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, pK_a 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.