On the origin of high Hückel molecular orbital degeneracy

Abstract

The previously observed high ("unusual") degeneracy of Hückel MOs in certain conjugated π-electron systems is rationalized by means of graph spectral theory. Rules are given, enabling the prediction of the degeneracy of MOs in the majority of cases of chemical interest. These generalize previously noticed regularities. It is shown that the high degeneracy of the MOs is not caused by summetry, but rather by certain peculiarities of the connectedness of the respective molecules.