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 res...