Consequences of Triplet Aromaticity in 4nπ-Electron Annulenes: Calculation of Magnetic Shieldings for Open-Shell Species

Abstract

The triplet states of 4nπ-electron annulenes, as suggested by Baird in 1972, should be regarded as aromatic rather than antiaromatic. Such triplet states show planarized, symmetrical geometries (see C9H9+ on the right), much lower energies than corresponding acylic triplet reference species, and evidence for diatropic ring currents (nucleus-independent chemical shifts and magnetic susceptibility exaltations).