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.     

Electrochemical oxidation of barbituric acids at low pH in the presence of chloride ion

Barbituric acid, 1-methylbarbituric acid and 1,3-dimethylbarbituric acid are electrochemically oxidized at the pyrolytic graphite electrode by way of a single voltammetric peak at pH 1 in the presence of chloride ion. At least four products are formed as a result of the reaction, the three major products, accounting for more than 80–90% of the oxidized barbituric acid, are the appropriately N-methylated 5,5′-dichlorohydurilic acids, 5,5-dichlorobarbituric acids and alloxans. The mechanism appears to proceed by an initial potential-controlling 1e/1H⁺ oxidation of the barbituric acids to give a barbituric acid radical. This can dimerize to hydurilic acid, which is then further electrochemically oxidized. However, this appears to be a minor route. The barbituric acid radical appears to be mainly further electrooxidized (1e) to a carbonium ion which further reacts with nucleophiles such as chloride ion to give 5-chlorobarbituric acid, or with water to give dialuric acid. Further electrochemical oxidation and chemical reactions of the latter species results in formation of the ultimate products.     

Mechanism of electrochemical reduction of oxamide analytical applications

Oxamide is electrochemically reduced at mercury electrodes between pH 5.6 and 11.6. The overall mechanism proceeds by an initial 2e reduction of the 1,2-carbonyl groups of oxamide to give a dianion. This then protonates, rearranges, and loses ammonia to glyoxylamide, which is reduced in a further 2e/2H⁺ reaction giving glycolamide as the ultimate product. The reaction thus proceeds by a typical e.c.e. mechanism. The overall homogeneous rate contant for the chemical reaction(s) interposed between the two charge-transfer steps was measured by peak voltammetric, potentiostatic and d.c. polarographic methods. The d.c. polarographic wave of oxamide between pH 5.6 and 11 provides the basis for a very simple analytical method for the determination of oxamide.     

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).     

Kinetics and mechanism of the electrochemical reduction of 2,2′-bipyridine: Part II. Consecutive isomerization processes of dihydrobipyridine

2,2′-Bipyridine is reduced by electrolysis at constant potential at a Hg electrode in the pH range 12<pH<14. A reversible 2e^?-, 2 H⁺-step was found. The first product Y is transformed in a fast irreversible reaction into a substance A, which undergoes slow consecutive reactions according to the scheme A agB agC. The mechanism was simulated by a computer program and the rate constants optimized. B is further reducible in a 2 e^?-, 2 H⁺-step, forming D. Therefore at long times of electrolysis only D remains. The substances Y, A, B, C are isomeric dihydrobipyridines. D is a tetrahydrobipyridine. The substances A, B, C and D could be isolated in solution, and partially identified by u.v.-spectroscopy, but polymerized if they were concentrated. The stable substance C is 2,5-dihydrobipy, Y probably N,N′-dihydrobipy. A reaction mechanism is proposed and it is shown that only two reaction paths remain possible, A being either the 1,4- or the 1,6-dihydrobipy.     

Electrochemical oxidation of HCOONa on Pt in acidic solutions

The electrooxidation of HCOONa was carried out over a wide range of pH on Pt. HCOO⁻ and its associated form of HCOOH do not show any difference in electrochemical behaviour. A voltammetric study demonstrates the formation of two kinds of poisoning species in the hydrogen (X₁) and double-layer (X₂) regions. Their dependences on the potential and pH were examined. Constant polarization measurements give the rate expression, i = k(α_H⁺)^−0.43 exp(0.4Fφ_n/RT), independent of the concentrations of HCOO⁻ and HCOOH. The rate-determining step is concluded to be HCOO⁻ (a) → COO⁻ (a)+H⁺ + e⁻ or HCOOH(a) → COOH(a)+H⁺ + e⁻. The negative reaction order with respect to H⁺ was explained through the retarding action of X₂. The nature of X₁ and X₂ is discussed.     

Chemi-ionization reactions of photo-excited K atoms

Ion production resulting from excitation of K atoms to m²P (m ? 6) levels has been observed in mixtures of K with various species. Mass spectrometric data provide direct evidence for the occurrence of K(m²P) + K(4 ²S) → K₂⁺ + e^? and are consistent with the formation of unstable KX⁺ ions from K(m²P) + X → KX⁺ + e^? for X = C₂H₄, CH₄, N₂.     

Electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode

The electrochemical oxidation of 4,5,6,7-tetrahydro-1H-pyrazolo[3,4-d] pyrimidine-4,6-dione (oxipurinol) at the pyrolytic graphite electrode (PGE) has been studied. Oxipurinol exhibits up to three voltammetric oxidation peaks at the PGE between pH 1–12. The first pH-dependent peak (peak I_a) is proposed to be an initial, irreversible 2e-2H⁺ reaction to give 5,6-dihydro-4H-pyrazolo[3,4-d] pyrimidine-4,6-dione. This primary product further reacts by two routes. The major route, accounting for ca. 90% of the latter compound, involves a Michael addition of water followed by further electrochemical oxidation and hydrolysis to give 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil. The minor route involves further electrochemical oxidation of 5,6-dihydro-4H-pyrazolo[3,4-d]-pyrimidine-4,6-dione in a 2e-2H⁺ reaction to give 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]-pyrimidine-3,4,6-trione.Decomposition and, generally, additional electrochemical reactions of 5,6-dihydro-5,6-dihydroxy-5-carboxy-6-diazenouracil result in the formation of alloxan, parabanic acid, 6-diazo-isobarbituric acid and 5′-hydroxy-5-carboxy-6,6′-azouracil. The two latter compounds have never previously been reported. Decomposition of 4,5,6,7-tetrahydro-3H-pyrazolo[3,4-d]pyrimidine-3,4,6-trione results in formation of uracil-5-carboxylic acid.Detailed reaction schemes have been proposed to explain the observed electrochemistry and the formation of the observed products.     

Semiempirical quantum chemical treatment of the standard reduction potentials of quinone and plastoquinone in water

The standard electrode potential for the quinone (Q)-hydroquinone (QH₂) couple in aqueous acidic media has been explicitly calculated. Molecular geometries of Q and QH₂ have been optimized. Protonation of Q, i.e., the formation of QH⁺ and QH, have been considered. Molecular geometries of these species have been thoroughly optimized. The energy of complexation of these molecules with water have been calculated by optimizing the structures of the hydrated complexes Q · 6H₂O, QH₂ · 6H₂O, QH⁺ · 6H₂O. and QH · 6H₂O. The ion–solvent interaction energy of QH⁺ · 6H₂O, in turn, has been calculated by considering the complex QH⁺ · 6H₂O…? 2H₂O, where the two extra water molecules approach the charge center of the complex QH⁺ · 6H₂O vertically from top and bottom of the quinonoid ring. The standard reduction potential calculated by the CNDO method, 0.8548 V, is somewhat larger than the experimental potential, 0.6998 V, at 25°C. But the INDO value, 0.7085 V, is in excellent agreement with the observed potential. The electrode potential for the plastoquinone (PQ)-plastohydroquinone (PQH₂) couple present in the aqueous pool in chloroplast has been calculated by the INDO method. The basic geometries of PQ, PQH⁺, and PQH_2/sb have been synthesized by adopting the optimized geometries of Q, QH⁺, and QH₂ and considering methyl substituents as well as an isoprenoid side chain containing up to 3 isoprene units with possible geometrical isomerism. The hydrated species PQ · 6H₂O, PQH⁺ · 6H₂O, and PQH₂ · 6H₂O are unstable compared to the isolated species PQ, PQH⁺, and PQH₂, respectively. In fact, we have found that the hydration of PQH⁺ and PQH₂ is much less extensive, and stability arises only when the hydroxyl groups in these two molecules are hydrogen-bonded to water molecules. But PQH⁺ is also stabilized through the association of two more water molecules in the vertical direction. For this reason, we have calculated the reduction potential of the PQ/PQH₂ system from the energies of the isolated molecules PQH₂ and the hydrated species PQH⁺ · 2H₂O. The computed standard reduction potential is 0.2785 V and it yields a potential of 0.07V at pH 7 at 25°C, which is in good agreement with the reduction potential 0.11 V observed for plastoquinone in the aqueous pool in chloroplast. © 1994 John Wiley & Sons, Inc.     

A faradaic impedance study of the reduction of oxygen from aqueous alkaline solution at the dropping mercury electrode

The reduction of oxygen to hydrogen peroxide at a dropping mercury electrode in an aqueous solution of 1 M KNO₃+0.04 M KOH (pH=12.35) has been studied by means of impedance measurements as a function of frequency and d.c. potential. The reaction appears to be nearly reversible in the dc sense, but quasi-reversible in the ac sense. The impedance data obey the Randles' equivalent circuit with the following apparent values for the kinetic parameters: standard heterogeneous rate constant k_sh^a=0.035 cm s^?1 and cathodic transfer coefficient α^a_c=0.22. The results are interpreted in terms of a two-step charge transfer mechanism with the step O₂+eO₂^? being rate-determining.     

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.     

Zundel‐like and Eigen‐like Hydrated Protons on a Platinum Surface

The nature of hydrated protons is an important topic in the fundamental study of electrode processes in acidic environment. For example, it is not yet clear whether hydrated protons are formed in the solution or on the electrode surface in the hydrogen evolution reaction on a Pt electrode. Using mass spectrometry and infrared spectroscopy, we show that hydrogen atoms are converted into hydrated protons directly on a Pt(111) surface coadsorbed with hydrogen and water in ultrahigh vacuum. The hydrated protons are preferentially stabilized as multiply hydrated species (H₅O₂⁺ and H₇O₃⁺) rather than as hydronium (H₃O⁺) ions. These surface‐bound hydrated protons may play an important role in the interconversion between adsorbed hydrogen atoms and solvated protons in solution.     

Hydration Energies of Protonated and Sodiated Thiouracils

Hydration reactions of protonated and sodiated thiouracils (2-thiouracil, 6-methyl-2-thiouracil, and 4-thiouracil) generated by electrospray ionization have been studied in a gas phase at 10 mbar using a pulsed ion-beam high-pressure mass spectrometer. The thermochemical data, ΔH ^o n, ΔS ^o n, and ΔG ^o n, for the hydrated systems were obtained by equilibrium measurements. The water binding energies of protonated thiouracils, [2SU]H⁺ and [6Me2SU]H⁺, were found to be of the order of 51 kJ/mol for the first, and 46 kJ/mol for the second water molecule. For [4SU]H⁺, these values are 3–4 kJ/mol lower. For sodiated complexes, these energies are similar for all studied systems, and varied between 62 and 68 kJ/mol for the first and between 48 and 51 kJ/mol for the second water molecule. The structural aspects of the precursors for hydrated complexes are discussed in conjunction with available literature data. Graphical Abstract

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Spectral and kinetic properties of intermediates induced by reaction of hydrated electrons with adenine,adenosine, adenylic acid and polyadenylic acid: A multicomponent analysis

The overlapping transient optical absorption spectra obtained from pulse radiolysis experiments in which the hydrated electron reacted with adenine, adenosine, 5'-adenosine monophosphate and polyadenylic acid were measured and analysed. The analysis consists of a multicomponent analysis which yields the number of different species and a set of orthonormal basis spectra. From this set of basis spectra together with a number of trial models for the possible kinetics of the different species the best model is determined by fitting the original data. In this way the molar absorptivity spectra and the kinetics of the species can be found. To describe all the spectra of the adenine-containing compounds in a consistent way we need, at most, three different species. The following reaction scheme is obtained: e^-_aq + A → A^-., A^-. + H₂O →2 A′H^. + OH^-, A^-. → A″H^. and A'H^.(A″H^.) + A'H^.(A″H^.). The radicals A'H^. and A″H^. are both protonated electron adducts, while P appears to be “stable” product with a decay time of the order of 1 day.     

Electrochemical reactions with protonations at equilibrium: Part VI. The homogeneous electron exchange reaction between a monoelectronic and A 1 e, 1 H+ system

The rate of homogeneous electron exchange between a 1 e redox couple and a 1 e, 1 H⁺ redox system is studied theoretically when the protonations are much faster than the electron exchange reactions, i.e. when they can be assumed to be at equilibrium. It is shown that the whole system is equivalent to a reaction between two simple 1 e couples, with apparent rate constants for the electron exchange. Variations of these constants are complex functions of the difference of the standard potentials of the monoelectronic system and those of the 1 e, 1 H⁺ system. They also vary with pH in a less complicated way. The reaction path and the reaction sequence (order of addition or loss of electrons and protons) are studied. A potential-pH diagram, which allows the results to be visualized, is given. It is shown that it is not possible to accelerate at the same time the reduction and oxidation of the members of the square scheme by the same monoelectronic system. Applications to redox polymer electrodes are discussed.     

Raman studies of the hydrated melt of FeCl36H2O

The hydrated melt of FeCl₃6H₂O has been investigated by laser Raman spectroscopy. Application of the background correction and band fitting computational methods has revealed that the hydrated melt predominantly contains an octahedral species, Fe(H₂O)₄Cl⁺₂, and a tetrahedral species, FeCl⁻₄. These are present in almost equal concentrations because the hydrated melt produces about twice the amount of contained FeCl⁻₄ species in the presence of excess CL⁻ ion. The relatively high electrical conductivity of the hydrated melt is consistent with the ionic species [Fe(H₂O)₄Cl₂]⁺ and [FeCl₄]⁻, rather than a bitetrahedral species, Fe₂Cl₆.     

Synthesis of thieno[3,4-b]pyrrole derivatives based on the reaction of 1-methyl-2-nitromethylenepyrrolidine with isothiocyanates

The reaction of 1-methyl-2-nitromethylenepyrrolidine with arylisothiocyanates in the presence of a base was found to proceed at the 3-СН₂ group of the nitroenamine followed by further cyclization to afford 4-R-imino-1-methyl-1,2,3,4-tetrahydro-6H-thieno[3,4-b]pyrrol-6-one oximes. Under analogous conditions, alkylisothiocyanates add to 1-methyl-2-nitromethylenepyrrolidine giving 1-methyl-2-nitromethylenepyrrolidine-3-carbothioic acid amides, and no cyclization was observed. A tentative mechanism for the formation of thieno[3,4-b]pyrrolе derivatives is proposed.     

Synthesis of 3-(aminooxoethyl)-6-methyl-1-(thiethan-3-yl)-pyrimidine-2,4-(1H,3H)-diones

A new approach to prepare 2-chloroacetamides has been developed, based on the reaction of chloroacetyl chloride with excess of secondary amines. Alkylation of 6-methyl-1-(thiethan-3-yl)pyrimidine-2,4(1H,3H)-dione with the synthesized 2-chloroacetamides in the presence of potassium carbonate has afforded N ³-acetamido-substituted 6-methyl-1-(thiethan-3-yl)pyrimidine-2,4(1H,3H)-diones.     

On Reactions of Oxygenated Cobalt(II) Complexes. XI. Note on the Mechanism of O2 Uptake by Cobalt(II) Chelates with Tetra- and Pentadentate Amines

The synthesis of two new polyamines containing 2-pyridyl and 6-methyl-(2-pyridyl) groups is described. The equilibria between H⁺ and Co²⁺ and the new ligand 1,9-di(2-pyridyl)-2,5,8-triazanonane (dptn) as well as the protonation of the hydroxo complexes of 1,6-di(2-pyridyl)-2,5-diazahexane-Co(II) (Co(dpdh) and 1-(6-methyl-2-pyridyl-6-(2-pyridyl)-2,5-diazahexane-Co(II) (Co(mdpdh)) have been studied in aqueous solution using the pH method. The coordination ability of the pyridine containing ligand dptn is compared with the chelating tendency of the analogous aliphatic amine (tetren). In spite of the lower basicity of the pyridine derivative the stability constants of its Co(II) complex is higher by a factor of thirty. The absorption spectra give evidence for a pseudooctahedral geometry of Co(dpdh) (H²O) and Co(dpdh)(H₂O)(OH)⁺. Oxygen-uptake measurements indicate the formation of binuclear peroxo species. The potentiometric equilibrium data indicate the presence of dibridged species (dpdh)Co(O₂, OH)Co(dpdh)³⁺ and (mdpdh)Co(O₂, OH)Co-(mdpdh)³⁺. The kinetics of the rapid O₂-uptake was measured over a wide pH range on a stopped-flow apparatus. For Co(dpdh)²⁺ and Co(mdpdh)²⁺ we found a second order rate constant independent of pH up to pH 9, but in more alkaline solutions it increases and reaches an upper limit around pH 12.3. The data could be fitted by a rate law of the form k₁ = (k′₁[H⁺] + k″₁ K_H) ([H⁺] + K_H)^?1. This variation with pH was explained by a rapid equilibrium Co(dpdh) (H₂O) ? Co(dpdh)(H₂O)(OH)⁺ + H⁺(K_H). The enhanced rate constants of the hydroxo species must arise from a rate determining H₂O replacement by O₂, dominated by Co-OH₂ bond breaking and the expected ability of an OH^? group to labilize neighboring H₂O molecules. The protonation constant of the hydroxo complex obtained by equilibrium measurements (pK_H = 11.19 ± 0.03) was in good agreement with that derived from kinetic data (11.12 ± 0.04). The hydrolysis of Co(dptn)(H₂O)²⁺ influences the rate of O₂-incorporation in a different way. In this system retardation occurs as a result of hydrolysis ascribed to the slower leaving of OH^? compared to H₂O. This was expected if a mechanism with rate determining H₂O replacements by O₂ holds.     

Oxygen reduction on a platinum cluster

The reaction Pt₅O₂(ads)+H⁺(aq)+e⁻→Pt₅–OOH is analyzed on the basis of density functional theory calculations. It is found that the electron transfer process takes place gradually as the hydronium ion gets close to the adsorbed oxygen. At a certain small O_ads–H⋯O_water distance, the barrier for proton and electron transfer becomes negligible. The effect of an electric field on the reaction is studied by charging the metal/adsorbate complex, and that of a solvation shell on the proton transfer process is explored by using the H₃O⁺(H₂O)₂ ion cluster to model the hydrated proton.