Double aromaticity in monocyclic carbon, boron, and borocarbon rings based on magnetic criteria

The double-aromatic character of selected monocyclic carbon, boron, and borocarbon rings is demonstrated by refined nucleus-independent chemical shift (NICS) analyses involving the contributions of individual canonical MOs and their out-of-plane NICS tensor component (CMO-NICSzz). The double aromaticity considered results from two mutually orthogonal Huckel p AO frameworks in a single molecule. The familiar a orbitals are augmented by the in-plane delocalization of electrons occupying sets of radial (rad) p orbitals. Such double aromaticity is present in B-3(-), C6H3+, C(6)(4+)6+, C4B44+, C-6, C5B2, C4B4, C2B8, B-10(2-), B-12, C-10, C9B2, C8B4, C7B6, C6B8, and C-14. Monocyclic C-8 and C-12 are doubly antiaromatic, as both the orthogonal pi and radial Huckel sets are paratropic. Planar C-7 and C-9 monocycles have mixed aromatic (a) and antiaromatic (radial) systems.