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Abstract: Recently assigned numerical expressions to qualitative benzenoid formulas of Clar have transformed Clar's theory into a quantitative model that allowed a structural characterization of aromaticity in benzenoid polycyclic hydrocarbons and construction of aromaticity maps for benzenoid polycyclic hydrocarbons. The key structural elements of the novel quantitative aromatic sextet theory of Clar are (1) Kekulé valence structures, (2) Pauling bond orders, and (3) Clar's aromatic sextet benzenoid rings. In the present contribution we go beyond Clar and consider local aromaticities of benzenoid rings in benzenoid polycyclic hydrocarbons. We have characterized each benzenoid ring by two descriptors: (1) their content of C = C bonds in the set of Kekulé valence structures, which is reflected in the ring bond orders, and by (2) the variations of CC bond orders within rings, which are zero for benzene. This allows us to introduce for any set of benzenoid hydrocarbons local aromaticity map, which allows one to graphically view similarities and dissimilarities of local aromaticity of different benzenoid rings. For such map, one can calculate the “distance” of each ring from the benzene, with coordinates (3.00, 0) for benzene RBO and variations in CC bonds, respectively. We show that the central ring in peropyrene is visibly less aromatic that the terminal sextets, as has been claimed by Gutman and Agranat, and similarly we show that the central ring of anthracene is unexpectedly more aromatic than the terminal rings, in having greater similarity in bond lengths to benzene ring than the terminal rings. This suggests limitations of the assumption of the Clar model that migrating sextets have equal local aromaticities. Finally, we examined a pair of non benzenoid hydrocarbons related to peropyrene, built from terminal benzenoid rings and naphthalene fragments. Surprisingly, we have found the terminal benzene rings in the two non benzenoid compounds to have unusually high ring bond orders, approaching that of benzene. [ABSTRACT FROM AUTHOR]

By analogy with benzenoids, diamond hydrocarbons (diamondoids for short) can be classified according to their dualists into catamantanes with acyclic dualists, perimantanes with dualists having 6-membered rings, and coronamantanes having larger rings that are not peripheries of 6-membered ring aggregates. Coronoid diamond hydrocarbons may be either tridimensional portions of the macromolecular diamond lattice with holes examined in an earlier article and here, or hollow smaller molecules (coronamantanes) also discussed in the present paper, with 3D or quasi-2D holes. In the case of lattices, the effect of external hydrogen atoms is negligible but it cannot be ignored in the case of coronamantanes. Both the outer and inner dangling bonds may be connected with hydrogen atoms, or some of the inner carbon atoms with dangling bonds may be replaced by heteroatoms (N, O, or S) which might be able to bind metallic cations with coordinative bonds. A convenient means of structure representation uses dualists of the diamondoids. The holes may be isolated as are "closed pores" in polymer foams, or they may communicate forming tunnels as for 'open pores' in 'sponge foams'. A special discussion is reserved for coronamantane analogs of the quasi-planar coronoids. In these cases the diamondoid is quasi-fiat and the quasi-2D hole or tunnel is accessible on both sides; it may be chiral or achiral. The diamondoid analog of kekulene is a cyclohexadecamantane C64H64, a challenge for the future. [ABSTRACT FROM AUTHOR]

Comparison is made between three different indices that characterise the individual rings of a wide range of condensed benzenoid hydrocarbons. Two of these ("π-electron partitions" and the six-centre delocalisation-indices that have been called "Δ6-values") have been introduced only recently as potential indicators of what might be called "local aromaticity", whilst the third ("topological π-electron ring-currents") was suggested as a possible discriminator in this regard nearly fifty years ago. Whilst linear correlations between ring currents and π-electron partitions within certain restricted classes of ring types are good (with correlation coefficients of up to 0.998), agreement between the two indices over all classes of ring types is poor. Predictions arising from a consideration of π-electron partitions and Δ6-values seem, on the other hand, to be in somewhat better accord. It is therefore concluded that, despite its superficially intuitive appeal, the ring-current index is out of step both with π-electron partitions and Δ6-values as a general indicator of so-called "local aromaticity". [ABSTRACT FROM AUTHOR]

According to the recently discovered PCP rule, the intensity of cyclic conjugation in the fivemembered rings of polycyclic conjugated hydrocarbons related to acenaphthylene and fluoranthene increases with the number of phenyl-cyclopentadienyl (PCP) fragments present in the molecule. The validity of this regularity was first observed by studying the energy effects of cyclic conjugation in the fivemembered rings of these systems. In order to show that the PCP rule is not an artifact of the method employed, it is necessary to confirm its validity by other approaches. We now show that the PCP rule is in perfect agreement with calculated topological p-electron ring-currents, ostensibly an independent, quantitative, theoretical - albeit, it is here argued, likewise also graph-theoretical - measure of cyclic conjugation in the individual rings of polycyclic p-electron systems. In addition, we show that the molecularstructure dependency of the ring currents in acenaphthylene and fluoranthene congeners also agrees with other, earlier-established, regularities for cyclic conjugation. [ABSTRACT FROM AUTHOR]

Generalized helicenes are defined. A theoretical analysis is presented for the intramolecular dehydrogenation of helicenes, bridging adjacent rungs of the helix with the formation of 4-, 5-, and/or 6-membered rings, yielding helical tubulenes to which Euler's formula for polyhedra no longer applies as such constructions are derived from Möbius strips and correspond to nonplanar graphs (i.e., have crossing lines when drawn in two dimensions). Intermolecular dehydrogenation of two parallel-superimposed polycyclic aromatic hydrocarbons with formation of similar rings leads to convex polyhedral carbon cages. Carbon helical nanotubes and some carbon cages that are different from fullerenes are also discussed. [ABSTRACT FROM AUTHOR]

Earlier studies revealed that benzo-annelation has a peculiar effect on the intensity of cyclic conjugation in the five-membered ring of fluoranthene congeners. Now, the analogous effect of benzocyclobutadieno-annelation was examined and it was found show it is opposite to the effect of benzo-annelation: a benzocyclobutadiene fragment in angular (resp. linear) position with regard to the five-membered ring, decreases (resp. increases) the intensity of cyclic conjugation in the five-membered ring. [ABSTRACT FROM AUTHOR]