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 NONAROMATIC ANALOG

Shielding tensors of the protons inside and outside the aromatic 1,8-d idehydro-[14]-annulene ring, and of the corresponding protons in a clo sely similar open chain, are computed ab initio using the localized or bital/local origin (LORG) method. Shielding tensors for the carbon nuc lei and for carbon and hydrogen in benzene also are reported. The anis otropy of the aromatic proton shielding tensors, and the large differe nce between the shielding of the inner and outer protons in the aromat ic annulene system demonstrated by Sondheimer and commonly described a s a ring current effect, are reproduced well. Proton shieldings in the open chain fall in an olefinic range. Carbon shieldings in the two sy stems fall into aromatic and olefinic ranges. Diamagnetic and paramagn etic contributions from sigma and pi electrons are extracted in a cent re of mass (COM) gauge, and also the various contributions to the perp endicular shielding component are analysed in a COM gauge, allowing di rect contact to ring current models. The analysis reveals a topologica l effect, unaffected by ring closure, making pi as well as signal elec tron contributions to the diamagnetic shielding negative (positive) fo r protons on the convex (concave) side of the annulene or open chain, with a similar but oppositely signed switch for the paramagnetic contr ibutions. 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 par t 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. Therefo re the conclusion is that the characteristic shielding anisotropy obse rved for aromatic protons is caused solely by an absence of paramagnet ic pi electron currents, all other shielding contributions being sensi tive to local topology but indifferent to aromatic character.