Topological effects displayed in absorption and photoelectron spectra

For topologically related pairs of isomers, S and T, a theorem we term TEMO (Topological Effects on Molecular Orbitals) has been derived; it organizes the MO pattern of the isomers so that in the range of two subsequent MO's of S there are alternately two and no MO's of T. It follows that depending on the number of electrons, the HOMO-LUMO separation in S and T differs in a topologically determined manner. The wave-lengths of the absorption maxima of the longest wave-length transitions in polycyclic aromatic hydrocarbons, polymethines, and heterocyclic compounds are in accord with TEMO. The photoelectron spectra of these compounds exhibit the TEMO pattern of their MO's even in cases where Koopmans' theorem fails.