title: Downfield proton chemical shifts are not reliable aromaticity indicators.
abstract: [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".
authors.affiliation: Center for Computational Chemistry, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2525, USA.
authors.forename: Chaitanya S
authors.initials: CS
authors.lastname: Wannere
authors.forename: Clémence
authors.initials: C
authors.lastname: Corminboeuf
authors.forename: Wesley D
authors.initials: WD
authors.lastname: Allen
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer
authors.forename: Paul von Ragué
authors.initials: Pv
authors.lastname: Schleyer