Qualitative Methods for Predicting the Ground States of Non-Kekule Hydrocarbons and the Effects of Heteroatom Substitution on the Ordering of the Electronic States

Classification of the NBMOs of a non-Kekule hydrocarbon as being disjoint or non-disjoint can be used to predict the spin of the ground state and the magnitude of the energy difference between it and the lowest excited state. If the NBMOs are non-disjoint, the bonding in the lowest excited states will be more localized than that in the ground state. The nature of the localized pi bonding in the lowest excited states allows one to predict qualitatively which will be most stabilized by hetero-atom substitution and whether it is possible that the attendant stabilization, relative to the ground state, is capable of giving the substituted molecule a ground state of different spin multiplicity. The same predictions can also be made using VB theory.