Achieving Accurate Reduction Potential Predictions for Anthraquinones in Water and Aprotic Solvents: Effects of Inter- and Intramolecular H-Bonding and Ion Pairing

Abstract

In this combined computational and experimental study, specific chemical interactions affecting the prediction of one-electron and two-electron reduction potentials for anthraquinone derivatives are investigated. For 19 redox reactions in acidic aqueous solution, where AQ is reduced to hydroanthraquinone, density functional theory (DFT) with the polarizable continuum model (PCM) gives a mean absolute deviation (MAD) of 0.037 V for 16 species. DFT(PCM), however, highly overestimates three redox couples with a MAD of 0.194 V, which is almost 5 times that of the remaining 16. These three molecules have ether groups positioned for intramolecular hydrogen bonding that are not balanced with the intermolecular H-bonding of the solvent. This imbalanced description is corrected by quantum mechanics/molecular mechanics (QM/MM) simulations, which include explicit water molecules. The best theoretical estimations result in a good correlation with experiments, V(Theory) = 0.903V(Expt) + 0.007 with an R 2 value of 0.835 and an MAD of 0.033 V. In addition to the aqueous test set, 221 anthraquinone redox couples in aprotic solvent were studied. Five anthraquinone derivatives spanning a range of redox potentials were selected from this library, and their reduction potentials were measured by cyclic voltammetry. DFT(PCM) calculations predict the first reduction potential with high accuracy giving the linear relation, V(Theory) = 0.960V(Expt) − 0.049 with an R 2 value of 0.937 and an MAD of 0.051 V. This approach, however, significantly underestimates the second reduction potential, with an MAD of 0.329 V. It is shown herein that treatment of explicit ion-pair interactions between the anthraquinone derivatives and the cation of the supporting electrolyte is required for the accurate prediction of the second reduction potential. After the correction, V(Theory) = 1.045V(Expt) − 0.088 with an R 2 value 0.910 and an MAD value reduced by more than half to 0.145 V. Finally, molecular design principles are discussed that go beyond simple electron-donating and electron-withdrawing effects to lead to predictable and controllable reduction potentials.

Experimental and Theoretical Reduction Potentials of Some Biologically Active ortho-Carbonyl para-Quinones

“…Calculated second reduction potentials had R 2 = 0.462 at the BHandHLYP and M06-2x levels and R 2 = 0.517 at the PBE0 level. In previous work, the quality of the calculated second reduction potential of quinones was shown to be poor, which can be associated with the difficult treatment of the dianion species [ 55 ]. Specifically, the ion pair formation [ 56 ] between the anion species and the cation of support electrolite tetabutilammonium (TBA + ) can exert an influence on the formation of dianion species [ 57 ].…”

Electrochemistry of bi-redox ionic liquid from solution to bi-functional carbon surface

“…In this part, aqueous solution containing 0.1 mol.L -1 HTFSI as supporting electrolyte was used for the electrochemical characterization. Under the acidic condition, the ferrocene moiety undergoes one electron oxidation process (donor-group), while anthraquinone involve a 2-electron and 2-proton reduction process (acceptor-group) [33]. The HTFSI electrolyte was selected because it has the same anion as in the attached layer, which avoids the anion exchange during the electrochemical measurements.…”

“…Their half-wave potentials are observed at 0.55 V and -0.7 V/SCE and were attributed to the redox reaction of Fc and AQ moieties, respectively. The Fc and the AQ redox response, in non-protic solution, follow a one electron pathway with the generation of Fc + and the radical anion AQ •-[33,34]. Next, several CV's were performed at different scan rates, from 0.01 to 100 V.s -1(Fig.…”

Theoretical and Experimental Investigation of the Stability Limits of Quinones in Aqueous Media: Implications for Organic Aqueous Redox Flow Batteries

“…Other recent studies have also demonstrated capability of different levels of DFT (with and without calibration and/or explicit solvation) to estimate the reduction potentials of organic molecules, including quinones. 4,22,27,28 The calculated reaction energies for both reduction and gem-diol reactions were benchmarked against experimental reduction potentials and gem-diol hydration equilibrium constants (Tables S1 and S2 with water as the solvent. 29 The PBE/6-31G(d) optimizations were implemented using Ter-aChem.…”