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

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Abstract
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.

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