PNNL

Abstract

Quinones participate in diverse electron transfer and proton-coupled electron transfer processes in chemistry and biology. An experimental study of common quinones reveals a non-linear correlation between the 1 e– and 2 e–/2 H+ reduction potentials. This unexpected observation prompted a computational study of 128 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 halogen substituents, charged substituents, intramolecular hydrogen bonding in the hydroquinone, 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.