The Hückel molecular orbital spectra (HMO) of Hückel- and Mobius-type cyclacenes having 3-15 benzenoid rings were investigated and it was found that certain eigenvalues are topologically invariant, whereas others exhibit topological periodicity. For example, Hückel-type cyclacenes having 2r (r: 2,3,4,...) benzenoid rings which possess nonbonding molecular orbitals (NBMO) and are thus expected to be open shell structures.
TÜRKER, Lemi and ERKOÇ, Şakir (1997) "Investigation of HMO Spectra of Hückel and Mobius Type Cyclacenes," Turkish Journal of Chemistry: Vol. 21: No. 3, Article 3. Available at: https://journals.tubitak.gov.tr/chem/vol21/iss3/3