Ring currents and magnetic properties of s-indacene, an archetypal paratropic, non-antiaromatic molecule

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of s-indacene have been evaluated by a series of different approximations and a large Gaussian basis set. An ab initio model of magnetic field induced current density was developed, showing that intense paramagnetic flow takes place within the π electrons. It causes strong upfield chemical shift of protons attached to hexagonal and pentagonal rings. The magnetic susceptibility tensor is typical of a diamagnetic molecule but is characterized by small anisotropy. Group-theoretical methods developed by Steiner and Fowler indicate that s-indacene can be classified as a 2p + 4d system, in which the paramagnetic circulation of two electrons in the highest occupied orbital of Bg symmetry overwhelms the diamagnetic flow of four electrons in occupied Au orbitals. Because of its unique structural and magnetic properties, s-indacene can be described as a limit system between the classes of aromatic and antiaromatic molecules.