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

Abstract

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 S0 and the lowest excited triplet T1 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 S0 and lowest excited triplet T1 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 T1 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.