, viz., the excited singlet (S) and triplet (T) states of a neutral molecule and the doublet (D) state of its radical cation. The predictions of the proposed singlet-doublet state correlation are found to be far superior than those of the SDT relations.