Origin of the Individual Basicity of Corrole NH-Tautomers: A Quantum Chemical Study on Molecular Structure and Dynamics, Kinetics, and Thermodynamics

Abstract

Free-base corroles exist as individual NH-tautomers that may differ in their spectral and chemical properties. The present paper focuses on the origin of the basicity difference between two AB(2)-pyrimidinylcorrole NH-tautomers, which has been tentatively attributed to differences in the weak out-of-plane distortions of the pyrrolenic ring between two NH-tautomers. Using DFT-geometry optimizations, we show that the pyrroles involved in the NH-tautomerization process are approximately in-plane, whereas the other two pyrroles are tilted out-of-plane in opposite directions. Alternative out-of-plane distortion patterns play a minor role, as revealed by ab initio molecular dynamics simulations. Given that the protonated corrole is a unique species, the energy difference between the two NH-tautomers equals the difference in protonation driving force between them. This energy difference increases with improved theoretical level of accounting for intermolecular interactions and dielectric screening of surface charges. The different charge distributions of the two NH-tautomers result in electrostatic potential distributions that effect a larger proton attraction in the case of the T1 tautomer than in the case of the T2 tautomer. In summary, our quantum chemical results show clearly a higher basicity of the T1 tautomer as compared to the T2 tautomer: The previously assumed pronounced out-of-plane tilt of the Ti-nonprotonated nitrogen is verified by ab initio molecular dynamics simulations. Together with analysis of the electrostatic potential distribution we show that the nonprotonated nitrogen is not only tilted stronger but also significantly more accessible for protons in the case of Ti as compared to T2. Additionally, the thermodynamic basicity is higher for Ti than for T2.