The Clar aromatic sextet theory predicts that the intensity of cyclic conjugation in chevron-type benzenoid hydrocarbons monotonically decreases along the central chain. This regularity has been tested by means of several independent theoretical methods (by the energy effects of the respective sixmembered rings, as well as by their HOMA, NICS, and SCI values, calculated at the B3LYP/6-311G(d,p) level of DFT theory). Our results show that the predictions of Clar theory are correct only for the first few members of the chevron homologous series, and are violated at the higher members. This indicates that Clar theory is not universally applicable, even in the case of fully conjugated benzenoid molecules.

A fluoranthene-type benzenoid hydrocarbon (FTBH) is a polycyclic conjugated system obtained by joining two ordinary benzenoid hydrocarbons so as to form a five-membered ring. The main differences between the π-electron properties of FTBHs and those of ordinary benzenoid hydrocarbons are caused by this five-membered ring. The most important structural factors influencing the π-electron conjugation in the five-membered ring of FTBHs were analyzed and established.

It is shown that some Kekuléan fluoranthenes are diradicals and that their ground state is a triplet. In the energetically less favorable singlet state, these hydrocarbons also exhibit pronounced diradical character. The diradical character y of the compounds under investigation was estimated using the unrestricted symmetry-broken (yPUHF) and complete active space (yNOON) methods. It was found that the yPUHF values better reproduce the diradical character of the investigated hydrocarbons. It was shown that singly occupied molecular orbital (SOMO) and SOMO-1 of a diradical structure occupy different parts of space with a small shared region, resulting in a spin density distribution over the entire molecule. The spatial diradical distribution in the singlet diradical structures was examined by inspecting the HOMOs and LUMOs for α and β spin electrons. It was shown that the α-HOMO and the β-LUMO (as well as the β-HOMO and the α-LUMO) occupy practically the same part of space. In this way, there are no unpaired electrons in a singlet diradical structure, yet two of them occupy different parts of space, thus allowing the π-electrons to delocalize.

Cyclic conjugation in the "empty" central ring of tetrabenzo- [bc,ef,op,rs]circumanthracene (TBCA) is stronger than in its neighboring "nonempty" rings, contradicting the predictions of Kekulè-structure-based theoretical models. Earlier examples of such anomalous cyclic conjugation were observed in highly strained, non-planar benzenoid systems. Because the molecule of TBCA is perfectly planar and strain-free, its cyclic conjugation pattern could be tested and verified by means of high-level, B3LYP/6- 311+G(d,p), ab initio DFT calculations

The effect of benzo- and benzocyclobutadieno-annelation on cyclic conjugation in terrylene and the higher members of the rylene homologous series is examined. Some peculiar regularities are established, that could not be observed in the case of perylene (the first member of the rylene series)