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 disjo int or non-disjoint can be used to predict the spin of the ground stat e and the magnitude of the energy difference between it and the lowest excited state. If the NBMOs are non-disjoint, the bonding in the lowe st 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 h eteroatom substitution and whether it is possible that the attendant s tabilization, relative to the ground state, is capable of giving the s ubstituted molecule a ground state of different spin multiplicity. The same predictions can also be made using VB theory.