Determination of the Assignment of Triplet States in Catacondensed Aromatic Hydrocarbons

Abstract

The assignment of the triplet states in benzene plays a central role in the determination of the assignment of triplet states of all the catacondensed aromatic hydrocarbons. In this paper two tests of the assignment of triplet states, based on (i) the prediction of triplet state energies in catacondensed aromatic hydrocarbons calculated using a perimeter model developed by Moffitt, and (ii) the prediction of the lowest singlet-triplet transitions in substituted benzenes calculated using perturbation theory, are developed and applied to existing data. A number of possible benzene triplet state assignments were considered. However, the above tests are in agreement with the generally accepted assignment in which the lowest triplet state in benzene as well as the linear catacondensed aromatic hydrocarbons is assigned to 3La (in the Platt notation). The first test (i) virtually rules out all alternative assignments. The second test (ii) is consistent with the favored assignment (lowest triplet state in benzene assigned 3La) and provides fairly strong evidence against the assignment which, on theoretical grounds, is considered the next most likely assignment (lowest triplet state in benzene assigned 3Lb). More crucial experiments are proposed which should lead to an even more definitive selection of the correct assignment of the triplet states in the catacondensed aromated hydrocarbons.