Local and Global Paratropic and Diatropic Ring Currents in Pyrene and Its Cyclopenta‐Fused Congeners
An ab initio distributed-origin coupled Hartree−Fock method has been used to compute the σ-, π- and total (σ+π)-current density maps of a family of polycyclic aromatic hydrocarbons comprising pyrene (1) and its externally cyclopenta-fused congeners 2−8. The number and the distribution of pentagons strongly affect the local patterns of induced currents, and global aromaticity is reduced as more pentagons are added to the pyrene perimeter. The pentagons in 2 and 3 have only minor effects on the pyrene core and contain cyclopenteno double bonds. In the case of the other species 4−8, in marked contrast, intense paratropic currents in the pentagons result in stepwise destruction of the 14π pyrene perimeter motif and the breaking up of the currents into single-ring contributions. The visualisation adds detail to the interpretation of Nucleus-Independent Chemical Shift (NICS) values, and the pattern of paratropic contributions can in fact, with the exception of 6 and 7, be explained qualitatively by simple Huckel−London π-electron theory.