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-NICS(zz)). The double aromaticity considered results from two mutually orthogonal Hückel p AO frameworks in a single molecule. The familiar pi 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) (-), C(6)H(3) (+), C(6) (4+), C(4)B(4) (4+), C(6), C(5)B(2), C(4)B(4), C(2)B(8), B(10) (2-), B(12), C(10), C(9)B(2), C(8)B(4), C(7)B(6), C(6)B(8), and C(14). Monocyclic C(8) and C(12) are doubly antiaromatic, as both the orthogonal pi and radial Hückel sets are paratropic. Planar C(7) and C(9) monocycles have mixed aromatic (pi) and antiaromatic (radial) systems.