A DFT Study of Electronic Inductive and Resonance Effects of Substituents on Concerted Two-proton Coupled Electron Transfer between Catechol Derivatives and Superoxide

The development of biomimetic electron transfer catalysts based on proton-coupled electron transfer (PCET), which is characterized by quinone–hydroquinone π-conjugation, represent a promising approach for achieving highly efficient artificial energy conversion. Herein, I report a density functional theory (DFT)-based analysis of the electronic inductive (I) and resonance (R) effects of substituents on concerted two-proton-coupled electron transfer (2PCET) between benzene-1,2-diol (catechol) derivatives and the superoxide radical anion (O2•−). In this study, I investigated the relationship between the type and number of substituents and their effects on 2PCET using 12 catechol derivatives. Four types of substituents—methyl (+I, +R), chloro (−I, +R), methoxy (−I, +R), and cyano (−I, −R)—were selected in mono-, di-, tri-, and tetra-substituted forms to isolate and analyze their electronic effects without additional functionalities. Our DFT results confirmed that substituent effects selectively enhance either proton or electron transfer along a sequential PCET pathway. Further analysis revealed that the R effect is the primary driving force for concerted 2PCET, where an increasing number of methyl or chloro substituents promotes the reaction, whereas cyano substituents suppress it. The I and R effects influence the electronic properties of catechol molecule in proportion to the number of substituents. However, free energy calculations indicated kinetic and thermodynamic deviations, suggesting that the substituents directly affected the two hydroxyl groups—the reaction sites of 2PCET—as well as their solvation environment.