title: Ring currents in tangentially p-p bonded sigma-aromatic systems.
abstract: We report a theoretical study of ring systems that delocalize electrons in a cyclic array of p orbitals arranged tangentially in sigma-bonding fashion. Sigma-bonded arrays are compared to conventional pi-bonded analogues with respect to orbital symmetry and aromatic/antiaromatic behavior. In a one-to-one correspondence between pi and tangential molecular orbitals of a cycle, local rotation turns each pi 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 pi-ring currents, the sigma-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 sigma-aromatic structures. Calculations show the wheel-like Si10C50H70 structure 6 as a stable, neutral aromatic molecule with a diatropic ring current following the sigma-bond path formed by Si 3p orbitals.
authors.affiliation: Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK. P.W.Fowler@sheffield.ac.uk
authors.forename: Patrick W
authors.initials: PW
authors.lastname: Fowler
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Rogowska
authors.forename: Alessandro
authors.initials: A
authors.lastname: Soncini
authors.forename: Mark
authors.initials: M
authors.lastname: Lillington
authors.forename: Leif P
authors.initials: LP
authors.lastname: Olson