Molecular Orbital Predictions of Organic Spectra

Abstract

The energies of the first two to five electronic transitions of 16 conjugated molecules are predicted as simple one-electron LCAO molecular orbital transitions (with overlap included). Assignments of the observed levels to the predicted levels are made. Qualitative agreement is good when S=0.25 and β=23,000 cm−1 for ring systems. It is shown that configuration interaction must be introduced to modify these energy predictions and to account for the intensities in some molecules (``round-field'') such as benzene, triphenylene, coronene, and porphine. This interaction is less important in other molecules (``long-field'') such as pyrene, biphenyl, and tetrahydroporphine. Empirical groupings of intensities and luminescence lifetimes are shown for the different singlet-singlet and singlet-triplet band types of round-field hydrocarbons and for related molecules containing conjugated hetero-atoms. The alteration of these groupings in long-field molecules, and under jj-coupling, is described.