Quinone 1 e^– and 2 e^–/2 H⁺ Reduction Potentials: Identification and Analysis of Deviations from Systematic Scaling Relationships

Quinones participate in diverse electron transfer and proton-coupled electron transfer processes in chemistry and biology. To understand the relationship between these redox processes, an experimental study was carried out to probe the 1 e– and 2 e–/​2 H+ reduction potentials of a number of common quinones. The results reveal a non-linear correlation between the 1 e– and 2 e–/​2 H+ reduction potentials. This unexpected observation prompted a computational study of 134 different quinones, probing their 1 e– reduction potentials, pKa values, and 2 e–/​2 H+ reduction potentials. The density functional theory calculations reveal an approximately linear correlation between these three properties and an effective Hammett constant associated with the quinone substituent(s). However, deviations from this linear scaling relationship are evident for quinones that feature intramolecular hydrogen bonding, halogen substituents, charged substituents, and/or sterically bulky substituents. These results, particularly the different substituent effects on the 1 e– versus 2 e–/​2 H+ reduction potentials, have important implications for designing quinones with tailored redox properties.醌类（quinones）广泛参与化学与生物学领域内各类电子转移及质子耦合电子转移过程。为明晰此类氧化还原过程间的关联机制，本研究通过实验测定了多种常见醌类的单电子（1 e–）与两电子/两质子（2 e–/2 H+）还原电势。实验结果表明，单电子还原电势与两电子/两质子还原电势之间存在非线性相关关系。这一意外发现促使研究团队针对134种不同醌类开展计算研究，测定其单电子还原电势、pKa值以及两电子/两质子还原电势。密度泛函理论（density functional theory）计算结果显示，上述三类性质与醌类取代基对应的有效哈米特常数（Hammett constant）之间存在近似线性相关关系。但对于带有分子内氢键、卤素取代基、带电取代基或空间位阻型取代基的醌类而言，其数据明显偏离该线性标度关系。本研究结果，尤其是取代基对单电子还原电势与两电子/两质子还原电势的差异化调控效应，对于设计具备定制化氧化还原特性的醌类具有重要指导价值。