On the Interpretation of Molecular Spectra in Odd-Membered Aromatic Compounds

Molecular-state functions, based on the perimeter model, were constructed for odd-membered aromatic molecules. The electronic spectra of the tropylium, benztropylium, and three dibenztropylium positive ions were calculated by first-order configuration interaction (i.e., by interaction between the four states of the first excited configuration) and subsequent perturbation of the ring by cross links. The results are compared with the experimental values and calculation of Heilbronner and Murrell. The heteroaromatic indene compounds indole, indolizine, and methyl pyrindine, as well as their aza-substituted derivatives were similarly investigated. The main features of these spectra are the splitting of the E(1u) and E(nu) perimetex degenerate states and the shifts of the state energies by the inductive effect. The latter depends mainly upon the difference in charge densities between ground and excited state at the sites of substitution in the molecule. The agreement with the observed spectra is generally good. However, in carbazole, quantitative agreement is not achieved, though the predicted features of the spectrum are observed.