A Model for Characterization of Successive Hydrogen Bonding Interactions with Electrochemically Generated Charged Species. The Quinone Electroreduction in the Presence of Donor Protons

Abstract

In this work, it is considered that charged molecules, such as the semiquinone (Q .À ) and the benzoquinone dianion (Q 2À ), interact with weak donor protons (DH) via hydrogen bonding through consecutive association steps which depend on the concentration of DH. On the basis of a reversible voltammetric behavior, a model is presented for determining integer numbers of associated molecules of DH and the thermodynamic constants (b i ) related with each association equilibrium. The electrochemical reduction of 1,4-benzoquinone (Q) in the presence of different aliphatic alcohols and diols, in dimethylsulfoxide (DMSO), illustrates the utility of the model. Based on analysis of half-wave potential variation according to the DH concentration, the association constants were determined for the Q .À and Q 2À complexes with the different alcohols studied and mole-fraction distribution diagrams were constructed. These diagrams explain the fractional values, produced using the typical model, widely reported in the literature, for study the hydrogen bonding between the electrochemically generated charged species with donor protons.

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

“…[1][2][3][4][5] Hydrogen-bonding and proton transfer are of key importance for controlling the reduction potential and reaction path. [5][6][7][8][9][10][11][12][13][14][15] In well-buffered aqueous media, quinone-hydroquinone couples provide familiar, reversible two-electron redox systems in which half-wave reduction potentials vary with the pH in a straightforward Nernstian manner. 16 On the other hand, in dry, neutral aprotic media, quinones typically show two cathodic waves corresponding to reversible formation of the radical anion and the dianion.…”

“…It is well documented that thermodynamic stabilization of these ions by hydrogen donors significantly affects the electrochemical behavior of quinones. [6][7][8][9][10][11][12][13][14][15] Clearly different types of behavior are observed for the electrochemistry of quinones in the presence of various kinds of additives. Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes.…”

“…Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes. [6][7][8][9][10][11][12][13][14][15]17 Much attention has been paid to the characteristics of the redox behavior of quinones by hydrogen-bonding with weakly interacting proton donors, such as CH3OH and C2H5OH, [6][7][8][9][10][11][12][13][14][15] which is implicated in controlling both intra-and intermolecular structures in biological systems and biological function as an active site of quinoenzymes. [3][4][5] Such studies demonstrate that the first and second reduction waves of quinones move towards less-negative potential values as the concentration of the proton donor is increased, with no loss of reversibility.…”

Redox chemistry of coenzyme Q—a short overview of the voltammetric features

“…However, the authors did not pay more efforts to clarify this phenomenon. More relevant aspects related to the coenzyme Q's redox chemistry in aqueous media can also be found in [34][35][36][37][38][39][40].…”

“…Keeping in mind all the possible redox transformations in which CoQs are undergoing in both aprotic or aqueous media [2,9,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47], several relevant theoretical models can be applied to describe their voltammetric behavior. In the last few years, several groups worked on the development of mathematical models of coupled redox reactions under conditions of cyclic and square-wave voltammetry.…”