Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

The kinetics of the p-benzoquinone/hydroquinone Q/QH₂ couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q^?, k_h3=1/6×10^?3 cm s^?1; QH^.QH^?, k_h5=0.11 cm s^?1; QH⁺QH^., k_h2?160 cm s^?1; k_h4 for the reaction QH₂^+.QH₂ is in the range 0.5–4 cm s^?1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H⁺e^?H⁺e^?, e^?H⁺H⁺e^?, and e^?H⁺e^?H⁺ (reverse sequence during the oxidation).     

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^? process, while oxygen can be fully reduced to water by a 4 e^?/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^?. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Cu(II) and Ni(II)‐1,10‐phenanthroline‐ 5,6‐dione‐amino acid ternary complexes exhibiting pH‐sensitive redox properties

Syntheses, and electrochemical properties of two novel complexes, [Cu(phendio)(L ‐Phe)(H₂O)](ClO₄) ·H₂O (1) and [Ni(phendio)(Gly)(H₂O)](ClO₄)·H₂O (2) (where phendio = 1,10‐phenanthroline‐5,6‐dione, L ‐Phe = L ‐phenylalanine, Gly = glycine), are reported. Single‐crystal X‐ray diffraction results of (1) suggest that this complex structure belongs to the orthorhombic crystal system. The electrochemical properties of free phendio and these complexes in phosphate buffer solutions in a pH range between 2 and 9 have been investigated using cyclic voltammetry. The redox potential of these compounds is strongly dependent on the proton concentration in the range of ? 0.3–0.4 V vs SCE (saturated calomel reference electrode). Phendiol reacts by the reduction of the quinone species to the semiquinone anion followed by reduction to the fully reduced dianion. At pH lower than 4 and higher than 4, reduction of phendio proceeds via 2e^?/3H⁺ and 2e^?/2H⁺ processes. For complexes (1) and (2), being modulated by the coordinated amino acid, the reduction of the phendio ligand proceeds via 2e^?/2H⁺ and 2e^?/H⁺ processes, respectively. Copyright © 2006 John Wiley & Sons, Ltd.     

Oxygen evolution on SrFeO3 electrode

Oxygen evolution reactions on SrFeO₃ were investigated in alkaline and acidic solutions. It was found that the catalytic activity for the oxygen evolution reaction in the alkaline solution is high. The following reaction steps (V)+Fe+2H₂O→(O)+FeOH₂+2H⁺+2e^? in acidic solution and FeOH+OH^?→FeO^?+H₂O in alkaline solution are presumed to be rate-controlling in the anodic evolution of oxygen on SrFeO₃ electrode, where (V) denotes oxygen vacancy on the electrode surface. The reaction mechanism and the catalytic property are discussed in connection to the band structure of the oxide.     

Ground state for two‐electron and electron‐muon three‐body atomic systems

In this article, the angular correlated configuration interaction method previously introduced by some of the authors is extended to three‐body atomic systems with general masses. A recently proposed angularly correlated basis set is used to construct ground state wave functions which: (i) satisfy exactly Kato cusp conditions at the two‐body coalescence points; (ii) have only linear coefficients; and (iii) show a fast convergency rate for the energy. The efficiency of the construction is illustrated by the study of the negatively charged hydrogen‐like systems (^∞H^?, T^?, D^?, ¹H^?, and Mu^?), neutral helium‐like systems (e^?e^{? ∞}He⁺²,e^?e^{? 4}He⁺², e^?e^{? 3}He⁺², e^?μ^{? ∞}He⁺², e^?μ ^?4He⁺², and e^?μ^{? 3}He⁺²), and positively charged lithium‐like systems (e^?e^{? ∞}Li⁺³, e^?e^{? 7}Li⁺³, e^?e^{? 6}Li⁺³, e^?μ^{? ∞}Li⁺³, e^?μ^{? 7}Li⁺³, and e^?μ^{? 6}Li⁺³). The ground state energies and other mean values are compared with those given in the literature, when available. Wave functions with a moderate number of (20 maximum) linear coefficients are given explicitly; they are sufficiently simple and accurate to be used in practical calculations of atomic collision in which multidimensional integrations are involved. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Oxidation of 2-aminoquinoline at a stationary pyrolytic graphite electrode

The oxidation of 2-aminoquinoline was studied at a stationary pyrolytic graphite electrode in methanol-phosphate buffer at 25°C using various electroanalytical techniques. In the entire pH range (2.2–10.4), 2-aminoquinoline is oxidized and exhibits a well defined oxidation peak following a 2e^?, 2H⁺ process to give, 2,2′-azoquinoline as the major product. The linear relationship between peak current at a pyrolytic graphite electrode and concentration indicated that 2-aminoquinoline can be determined in the concentration range 0.1–1.0 mM. On the basis of cyclic voltammetry, spectral studies and controlled-potential coulometry, a mechanism of the electrode process is proposed.     

A Functional Model of Cytochrome c Oxidase: Thermodynamic Implications

The environment of the Cu ^I ion in the distal ligand group decides the fate of the reduction of O₂ by the two analogues 1 and 2 of the heme a₃ Cu_B center in cytochrome c oxidase. The fourfold coordination by N in 1 favors the Cu^II oxidation state and leads to a 4 e⁻–4 H⁺ reduction and the formation of H₂O under physiological conditions, while with 2 a 2 e⁻–2 H⁺ reduction occurs to form the cytotoxic H₂O₂.     

Inverse Kinetic Solvent Isotope Effect in TiO2 Photocatalytic Dehalogenation of Non‐adsorbable Aromatic Halides: A Proton‐Induced Pathway

An efficient redox reaction between organic substrates in solution and photoinduced h⁺_vb/e^?_cb on the surface of photocatalysts requires the substrates or solvent to be adsorbed onto the surface, and is consequentially marked by a normal kinetic solvent isotope effect (KSIE≥1). Reported herein is a universal inverse KSIE (0.6–0.8 at 298 K) for the reductive dehalogenation of aromatic halides which cannot adsorb onto TiO₂ in a [D₀]methanol/[D₄]methanol solution. Combined with in situ ATR‐FTIR spectroscopy investigations, a previously unknown pathway for the transformation of these aromatic halides in TiO₂ photocatalysis was identified: a proton adduct intermediate, induced by released H⁺/D⁺ from solvent oxidation, accompanies a change in hybridization from sp² to sp³ at a carbon atom of the aromatic halides. The protonation event leads these aromatic halides to adsorb onto the TiO₂ surface and an ET reaction to form dehalogenated products follows.     

Antioxidant Pathways and One‐Electron Oxidation of Dopamine and Cysteine in Electrospray and On‐Line Electrochemistry Electrospray Ionization Mass Spectrometry

Dopamine [DA]⁺ (m/z 154), DA dimer [2DA‐H]⁺ (m/z 307) and DA quinone [DAQ]⁺ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e^?, 1H⁺ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e^?, 2H⁺ oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys]⁺ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e^? pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.     

524—NAD/NADH as a model redox system: Mechanism,mediation, modification by the environment

The biologically important redox couple, β-nicotinamide adenine dinucleotide/1,4,β-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e^?) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation—deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD⁺ proceeds through two 1 e^? steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e^? anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH^+?, loses a proton (first order reaction) to form a neutral radical, NAD^?, which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NAD^+? (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD⁺.     

Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

In order to use H₂ as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H₂ into 2 H⁺ and 2 e^?. Herein, we report a metal‐free approach to catalyze the oxidation of H₂ by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H₂ with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC‐stabilized borenium cation [(IiPr₂)(BC₈H₁₄)]⁺ (IiPr₂=C₃H₂(NiPr)₂, NHC=N‐heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H₂ oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol^?1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C₆F₅)₃, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.     

Binding Matter with Antimatter: The Covalent Positron Bond

We report sufficient theoretical evidence of the energy stability of the e⁺?H₂^2? molecule, formed by two H^? anions and one positron. Analysis of the electronic and positronic densities of the latter compound undoubtedly points out the formation of a positronic covalent bond between the otherwise repelling hydride anions. The lower limit for the bonding energy of the e⁺?H₂^2? molecule is 74 kJ mol^?1 (0.77 eV), accounting for the zero‐point vibrational correction. The formation of a non electronic covalent bond is fundamentally distinct from positron attachment to stable molecules, as the latter process is characterized by a positron affinity, analogous to the electron affinity.     

Electrochemistry of 6-methyl-5,6,7,8-tetrahydropterin

The electrochemical oxidation of 6-methyl-5,6,7,8-tetrahydropterin (6-MTHP), the most effective of the synthetic aromatic amino acid hydroxylase pseudo cofactors, has been studied in aqueous solution over a wide pH range at a pyrolytic graphite electrode. The first electrooxidation of 6-WTHP occurs by a quasi-reversible 2e-2H⁺ reaction giving an unstable quinonoid-dihydropterin. The latter undergoes a first order chemical follow-up reaction yielding 6-methyl-7,8-dihydropterin (6-MDHP). At pH values ?5.6 6-MDHP forms an equilibrium mixture of a covalently hydrated species and non-hydrated species. The covalently hydrated form of 6-MDHP is electrooxidized in a 2e-2H⁺ quasi-reversible reaction to another unstable quinonoid that appears to undergo a two-step rearrangement to 6-methylpterin. Non-hydrated 6-MDHP is electrooxidized at the most positive potential in an irreversible 2e-2H⁺ reaction giving 6-methylpterin.     

Electrochemical oxidation of 9-methylxanthine

The electrochemical oxidation of 9-methylxanthine proceeds via four voltammetric oxidation peaks at the pyrolytic graphite electrode. The first voltammetric oxidation peak (peak I_a) is a 1e reaction giving a radical which dimerizes to 8,8′-bi-9-methyl-9H-purine-2,6-(1H, 3H)-dione. Peak II_a is a further 2e electrooxidation of the peak I_a dimer to another yellow dimer 8,8′-bi-9-methyl-9H-purine-2,6-(1H)-dione-3,5-(3H)-diiminylidene. This dimer is not very stable and it hydrolyzes to 1-methyl allantoin. Peak III_a is an adsorption pre-peak to peak IV_a which corresponds, overall, to a direct 4e?4H⁺ electrooxidation of 9-methylxanthine to an unstable diimine of 9-methyluric acid. Hydrolysis of this diimine leads to a variety of ultimate products.     

Ion Implantation in Semiconductors: Science and Technology,S. Namba (Ed.), Plenum Press,New York (1975)

A study has been made of the polarographic behaviour of 4-thiouracil and its N-methylated derivatives, and of 4-thiouridine. Similar electrochemical properties are exhibited by all the compounds with a 2-keto-4-thione structure. In acid medium they give a typical catalytic hydrogen discharge wave, which disappears completely at pH values above 6. At pH values above 4.2 a second catalytic wave appears. Maximal catalytic activity is exhibited at pH 7.1, and decreases with increasing pH. Both catalytic waves possess pronounced surface characteristics, most likely due to adsorption of the molecules with differing orientations on the electrode surface. The second catalytic wave overlaps the reduction wave, which is placed in evidence under conditions where the catalytic effect is absent. The reduction wave is a 4e^?/4H⁺ process involving reduction of the 4-thiouracil ring to 5,6-dihydropyrimidone-2. The same product is formed by a 2e^?/2H⁺ reduction of 5,6-dihydro-4-thiouracil. The potential applications of the electrochemical properties of 4-thiouracil to studies on tRNA structure are discussed.     

Theoretical study of the reaction Ne + H+2 → NEH+ + H in the 2A′ ground state

A three-dimensional potential energy surface for the ²A′ ground state of the system (Ne? H₂)⁺ (²Σ⁺ in collinear geometry) has been calculated at SCF and CEPA levels. This surface describes the abstraction reaction which is endoergic by 0.57 eV (ΔH⁰₀) and has been studied recently by different experimental groups at low collision energies. Our CEPA calculations yield an endoergicity of 0.55 eV (ΔH⁰₀). The ²A′ surface has a minimum at collinear geometry with R_Ne—H = 2.29 a₀ and R_{H? H} = 2.08 a₀ and a well depth of 0.49 eV relative to Ne + H⁺₂. The effects of electron correlation on the shape of the surface and on the well depth are discussed. An analytic fit of the collinear part of the surface has been constructed based on Simon's proposal of using polynomials in the coordinates (R? R_e)/R instead of (R? R_e). The fitted potential is used for quantum mechanical scattering calculations with the finite element method (FEM ). Preliminary results for reaction probabilities for H⁺₂ in different vibrationally excited states are given and compared to the experimental results.     

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

OH⁺ is an extraordinarily strong oxidant. Complexed forms (L? OH⁺), such as H₂OOH⁺, H₃NOH⁺, or iron–porphyrin‐OH⁺ are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L? OH⁺ towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L? OH⁺. With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas‐phase pseudo‐first‐order reaction rates of H₂OOH⁺, H₃NOH⁺, and protonated 4‐picoline‐N‐oxide towards isobutane and different halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂ have been determined by means of Fourier transform ion cyclotron resonance meaurements. Reaction rates for H₂OOH⁺ are generally fast (7.2×10^?10–3.0×10^?9 cm³ mol^?1 s^?1) and only in the cases HCF₃ and CF₄ no reactivity is observed. In contrast to this H₃NOH⁺ only reacts with tC₄H₉Cl (k_obs=9.2×10^?10), while 4‐CH₃‐C₅H₄N‐OH⁺ is completely unreactive. While H₂OOH⁺ oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH₂OH, a series of alkanes C_nH_2n+2 (n=1–4), and halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂, were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C_nH_2n+1⁺ (n=1–4) and C_nH_2nCl⁺ (n=1–4). The hypersurface for the reaction of H₂OOH⁺ with CH₃Cl and C₂H₅Cl was calculated at the B3 LYP, MP2, and G3_m* level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

Picomolar Detection of Hydrogen Peroxide at Glassy Carbon Electrode Modified with NAD+ and Single Walled Carbon Nanotubes

Potential cycling was used for oxidation of NAD⁺ and producing an electroactive redox couple which strongly adsorbed on the electrode surface modified with single walled carbon nanotubes (SWCNTs). Modified electrode shows a pair of well defined and nearly reversible redox peaks at pH range 1–13 and the response showed a surface‐controlled electrode process. The surface coverage and heterogeneous electron transfer rate constant (k_s) of adsorbed redox couple onto CNTs films were about 6.32×10^?10 mol cm^?2 and 2.0 (±0.20) s^?1, respectively, indicating the high loading ability of CNTs toward the oxidation product of NAD⁺ (2,8‐dihydroxy adenine dinucleotide) and great facilitation of the electron transfer between redox couple and CNTs immobilized onto electrode surface. The modified electrode exhibited excellent electrocatalytic activity for H₂O₂ reduction at reduced overpotential. The catalytic rate constant for H₂O₂ reduction was found to be 2.22(±0.20)×10⁴ M^?1 s^?1. The catalytic reduction current allows the amperometric detection of H₂O₂ at an applied potential of ?0.25 V vs. Ag/AgCl with a detection limit of 10 pM and linear response up to 100 nM and resulting analytical sensitivity 747.6 nA/pM. The remarkably low detection limit (10 pM) is the lowest value ever reported for direct H₂O₂ determination on the electrodes at pH 7. The modified electrode can be used for monitoring H₂O₂ without the need for an enzyme or enzyme mimic. The proposed method for rapid amperometric detection of H₂O₂ is low cost and high throughput. Furthermore, the sensor can be used to any detection scheme that uses enzymatically generated H₂O₂ as a reactive product in biological systems.     

Fast atom bombardment mass spectrometry of ionic gold(I) and gold(III) carbene derivatives: An investigation of the formation of [M – H]+ species

Fast atom bombardment (FAB) mass spectrometry of the gold(I) and gold(III) derivatives, {Au[C(Y)–NHAr]₂}⁺X^? and {Au[C(Y)–NHAr]₂I₂} ⁺ X^? (Y =? OC₂H₅ or ? NHAr; X^? = CIO₄^? or BF₄^?; Ar = p-CH₃? C₆H₄) has led to the detection, for the alkoxyamino derivatives only, of [M–H]⁺˙ molecular species. The mechanism of the formation of these unusual species has been studied with respect to the oxidation state of gold, nature of the matrix, matrix acidity and ligand structure. The energetics of two possible alternative mechanisms has been studied by means of ab initio theoretical calculations. Both experimental and theoretical data indicate that [M–H]⁺˙ formation is due to the reaction of M⁺ with H⁺-philic and/or H˙-philic species produced from the matrix by FAB. Whatever the operative mechanism, the [M–H]⁺˙ formation is to be considered a FAB-induced oxidative process.