On the Lack of Ring-Current Aromaticity of (Heteroatom) [N]Radialenes and their Dianions

Current-density maps, calculated at the ab initio RHF//6–31G**/CTOCD-DZ level, show no significant π ring current in planar equilateral geometries of neutral and dianionic [N]radialenes, oxocarbons and thiocarbons CNYNq− (Y=CH2, O, S; N=4, 5, 6; q=0 (1 a–12 a), 2 (1 b–12 b)). Only the N=3 deltate dianions C3Y32− (Y=CH2, O, S (1 b, 5 b and 9 b)) have discernible π ring current, and then with at most 20–25 % of the strength of the standard benzene current. On the magnetic criterion, lack of current is definitive evidence against aromaticity. Pictorial molecular-orbital analysis within the ipsocentric approach shows this to be an inevitable consequence of the nodal structure of the π and π* orbitals of [N]radialene-like systems. On grounds of angular-momentum symmetry, spatial distribution, or both, the HOMO–LUMO excitation does not contribute a significant central diamagnetic ring current.