The protoisomerization (isomerization induced by protonation) mechanisms of indigo as well as indigo di- and monoimine derivatives were investigated using computational chemistry. Both density functional theory (M06-2X) and wave function theory (GMC-QDPT) methods were used to obtain reliable results. A solvation model (C-PCM with CH2Cl2 solvent) was employed to mimic the actual environment of the isomerization. The calculations reveal that the protoisomerizations of both the indigo and its imine derivatives are thermodynamically favorable. However, the energy barriers for rotating the double bonds in the derivatives are found to be lower than the one for indigo. More importantly, the imine derivatives undergo one-step isomerization, whereas a two-step process is predicted for indigo itself. The computational results provide insightful explanation for the different protoisomerization propensities of the parent indigo and its imine derivatives observed in experiment.

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Journal Journal of Physical Chemistry A
Hajjar, L. (Laurence), Hicks, R.G. (Robin G.), & Zeng, T. (2016). A Computational Study of the Protoisomerization of Indigo and Its Imine Derivatives. Journal of Physical Chemistry A, 120(38), 7569–7576. doi:10.1021/acs.jpca.6b07838