Understanding proton magnetic shielding in the benzene molecule

According to the ring current model quoted in textbooks of nuclear magnetic resonance spectroscopy, the downfield chemical shifts of hydrogen nuclei in aromatic molecules is due to intense delocalized currents induced in the pi-electron cloud by a magnetic field at right angles to the molecular plane. By using the Biot-Savart law, it was found that the delocalized ring currents deshield the out-of-plane component of proton shielding tensor via an essentially local mechanism taking place in the close vicinity of protons. The pi ring currents over distant carbons shield the protons. pi and sigma electrons deshield benzene protons via different mechanisms clearly observed in plots of the shielding density function defined in the text. These results provide a novel interpretation of the phenomenology and suggest that the familiar model for interpreting chemical shifts of aromatics should be revised. (C) 2004 Published by Elsevier B.V.