Author:

Keywords:

effective core potentials, gaussian-type basis, molecular-orbital methods, ring currents, organic-molecules, current-density, allyl cation, basis-sets, metal, homoaromaticity, Science & Technology, Physical Sciences, Chemistry, Organic, Chemistry, EFFECTIVE CORE POTENTIALS, GAUSSIAN-TYPE BASIS, MOLECULAR-ORBITAL METHODS, RING CURRENTS, ORGANIC-MOLECULES, CURRENT-DENSITY, ALLYL CATION, BASIS-SETS, METAL, HOMOAROMATICITY, 0304 Medicinal and Biomolecular Chemistry, 0305 Organic Chemistry, Organic Chemistry, 3404 Medicinal and biomolecular chemistry, 3405 Organic chemistry

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 d 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.