No more doubts about four NH-tautomers formation: 5,10-diaryl-corroles case theoretical study

The possibility of four NH-tautomers formation in asymmetrically substituted 5,10-diaryl-corrole free bases has been studied. The molecular conformation optimization has been carried out for four NH-tautomers in both the ground singlet S 0 and the lowest excited triplet T 1 states with the density functional theory, and the sets of integral and local structure parameters have been evaluated. The results unambiguously demonstrate that each of the four NH-tautomers has its unique molecular conformation. However, the ground state energies of the NH-tautomers were found to differ strongly as a function of the rotation degree of freedom of aryl substituents. Almost the same ground state energies of all NH-tautomers have been found for the substitution with sterically constrained mesityl groups. In contrast, a large energy gap has been revealed between two pairs of NH-tautomers in case of substitution with freely rotating phenyl groups. Therefore, in the former case, the relative populations of all four NH-tautomers are of the same order of magnitude and all four NH-tautomers coexist, but only two NH-tautomers are expected in the latter at room temperatures. The aromaticity degree and the [Formula: see text]-conjugation pathways have been evaluated for each of four NH-tautomers in both ground S 0 and lowest excited triplet T 1 states, and similar behavior was found for the pairs of NH-tautomers those protons are localized either in the dipyrromethene fragment or in the dipyrrole fragment of the macrocycle. All NH-tautomers were found to be aromatic in the ground state, but the inversion of aromaticity takes place in the lowest triplet T 1 state for all of them. Finally, the energies of the four frontier molecular orbitals have been compared and analyzed in the framework of possible spectral differences between the NH-tautomers.