σ-Aromaticity in [formula omitted]: Insights from ring-current maps

Abstract

Putative σ-aromaticity of the clusters H 3 + and Li 3 + is investigated by computation of ring-current maps in the ab initio ipsocentric approach. Although H 3 + shows a marked diatropic ring-current and can be considered σ aromatic on the magnetic criterion, Li 3 + shows no global current and is non-aromatic on this criterion, in spite of its electron count and negative NICS value. The difference in magnetic response is interpreted in terms of orbital energies using the ipsocentric model: in Li 3 + the diatropic HOMO–LUMO excitation is opposed by paratropic excitations to higher unoccupied orbitals; in H 3 + this cancellation does not occur.