Ab initio study of the Pauling-London-Pople (ring current) effect: LORG calculation and analysis of the NMR shielding tensors in a Sondheimer aromatic annulene and a non-aromatic analogue

Shielding tensors of the protons inside and outside the aromatic 1,8-didehydro-[14]-annulene ring, and of the corresponding protons in a closely similar open chain, are computed ab initio using the localized orbital/local origin (LORG) method. Shielding tensors for the carbon nuclei and for carbon and hydrogen in benzene also are reported. The anisotropy of the aromatic proton shielding tensors, and the large difference between the shielding of the inner and outer protons in the aromatic annulene system demonstrated by Sondheimer and commonly described as a ring current effect, are reproduced well. Proton shieldings in the open chain fall in an olefinic range. Carbon shieldings in the two systems fall into aromatic and olefinic ranges. Diamagnetic and paramagnetic contributions from sigma and pi electrons are extracted in a centre of mass (COM) gauge, and also the various contributions to the perpendicular shielding component are analysed in a COM gauge, allowing direct contact to ring current models. The analysis reveals a topological effect, unaffected by ring closure, making pi as well as signal electron contributions to the diamagnetic shielding negative (positive) for protons on the convex (concave) side of the annulene or open chain, with a similar but oppositely signed switch for the paramagnetic contributions. Relating to ring current concepts, the analysis provides two main results. No evidence is found for a ring-closure (ring current) effect for the sigma electron contributions or for the diamagnetic part of the pi electron contributions, whereas the ring closure effect on the perpendicular paramagnetic pi electron contributions is to reduce them to almost nothing, in accord with Pauling's assumptions. Therefore the conclusion is that the characteristic shielding anisotropy observed for aromatic protons is caused solely by an absence of paramagnetic pi electron currents, all other shielding contributions being sensitive to local topology but indifferent to aromatic character.