Ring currents in tangentially p-p bonded σ-aromatic systems

We report a theoretical study of ring systems that delocalize electrons in a cyclic array of p orbitals arranged tangentially in σ-bonding fashion. σ-Bonded arrays are compared to conventional π-bonded analogues with respect to orbital symmetry and aromatic/antiaromatic behavior. In a one-to-one correspondence between π and tangential molecular orbitals of a cycle, local rotation turns each π to a tangential basis function, changing bonding interactions to antibonding and inverting the order of filling of molecular orbitals. The ipsocentric ring-current mapping approach is used to evaluate aromaticity on the magnetic criterion. As for conventional π-ring currents, the σ-ring current in tangential p-p bonded systems is dominated by the HOMO-LUMO transition, corresponding to circulation of four electrons in diatropic (4n + 2)-electron cycles but two in paratropic (4n)-electron cycles. The systems examined here utilize either C 2p or Si 3p orbitals for delocalization. Although interchangeable with C with respect to the fundamental orbital symmetry and ring-current rules, Si bonds at greater internuclear distances, a feature that allows easier design of potentially stable σ-aromatic structures. Calculations show the wheel-like Si 10C50H70 structure 6 as a stable, neutral aromatic molecule with a diatropic ring current following the σ-bond path formed by Si 3p orbitals. © 2006 American Chemical Society.