Downfield proton chemical shifts are not reliable aromaticity indicators.
[structure: see text] The downfield chemical shifts of arene hydrogens (delta (1)H) are due only in part to the pi ring current contribution [sigma(//)(pi)]; local framework effects are equally important. Neither proton chemical shifts nor even sigma(||)(pi) tensor elements, per se, are reliable aromaticity indicators. Unsaturated polycyclic hydrocarbons with nonaromatic quinoid structures have delta (1)H and sigma(//)(pi) values in the "aromatic range". Conversely, numerous aromatic protons, including those in five-membered ring heterocycles, resonate in the "nonaromatic range".
Wannere
Chaitanya S
CS
Center for Computational Chemistry, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2525, USA.
Corminboeuf
Clémence
C
Allen
Wesley D
WD
Schaefer
Henry F
HF
Schleyer
Paul von Ragué
Pv