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. 2019 Sep 6;123(38):8226–8233. doi: 10.1021/acs.jpca.9b06858

Scheme 2. Protonation on One of the Two Identical Oxygen Atoms in Indigo Allows for Mesomeric Structures Having a Single Bond Connecting the Two Subunits, Suggesting More Facile Trans-to-Cis Isomerization of the Protonated Species (Similar Resonance Structures Can Be Drawn for Isoindigo).

Scheme 2