Acid/base and hydrogen bonding effects on the proton-coupled electron transfer of quinones and hydroquinones in acetonitrile: Mechanistic investigation by voltammetry, 1H NMR and computation

Abstract

This report seeks to address the role of hydrogen bonding with Brønsted acids and bases in proton-coupled electron transfer (PCET) as it pertains to concerted or stepwise pathways of quinone (Q) and hydroquinone (QH 2) electrochemistry. This study was performed using a series of techniques that included cyclic voltammetry (CV), digital simulations, computational chemistry and 1H NMR. Hydrogen bonding was inferred by a decrease in diffusion coefficient ( D) values measured using a pulsed gradient echo- (PGE-) 1H NMR technique. Changes of 40.8% and 37.9% in D values were only noted after the addition of two equivalents of acetate to 1,4-hydroquinone (1,4-QH 2) and catechol (1,2-QH 2), respectively. In contrast, the D values for the addition of selected amines (pyridine, N,N-diisopropylethylamine and triethylamine) changed only 3.2% on average. Quantum mechanical calculations were conducted to determine the p K a of all quinoid species to serve as a starting point for the determination of equilibrium constants in voltammetric simulations. Simulations indicate that 1,4-benzoquinone undergoes stepwise electron–proton transfer upon addition of acetic acid, N-ethyldiisopropylammonium perchlorate and pyridinium nitrate and were simulated without the presence of hydrogen bonds. The QH 2 compounds show stepwise proton–electron transfers after addition of the both the conjugate amines and acetate.