Molecular Aromaticity from a Magnetic Perspective

The aromaticity concept was initially introduced to describe compounds that resemble benzene. Aromaticity was later extended to describe general ring-shaped molecules with delocalized conjugated bonds having enhanced thermodynamic stability, small bond length alternation, particular reactivity, and distinct magnetic properties. In this Perspective, we focus on various aspects of the magnetic criterion of aromaticity. Molecular aromaticity is assessed by using calculations of the magnetically induced current density (MICD) susceptibility. The degree of aromaticity is determined by calculating the strength of the magnetically induced ring current (MIRC), and the aromatic nature is obtained from the circulation direction of the MIRC vortex. The size limits of aromaticity and antiaromaticity are investigated by calculating the degree of (anti)aromaticity of trans-annulene rings of increasing size. We show how magnetic shielding densities that are calculated from MICD susceptibilities can be used to understand the spatial origin of nuclear magnetic resonance (NMR) shielding tensors. We consider the topology of molecular structures and how it affects MICD. Orbital contributions to the MIRC, group theoretical aspects, and aromatic bonding of antiaromatic rings are also discussed.