Mapping the modification of ring currents induced by cyclopentafusion on a naphthalene core

An analysis of the σ-, π- and total current density maps, calculated using a distributed-origin coupled Hartree-Fock method, of the non-alternant cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAH) acenaphthylene (1), pyracylene (2), fluoranthene (3) and cyclopenta[cd]fluoranthene (4) shows that, whereas the presence of a single pentagon in 1 and 3 has only minor effects on the core naphthalene unit, the strong cooperative effect of the two pentagons connected by a formal double bond in 2 and 4 results in intense paratropic currents in the pentagons, and markedly reduced aromaticity according to conventional criteria. The accuracy of the all-electron maps is supported by calculation of ¹H and ¹³C NMR chemical shifts in agreement with experiment and their interpretation is consistent with nucleus-independent chemical shifts (NICS values). It is shown that the pattern of paratropic contributions can be explained qualitatively within Hückel-London π-electron theory.