Prediction of the Lowest-Energy Non-Koopmans Doublet States and the First Ionization Potentials of Polycyclic Aromatic Systems from Their Triplet-State Energies

This paper presents some new relations connecting the triplet-state energy of an alternant aromatic hydrocarbon with that of its excited doublet state as well as its first ionization potential. The energy relation between the doublet and triplet states is basically a simplified version of the SDT relation but the predictive power of the proposed correlation is considerably better in spite of the drastic assumptions made in the model. Incorporation of molecular size in the doublet-triplet correlation has no appreciable effect on the predictions, but its inclusion in the ionization potential vs. triplet-state energy relation leads to a significant improvement in the results. It is explained on the basis of our new finding that the Coulomb term appearing in the expression for the ionization potential behaves in the same manner as the molecular size, which thus accounts for electron interaction in the model. The proposed correlations provide a simple but powerful means to estimate the first ionization potentials and excited doublet-state energies of polycyclic aromatic hydrocarbons directly from the knowledge of their triplet-state energies.