Figure 4.

Computed TS barriers for trans-to-cis isomerization and optimized TS geometries of protonated isoindigo (a) and indigo (b). Values given for the TS barriers are in kJ mol^–1 relative to the trans-form of the protonated molecules. For the protonated systems, unlike for the neutral molecules, cis is the minimum-energy isomer. Although both cis-isomers are positioned at the same energy in the plot, protonated cis-isoindigo is 46.3 kJ mol^–1 more stable than protonated cis-indigo as a consequence of the stronger shared-proton interaction induced by the smaller OO distance (see structures in Figures 1 and 2).