Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

The downfield chem. shifts of arene hydrogens (delta 1H) are due only in part to the pi ring current contribution [sigma.dblvert.(pi)]; local framework effects are equally important. Neither proton chem. shifts nor even sigma.dblvert.(pi) tensor elements, per se, are reliable aromaticity indicators. Unsatd. polycyclic hydrocarbons with nonarom. quinoid structures have delta 1H and sigma.dblvert.(pi) values in the "arom. range". Conversely, numerous arom. protons, including those in five-membered ring heterocycles, resonate in the "nonarom. range". [on SciFinder (R)]