Comparison of Exciton and Molecular Orbital Calculations on n→π* Transitions

In this paper we have compared the application of the exciton and the molecular orbital methods to the calculation of n→π* transitions in N-heteroaromatic molecules. A number of the features of the n→π* transitions were treated by these two methods, including energies of the (n, π*) states, the intensities of the allowed n→π* transitions, the ordering of the allowed and forbidden transitions and singlet-triplet splittings. From a comparison of experiment and theory, it is concluded that the assumption, which is made in the exciton calculations, that n→π* transitions are essentially localized ``atomic-like'' transitions, is a poor initial approximation and leads to a number of incorrect predictions. The assumption that the promoted lone-pair electron is delocalized over the pi-aromatic system appears to be a much better initial approximation, and when the effect of electron repulsion between lone-pair electrons is included in the MO calculations, there is very good agreement between the theoretical predictions and experimental observations on n→π* transition in the mono- and polycyclic azines. The relation between MO theory in the one-electron approximation and MO theory in which electron repulsion between the lone-pair electrons has been included and is also discussed.