σ-Aromaticity in H3+ and Li3+: Insights from ring-current maps

Putative s-aromaticity of the clusters H3+ and Li3+ is investigated by computation of ring-current maps in the ab initio ipsocentric approach. Although H3+ shows a marked diatropic ring-current and can be considered s aromatic on the magnetic criterion, Li3+ 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 Li3+ the diatropic HOMO-LUMO excitation is opposed by paratropic excitations to higher unoccupied orbitals; in H3+ this cancellation does not occur.