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)

The Kekulé structure count K of fluoranthene congeners is studied. It is shown that for such polycyclic conjugated π-electron systems, either K = 0 or K ≥ 3. Moreover, for every t ≥ 3, there are infinitely many fluoranthene congeners having exactly t Kekulé structures. Three classes of Kekuléan fluoranthenes are distinguished: i) Φ 0 -fluoranthene congeners in which neither the male nor the female benzenoid fragment has Kekulé structures, ii) Φ m -fluoranthene congeners in which the male benzenoid fragment has Kekulé structures, but the female does not, and iii) Φ fm -fluoranthene congeners in which both the male and female benzenoid fragments have Kekulé structures. Necessary and sufficient conditions are established for each class, Φ = Φ 0 , Φ m , Φ fm , such that for a given positive integer t, there exist fluoranthene congeners in Φ with the property K = t.

A few years ago, a method was proposed for assessing the π-electron content (EC) of rings in heteroatom-containing benzenoid molecules. In this work, the effect of two nitrogen atoms at the opposite sides of a linear benzenoid molecule on the partitioning of its π-electrons was investigated. The results obtained can be explained by means of resonance-theoretical arguments.

Several currently used models for assessing the extent of cyclic conjugation in benzenoid hydrocarbons, all based on Kekul?-type structural formulas predict that there is no cyclic conjugation in the central, "empty", ring of perylene and its annelated derivatives. In this paper it is shown that in some annelated perylenes the cyclic conjugation in the "empty" ring (measured by its energy-effect) may be unexpectedly high. Therefore, in the case of these annelated perylenes, the Kekul?- structure-based models fail. The cause for such an "anomalous" behavior of annelated perylenes is discussed.