On the Cycle-Dependence of Topological Resonance Energy

Effect of benzocyclobutadieno-annelation on cyclic conjugation in fluoranthene congeners

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.

Cyclic conjugation in benzo-annelated triphenylenes

Cyclic conjugation in benzo-annelated triphenylenes was studied by means of the energy effect (ef) and the ?-electron content (EC) of the six-membered rings. A regularity that was earlier discovered in the case of acenaphthylene and fluoranthene congeners is now shown to hold also for benzo-annelated triphenylenes: Benzenoid rings that are annelated angularly with regard to the central six-membered ring Z0 of triphenylene increase the intensity of the cyclic conjugation in Z0, whereas linearly annelated benzenoid rings decrease the cyclic conjugation in Z0. The efand EC-values are strongly correlated, yet in a non-linear manner.

Effect of a ring on the cyclic conjugation in another ring: Applications to acenaphthylene-type polycyclic conjugated molecules

In a recent work, a method was developed for assessing the influence ief(G,Z0|Z1) of a ring Z1 on the energy effect of another ring Z0 in a polycyclic conjugated molecule G. Herein, a report is given of detailed numerical investigations of ief(G,Z0|Z1) aimed at the elucidation of the influence of various six-membered rings on the intensity of cyclic conjugation in the fivemembered ring of acenaphthylene-type molecules. The earlier discovered regularities for cyclic conjugation in acenaphthylene-type molecules (in particular, the PCP rule and the linear rule) could thus not only be rationalized, but also a number of hitherto concealed regularities could be envisaged.

The close relation between cyclic delocalization, energy effects of cycles and aromaticity

New evidence questioning the multidimensionality of the aromaticity phenomenon exemplified in what is called orthogonality between the classical (structural and energetic) and magnetic aromaticity indices and measures is reported. For this purpose, the recently proposed methodologies for the quantitative characterization of the energy benefits associated with the cyclic arrangement of mobile π-electrons in polycyclic aromatic hydrocarbons are compared with the indices characterizing the extent of cyclic delocalization in the corresponding conjugated circuits. The reported close correlation between both types of indices implies that no discrepancies between classical and magnetic aromaticity measures exist provided the comparison is based on the indices of inherently local nature and/or the interfering contributions of contaminating conjugated circuits is properly taken into account in the description of aromaticity measures like topological resonance energy (TRE) or nucleus independent chemical shift (NICS).

On π-electron conjugation in the five-membered ring of fluoranthene-type benzenoid hydrocarbons

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 ?-electron properties of FTBHs and those of ordinary benzenoid hydrocarbons are caused by this five-membered ring. The most important structural factors influencing the ?-electron conjugation in the five-membered ring of FTBHs were analyzed and established.

Verifying the PCP-rule by five-center bond indices

According to the recently discovered PCP-rule, the (stabilizing) energy- effect of the cyclic conjugation in the 5-membered ring of acenaphthyleneand fluoranthene-type polycyclic conjugated hydrocarbons increases with the number of phenyl-cyclopentadienyl (PCP) fragments present in the molecule. It is now shown that the validity of the PCP-rule is also supported by the 5-center bond indices, an independent quantitative theoretical measure of cyclic conjugation in 5-membered rings.

Magnetic Resonance Energies of Heterocyclic Conjugated Molecules

Magnetic resonance energy (MRE), derived from ring-current diamagnetic susceptibility, can be interpreted as a kind of aromatic stabilization energy. For polycyclic conjugated hydrocarbons, this quantity correlates well with topological resonance energy (TRE). MREs for typical heterocyclic conjugated molecules were then calculated and analyzed. It was found that even for heterocycles MRE highly correlates with TRE. Thus, the MRE concept has been firmly established as a reliable indicator of aromaticity, which mediates magnetic criteria of aromaticity with energetic ones. The conformity of heterocycles to the rule of topological charge stabilization can be checked using not only TRE but also MRE.