On the lack of ring-current aromaticity of (heteroatom) [N]radialenes and their dianions

Abstract

Current-density maps, calculated at the ab initio RHF//6-31G**/ CTOCD-DZ level, show no significant π ring current in planar equilateral geometries ofneutral and dianionic [N]radialenes, oxocarbons and thiocarbons CNYNq- (Y=CH2, O, S; N=4, 5, 6; q=0 (1a-12 a), 2 (1b-12b)). Only the N=3 deltate dianions C3Y3 2- (Y=CH2, O, S (1b, 5b and 9b)) have discernible π ring current, and then with at most 20-25% ofthe strength ofthe standard benzene current. On the magnetic criterion, lack ofcurrent is definitive evidence against aromaticity. Pictorial molecular-orbital analysis within the ipsocentric approach shows this to be an inevitable consequence ofthe nodal structure ofthe π and π* orbitals of[ N]radialene-like systems. On grounds ofangular -momentum symmetry, spatial distribution, or both, the HOMO-LUMO excitation does not contribute a significant central diamagnetic ring current