Sequential Electron Proton Transfer Research Articles

Theoretical Study of Radical Scavenging Antioxidant Activity of Coumarin Derivatives in Licorice Using DFT

Nowdays, much research is focused on exploring antioxidant activity based on natural compounds. Licorice, a traditional natural product rich in antioxidant components, has been widely used in clinical medicine. However, there is a lack of theoretical studies on the active components in licorice. In this context, the antioxidant activity of several common active components of coumarin derivatives in licorice was investigated using a density flooding theory (DFT) approach. The free radical scavenging ability of the compounds was achieved by hydrogen atom transfer (HAT), sequential electron transfer proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET) mechanisms. The effect of solvent on antioxidant activity was also investigated. It was found that in the gas and benzene phases, the compounds studied preferred to undergo the HAT mechanism, while SPLET was favoured in polar solvents. In addition to the above mechanism, the HOMO and LUMO energies of the compounds as well as molecular descriptors and electrostatic potential maps were calculated and applied to predict the antioxidant activity of the compounds. The above studies allow a better understanding of the mechanism of free radical scavenging activity of coumarin derivatives from licorice.

Four Generations of Volcano Plots for the Oxygen Evolution Reaction: Beyond Proton-Coupled Electron Transfer Steps?

ConspectusDue to its importance for electrolyzers or metal-air batteries for energy conversion or storage, there is huge interest in the development of high-performance materials for the oxygen evolution reaction (OER). Theoretical investigations have aided the search for active material motifs through the construction of volcano plots for the kinetically sluggish OER, which involves the transfer of four proton-electron pairs to form a single oxygen molecule. The theory-driven volcano approach has gained unprecedented popularity in the catalysis and energy communities, largely due to its simplicity, as adsorption free energies can be used to approximate the electrocatalytic activity by heuristic descriptors.In the last two decades, the binding-energy-based volcano method has witnessed a renaissance with special concepts being developed to incorporate missing factors into the analysis. To this end, this Account summarizes and discusses the different generations of volcano plots for the example of the OER. While first-generation methods relied on the assessment of the thermodynamic information for the OER reaction intermediates by means of scaling relations, the second and third generations developed strategies to include overpotential and kinetic effects into the analysis of activity trends. Finally, the fourth generation of volcano approaches allowed the incorporation of various mechanistic pathways into the volcano methodology, thus paving the path toward data- and mechanistic-driven volcano plots in electrocatalysis.Although the concept of volcano plots has been significantly expanded in recent years, further research activities are discussed by challenging one of the main paradigms of the volcano concept. To date, the evaluation of activity trends relies on the assumption of proton-coupled electron transfer steps (CPET), even though there is experimental evidence of sequential proton-electron transfer (SPET) steps. While the computational assessment of SPET for solid-state electrodes is ambitious, it is strongly suggested to comprehend their importance in energy conversion and storage processes, including the OER. This can be achieved by knowledge transfer from homogeneous to heterogeneous electrocatalysis and by focusing on the material class of single-atom catalysts in which the active center is well defined. The derived concept of how to analyze the importance of SPET for mechanistic pathways in the OER over solid-state electrodes could further shape our understanding of the proton-electron transfer steps at electrified solid/liquid interfaces, which is crucial for further progress toward sustainable energy and climate neutrality.

Transition from Sequential to Concerted Proton-Coupled Electron Transfer of Water Oxidation on Semiconductor Photoanodes.

Accelerating proton transfer has been demonstrated as key to boosting water oxidation on semiconductor photoanodes. Herein, we study proton-coupled electron transfer (PCET) of water oxidation on five typical photoanodes [i.e., α-Fe2O3, BiVO4, TiO2, plasmonic Au/TiO2, and nickel-iron oxyhydroxide (Ni1-xFexOOH)-modified silicon (Si)] by combining the rate law analysis of H2O molecules with the H/D kinetic isotope effect (KIE) and operando spectroscopic studies. An unexpected and universal half-order kinetics is observed for the rate law analysis of H2O, referring to a sequential proton-electron transfer pathway, which is the rate-limiting factor that causes the sluggish water oxidation performance. Surface modification of the Ni1-xFexOOH electrocatalyst is observed to break this limitation and exhibits a normal first-order kinetics accompanied by much enhanced H/D KIE values, facilitating the turnover frequency of water oxidation by 1 order of magnitude. It is the first time that Ni1-xFexOOH is found to be a PCET modulator. The rate law analysis illustrates an effective strategy for modulating PCET kinetics of water oxidation on semiconductor surfaces.

Efficient O-O Coupling at Catalytic Interface to Assist Kinetics Optimization on Concerted and Sequential Proton-Electron Transfer for Water Oxidation.

A catalyst kinetics optimization strategy based on tuning active site intermediates adsorption is proposed. Construction of the M-OOH on the catalytic site before the rate-determining step (RDS) is considered a central issue in the strategy, which can optimize the overall catalytic kinetics by avoiding competition from other reaction intermediates on the active site. Herein, the kinetic energy barrier of the O-O coupling for as-prepared sulfated Co-NiFe-LDH nanosheets is significantly reduced, resulting in the formation of M-OOH on the active site at low overpotential, which is directly confirmed by in situ Raman and charge transfer fitting results. Moreover, catalysts constructed from active sites of highly efficient intermediates make a reliable model for studying the mechanism of the OER in proton transfer restriction. In weakly alkaline environments, a sequential proton-electron transfer (SPET) mechanism replaces the concerted proton-electron transfer (CPET) mechanism, and the proton transfer step becomes the RDS; high-speed consumption of reaction intermediates (M-OOH) induces sulfated Co-NiFe-LDH to exhibit excellent kinetics.

Effect of Explicit Water Molecules on the Electrochemical Hydrogenation of CO2 on Sn(112)

Water is typically treated as an implicit solvent in modeling electrochemical reactions in an aqueous environment. Such treatment may not be adequate, as a series of concerted or sequential proton-electron transfer steps that explicitly involve water molecules are likely to play important roles in a reaction, such as the electrochemical hydrogenation of CO2. Herein, we use the electrochemical hydrogenation of CO2 on the Sn(112) surface as a model, and employ the density functional theory (DFT) method to examine the effect of up to 12 explicit water molecules on the stability of the hydrogenation intermediates. Our results show that six water molecules are needed to account for the local interaction between an intermediate and the water solvent. Furthermore, the hydrogen bonding interaction between the explicit water molecules and intermediates causes a significant stabilization to the O-containing intermediates, such as the HCOO* and CHO* + OH* species. The inclusion of explicit water molecules also altered the prediction of the potential-limiting step from the formation of H* atoms without the explicit water molecules to the formation of H2COO* in the presence of water molecules and increased selectivity towards methane. This work provides useful insights into the electrocatalytic hydrogenation of CO2, emphasizing the importance of including explicit water molecules to account for the hydrogen bonding interaction between solvent water molecules and the reaction intermediates.

Inverted Region in Electrochemical Reduction of CO2 Induced by Potential-Dependent Pauli Repulsion.

Electrochemical CO2 reduction reaction (eCO2RR) is of great significance to energy and environmental engineering, while fundamental questions remain regarding its mechanisms. Herein, we formulate a fundamental understanding of the interplay between the applied potential (U) and kinetics of CO2 activation in eCO2RR on Cu surfaces. We find that the nature of the CO2 activation mechanism in eCO2RR varies with U, and it is the sequential electron-proton transfer (SEPT) mechanism dominant at the working U but switched to the concerted proton-electron transfer (CPET) mechanism at highly negative U. We then identify that the barrier of the electron-transfer step in the SEPT mechanism exhibits an inverted region as U decreases, which originates from the rapidly rising Pauli repulsion in the physisorption of CO2 with decreasing U. We further demonstrate catalyst designs that effectively suppress the adverse effect of Pauli repulsion. This fundamental understanding may be general for the electrochemical reduction reactions of closed-shell molecules.

Visible light-mediated C (sp3)-H bond functionalization inside an all-organic nanocavity.

Ultrafast C-H bond activation and functionalization in confinement using visible light will enable engineering chemical reactions with extraordinary speed and selectivity. To provide a transition metal-free route, here we demonstrate C-H bond activation reactions on poly-aromatic hydrocarbons (PAH) in all-organic cationic nanocage ExBox4+ for the first time. Visible light excitation in the host-guest charge transfer (CT) state allows the formation of oxidized photoproducts with high selectivity. Mechanistic understanding of this CT-mediated photoreaction using femtosecond broadband transient absorption revealed a few ∼100 ps timescale for C-H bond breaking on the attached -CH3 group via sequential electron transfer and proton transfer steps. We envision that our photosensitizer-free method will open up new avenues to pursue organic reactions using cavities that could serve both as photoredox catalysts and hosts for reactive reaction intermediates.

Two Theorems and Important Insight on How the Preferred Mechanism of Free Radical Scavenging Cannot Be Settled. Comment on Pandithavidana, D.R.; Jayawardana, S.B. Comparative Study of Antioxidant Potential of Selected Dietary Vitamins; Computational Insights. Molecules 2019, 24, 1646.

Totally ignoring that the five enthalpies of reaction-bond dissociation enthalpy (BDE), adiabatic ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA), and electron transfer enthalpy (ETE)-characterizing the three free radical scavenging mechanisms-direct hydrogen atom transfer (HAT), sequential electron transfer proton transfer (SET-PT), and stepwise proton loss electron transfer (SPLET)-are not independent of each other, a recent publication on the antioxidant activity of dietary vitamins compared various vitamins and "found" different quantities, which should be strictly equal by virtue of energy conservation. Aiming to clarify this point, as well as to avoid such mistakes in future studies and to unravel errors in the previous literature, in the present paper we formulate two theorems that any sound results on antioxidation should obey. The first theorem states that the sums of the enthalpies characterizing the individual steps of SET-PT and SPLET are equal: IP+PDE = PA+ETE (=H2). This is a mathematical identity emerging from the fact that both the reactants and the final products of SET-PT and SPLET are chemically identical. The second theorem, which is also a mathematical identity, states that H2 - BDE = IPH > 0, where IPH is the ionization potential of the H-atom in the medium (e.g., gas or solvent) considered. Due to their general character, these theorems may/should serve as necessary sanity tests for any results on antioxidant activity, whatever the method employed in their derivation. From a more general perspective, they should represent a serious word of caution regarding attempts to assign the preferred free radical scavenging pathway based merely on thermochemical descriptors.

Regulation of the electrocatalytic nitrogen cycle based on sequential proton–electron transfer

Regulation of the electrocatalytic nitrogen cycle based on sequential proton–electron transfer

Chemical and electrochemical water oxidation catalyzed by heteroleptic Ru(III) complexes of anionic 2,6 pyridine dicarboxylate ligand: Experimental and theoretical study

Herein we report two new mononuclear heteroleptic Ruthenium complexes, 1 and 2 with anionic N, O donor pyridine dicarboxylate (pdc2-) ligand along with neutral bipyridine moieties. The general formula of the complexes are [RuIII(pdc2-)(L1)Cl](1), [RuIII(pdc2-)(L2)Cl](2) (Where L1 = bipyridine, L2 = 4,4′-dimethylbipyridine). Synthesis, characterization and water oxidation catalytic properties of both complexes were investigated. A combined theoretical (DFT) and experimental study were carried out to investigate the catalytic (in presence of CAN) and electrocatalytic activity of 1 and 2 for water oxidation in aqueous solution at pH ∼ 1. During electrocatalysis, the catalytic activity wave has been detected in acetonitrile/water 1:9 (v/v) solution in the range 1.39–1.41 V vs SCE. pH based electrochemistry reveals that, water oxidation (at pH = 1) takes place via sequential proton-electron transfer process to form a RuV = O moiety, which oxidises water through I2M pathway. For water oxidation by CAN, both the molecules show appreciably high TOF values 3.47971 x10-4 s−1 and 1.183 × 10–3 s−1 for 1 and 2 respectively in comparison to similar molecules [1,2].

Phytate Coordination-Enhanced Electrocatalytic Activity of Copper for Nitroarene Hydrogenation through Concerted Proton-Coupled Electron Transfer.

Coupling acid-electrolyte proton exchange membrane fuel cells for electricity generation and cathodic hydrogenation for valuable chemical production shows great potential in energy and chemical industry. The key for this promising approach is the identification of cathode electrocatalysts with acid resistance, high activity, and low fabrication cost for practical applications. Among various promising cathodic candidates for this integrative approach, the easily available and cheap Cu suffers from low acidic hydrogenation activity due to kinetically arduous proton adsorption/activation. Inspired by the kinetic advantages of the concerted proton-coupled electron transfer (CPET) over the sequential proton-electron transfer process, herein, we use phytate coordination on Cu surface to overcome the kinetic bottleneck for proton adsorption/activation through the CPET pathway in an acidic half-cell setup; this leads to 1 order of magnitude activity enhancement (36.94-fold) for nitrobenzene hydrogenation. Mechanistic analysis confirms that phytate, as proton acceptor, induces the CPET process and overcomes the above kinetic limitations by tuning the d-band center and concentrating protons on the Cu surface. Consequently, the CPET process facilitates the formation of active hydrogen intermediates for efficient cathodic hydrogenation. This work provides a promising approach to integrate electricity generation and chemical production.

Theoretical insights into the radical scavenging activity of glipizide: DFT and molecular docking studies

Glipizide is an N-sulfonylurea compound used in the treatment of hyperglycemia in patients with type 2 diabetes mellitus. In the present study, DFT-based computational methods and molecular docking studies have been performed to systematically evaluate the radical scavenger behavior of the title molecule. Structural characteristics such as molecular descriptors, frontier molecular orbitals, molecular potential mapping, and Mulliken charge population have been investigated. Thermodynamic parameters like proton affinity (PA), ionization potential (IP), bond dissociation energy (BDE), electron transfer enthalpy (ETE), and proton dissociation enthalpy (PDE) related to three antiradical mechanisms namely hydrogen atom transfer (HAT), sequential electron transfer proton transfer (SETPT) and sequential proton loss electron transfer (SPLET) have been studied. Also, molecular docking studies have been carried out to have a theoretical understanding of the molecular mechanism and for the elucidation of binding mode/modes of a compound targeted through non-covalent interactions. The obtained results are of great significance in better understanding the reaction mechanism of the title molecule and opens a new perspective for the design of potent antioxidant agents.

Mass spectrometric observation on free radicals during electrooxidation of dopamine

It has been challenging to directly observe the o-semiquinone radicals and transient intermediates produced during the oxidation of dopamine (DA). To achieve this goal, we developed an electrochemistry-neutral reionization-mass spectrometry (EC-NR-MS) technique for online studying the electrooxidation process of DA. The EC-NR device mainly composed by a self-designed EC flow cell, a sonic spray ionization source, a heating tube, an ion deflector and an electrospray ionization source. By precisely controlling the oxidation potential at 0.55 V, a series of reaction products include o-quinone (DAQ), Leukodopaminochrome (LDAC), Dopaminochrome (DAC), 5,6-dihydroxyindole (DHI) and DA dimer clearly appeared in the MS spectrum. Based on the ion deflector of EC-NR setup, the neutral o-semiquinone radical DA● and neutral Leukodopaminochrome radical LDAC● were successfully extracted from these ionic products and allowed to be detected by MS. Such finding was further confirmed by spin trapping experiment. The formation of o-semiquinone radical anion (DA●-) has been identified as its metal complex with Zn2+. These results provide conclusive evidence for the DA oxidation mechanism, which consists of proton coupled electron transfer and sequential proton-electron transfer steps.

Biological Evaluation of 4-(1H-triazol-1-yl)benzoic Acid Hybrids as Antioxidant Agents: In Vitro Screening and DFT Study

Fourteen triazole benzoic acid hybrids were previously characterized. This work aimed to screen their in vitro antioxidant activity using different assays, i.e., DPPH (1,1-diphenyl-1-picrylhydrazyl), reducing the power capability, FRAP (ferric reducing antioxidants power) and ABTS (2,2′-azino-bis(3-ethylben zothiazoline-6-sulfonate) radical scavenging. The 14 compounds showed antioxidant properties in relation to standard BHA (butylated hydroxylanisole) and Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid). Higher antioxidant activity was observed by the parent (1) at a concentration of 100 µg/mL (89.95 ± 0.34 and 88.59 ± 0.13%) when tested by DPPH and ABTS methods in relation to BHA at 100 µg/mL (95.02 ± 0.74 and 96.18 ± 0.33%). The parent (2) demonstrated remarkable scavenging activity when tested by ABTS (62.00 ± 0.24%), however, 3 was less active (29.98 ± 0.13%). Compounds 5, 6, 9, and 11 exhibited good scavenging activity compared to 1. DFT studies were performed using the B3LYP/6-311++g (2d,2p) level of theory to evaluate different antioxidant descriptors for the targets. Three antioxidant mechanisms, i.e., hydrogen atom transfer (HAT), sequential electron transfer proton transfer (SETPT) and sequential proton loss electron transfer (SPLET) were suggested to describe the antioxidant properties of 1–14. Out of the 14 triazole benzoic acid hybrids, 5, 9, 6, and 11 showed some good theoretical results, which were in agreement with some experimental outcomes. Based on the computed (PA and ETE) and (BDE and IP) values in (SPLET) and (HAT and SETPT) mechanisms, respectively, compound 9 emerged has having good antioxidant activity.

A detailed DFT-based study of the free radical scavenging activity and mechanism of daphnetin in physiological environments

Daphnetin, a biologically active coumarin derivative found in plants of the genus Daphne, is a potent antioxidant phenolic compound. The present work describes the mechanisms and kinetics of the HO, NO, HOO, and NO2 scavenging activities of daphnetin in physiological environments using quantum chemistry calculations. The main antiradical mechanisms have been studied: formal hydrogen transfer (FHT), sequential electron transfer proton transfer (SETPT), sequential proton loss electron transfer (SPLET), and radical adduct formation (RAF). Besides its good HO scavenging activity in physiological environments, daphnetin is expected to exhibit good HOO and NO2 scavenging activities in water with koverall = 1.51 × 107 and 4.79 × 108 M−1s−1, respectively. The FHT mechanism decides the HO scavenging activity in aqueous solution, as well as HO, HOO, and NO2 scavenging activities in lipid media, while SPLET is the primary mechanism in water for HOO and NO2 scavenging activities. The theoretical predictions were found to be in good agreement with the available experimental data, which supports the reliability of the calculations.

Are thymol, rosefuran, terpinolene and umbelliferone good scavengers of peroxyl radicals?

DFT-based computational calculations have been used to investigate the hydroperoxyl radical scavenging activity of four essential oil constituents namely thymol (Thy), rosefuran (Ros), terpinolene (Ter), and umbelliferone (Umb). Different reaction mechanisms including formal hydrogen transfer (FHT), radical adduct formation (RAF), sequential proton loss electron transfer (SPLET), and sequential electron transfer proton transfer (SETPT) have been examined in the gas phase and physiological environments. It was found that the HOO radical scavenging activity of these compounds is strongly influenced by the environment, which becomes more important in water than pentyl ethanoate. According to the overall reaction rate constants, the phenolic compounds Thy and Umb are predicted to exhibit excellent activity in aqueous solution. Umb with an overall rate constant of 1.44 × 108M−1s−1 at physiological pH is among the best HOO radical scavengers in water with activity comparable to that of caffeic acid, higher than those of ascorbic acid, guaiacol and eugenol, and much higher than that of Trolox.

Selective Electrochemical Alkaline Seawater Oxidation Catalyzed by Cobalt Carbonate Hydroxide Nanorod Arrays with Sequential Proton-Electron Transfer Properties

Seawater oxygen evolution is one of the promising energy conversion technologies for large-scale renewable energy storage. It requires efficient catalysts to accelerate the oxygen evolution reaction (OER) for sustained water oxidation, avoiding chlorine evolution under acidic conditions or hypochlorite formation in alkaline solutions. Conventional metal oxide-based OER catalysts follow the adsorbate evolution mechanism that involves concerted proton-electron transfer steps at the active sites. Thus, on the scale of reversible hydrogen electrode, their catalytic activity is independent of the pH of electrolytes. In the present study, nanostructured cobalt carbonate hydroxide (CoCH) with sequential proton-electron transfer properties was tested as a catalyst for seawater oxygen evolution. CoCH exhibited pH-dependent water oxidation activities, thereby providing larger potential and current operating windows for selective water oxidation compared to the catalysts with pH-independent OER activities. The operating window can be further expanded by increasing the pH of the electrolyte.

Atomic-scale evidence for highly selective electrocatalytic N−N coupling on metallic MoS2

Molybdenum sulfide (MoS2) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS2 significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS2 has a protonation site with a pKa of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS2 and induces sequential proton-electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS2, expanding the application of TMDs to selective electrocatalysis.

Developing micro-kinetic model for electrocatalytic reduction of carbon dioxide on copper electrode

A micro-kinetic model combining electrochemical rate theory and first-principles simulation is developed to study the influences of solution pH and electrode potential on reaction rate, reaction pathways, and product distribution of electrocatalytic CO2 conversion. Two critical issues involved in electrochemical reaction mechanism are investigated: 1) competing concerted and sequential proton-electron transfer pathways, 2) competing thermodynamics-controlled and kinetics-controlled pathways. Our results show that the electrochemical reduction of CO2 to CO and HCOOH adopts a thermodynamics-controlled CPET mechanism at low pH, while follows a kinetics-controlled SPET mechanism at high pH. The electrocatalytic activity and selectivity can be effectively modulated by manipulating of solution pH and electrode potential. It is demonstrated that HCOOH is the main product at low overpotential while CO becomes the main product at high overpotential. In addition, increasing pH is conducive to improving the Faradic efficiency of HCOOH production and suppressing the hydrogen evolution reaction.

Combined experimental and theoretical studies of the structure-antiradical activity relationship of heterocyclic hydrazone compounds

The present paper describes combined experimental and theoretical studies of twelve heterocyclic compounds bearing hydrazone functional group (1–12). Experimentally, the free radical scavenging activity of all the compounds have been investigated using DPPH, ABTS, and DMSO alkaline assays. The obtained results showed that these compounds are potent antiradical agents. The best results have been obtained with the imidazole derivatives 5 and 12, which showed higher antiradical activity than the BHT standard in DPPH and ABTS assays. Theoretical calculations, by means of DFT/B3LYP approach, were carried out in order to explain the experimental results. Some structural characteristics such as molecular descriptors, highest occupied molecular orbital, and molecular electrostatic potential mapping have been computed. The mechanisms, HAT (hydrogen atom transfer), SETPT (sequential electron transfer proton transfer), and SPLET (sequential proton loss electron transfer) have been studied and discussed. In addition, the reactions toward HO•, MeO•, HOO•, and MeOO• radicals have been investigated. As results, the radical scavenger properties of the studied compounds were confirmed. The HAT was found to be the thermodynamically dominant mechanism in the gas-phase for all the studied compounds, whereas the SPLET mechanism is the thermodynamically favorable pathway in ethanol. A good correlation with DPPH and ABTS assays was found between experimental and theoretical results.