Why Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

TLDR

Traces of magnetizability, magnetic shielding at hydrogen nuclei, and nucleus‑independent chemical shift are unreliable aromaticity quantifiers for planar conjugated hydrocarbons. A reliable aromaticity measure is given by out‑of‑plane tensor components, whose magnitude reflects pi‑ring currents. The study shows that nucleus‑independent chemical shift fails to capture aromaticity, as demonstrated for the illustrated molecule with a diatropic pi‑current, while confirming the ring‑current model’s validity. Structure details are provided in the text.

Abstract

Traces of magnetizability, traces of magnetic shielding at the hydrogen nuclei, and nucleus-independent chemical shift are not reliable aromaticity quantifiers for planar conjugated hydrocarbons. A measure of aromaticity is provided by the out-of-plane tensor components, whose magnitude is influenced by the pi-ring currents. The failure of nucleus-independent chemical shift in this regard was proved for the molecule shown in the abstract graphic, sustaining a diatropic pi-current. The validity of the ring-current model is reaffirmed. [structure: see text]