Standard Oxidation Potentials Research Articles

Optical Oxidation Potential: a New Measure for Reducing Power

A new spectroscopy for solution, designated Photoelectron Emission Spectroscopy, has been developed. Since this technique is applicable to almost any solution and provides the ionization potential of solutes, the ionization potentials can be measured for solutes which give neither a reproducible oxidation potential by electrochemistry nor the ionization potential in a vacuum by photoelectron spectroscopy. Since photoionization is a one-electron oxidation reaction, the ionization potential in solution is very similar to the oxidation potential in character. Therefore, the ionization potential measured by the present method is designated as the optical oxidation potential. In this review the optical oxidation potential values are given for organic molecules, metal complexes and halide anions, and are compared with the electrochemical oxidation potentials and the results by other spectroscopies. In order to make proceed an oxidation reaction at an electrode, there is a limit for the electrode potential, i.e. no higher potential is necessary to accelerate the reaction, which is the optical oxidation potential proposed here, while there exists a lower limit known as the standard oxidation potential. The difference between these two oxidation potentials provides a kinetically important parameter, the reorganization energy for electron-transfer reaction. The present spectroscopic method has proved to be a technique which can provide a new measure of the reducing power of solvated species and new insights into the solvation energy and solvation structure. Its application to the analysis of an air/solution interface is also described.

From oligomers to polymer. An insight into the electrochemical doping—dedoping of polythiophene from electrochemical data on thiophene oligomers

Thiophene oligomers nT ( n = 4, 5, 6) were synthesized and their cyclic voltammograms (Cvs) recorded in acetonitrile and nitrobenzene solutions. The polymers formed behave like polythiophene (PT), with an anodic peak shifted considerably towards positive potentials. A simple model based on the substantial change in the conductivity of the polymer between its neutral and oxidized states is developed. The strongly asymmetrical CV of PT is interpreted in terms of the polaron—bipolaron theory. The model also led us to conclude that the standard oxidation potential E ox of the polymer is given by the reduction peak potential. An analysis of the CV of 4T, 5T and 6T allows the standard oxidation potentials of 5T, 6T, 8T and 10T to be estimated. Extrapolation to an infinite chain gives E ox = 0.48 V/SCE.

Standard oxidation potentials of methylbenzenes in acetonitrile

Combined use of ultramicroelectrodes and of an ultrafast potentiostat equipped with on-line ohmic drop compensation has allowed the determination of st ( E Bz-CH 3 0 = 1.58 (hexamethylbenzene), 1.69 (pentamethylbenzene), 1.75 (durene) and 1.77 V (1,2,3,4-tetramethylbenzene) vs. a saturated rate constants for the deprotonation of the corresponding cation radicals by a series of pyridine bases. These latter values compare satisfactorily wit

Large Reorganization Energies Associated with Electron-Transfer Reactions of Permethylated Catenates of Group 4B Elements

Abstract Large reorganization energies (38 ± 5 kcal mol−1) are found to be required for electron-transfer reactions of permethylated catenates of group 4B elements. With such large reorganization energies the standard oxidation potentials of permethylated catenates of group 4B elements in acetonitrile are about 1 V lower than the corresponding peak potentials observed in the electrochemical oxidation.

Mechanistic and energetic aspects of aromatic nitration with NO +2 in sulfolane

The electrooxidation of several commonly used aromatics has been studied in sulfolane at 303 K by means of the cyclic voltammetric (CV) technique (at a Pt electrode). It has been shown that the anodic peak potentials ( E a p) of the irreversible cyclic voltammograms obtained for these aromatics correlate well with the ionization potential ( I p), and that the CV peak shift ( E a p - E ° (ArH .+/ArH)) stems from the electrochemical kinetics for the EC mechanism for arene oxidation. Taking into account previous work on the theoretical analysis of the electrochemical kinetics, the peak potential E a p can be related to the standard oxidation potential of the ArH .+/ArH couple. Furthermore, we also examined the interactions of these aromatics with the electron acceptor NO + 2 using the potentiometric technique (employing a silver/silver chloride indicator electrode) as follows: NO 2Cl + ArH + AgClO 4 → ArNO 2 + HClO 4 + AgCl(s) (I) Analysis of the potentiometric curves using Nernst's law allowed us to determine the standard potential for reaction (I) and the Gibbs energy differences Δ G(II) and Δ G(III) corresponding to the nitration processes (NO + 2, ClO − 4) + ArH ⇌ ArNO 2 + HClO 4 (II) and NO + 2 +ArH ⇌ ArNO 2 + H + (III) The feasibility of the outer-sphere electron transfer NO + 2 + ArH ⇌ NO . 2 + ArH .+ (IV) has been investigated by application of the Marcus theory in order to gain further insight into the energetics of this ET process and to characterize the intermediate stages through which the nitration reaction may proceed in sulfolane.

The oxidation potential of 1,4-diaminobenzene: Calculation versus experiment

The oxidation potential of 1,4-diaminobenzene has been calculated theoretically using a combination of ab-initio calculations and molecular dynamics simulations. An experimental determination of the same quantity is presented. Data obtained from cyclic voltammetric experiments, at a platinum electrode, were consistent with an EC electrode reaction mechanism. Cyclic voltammograms recorded at high voltage scan rates ( v ⪢ 500 mV s −1), such that the following homogeneous kinetics were outrun, indicated the oxidation to be quasi-reversible and this enabled the standard oxidation potential to be estimated. Comparison of the theoretical and experimental values for the oxidation potential showed agreement to within 25 mV. The level of agreement between theory and experiment was considered to be highly satisfactory.

Lower oxidation states of lanthanides and actinides. Prospects of their use in analytical practice

Physico-chemical characteristics of Ln2+ and An2+ such as standard oxidation potentials EM3+M2+, hydration energies, ionic radii were obtained by the cocrystallization method. A number of energy parameters for Ln2+ and U3+ were obtained by optical spectroscopy on the basis of fd-spectra in such solvents as H2O, C2H5OH, HMPA, THP and others. A new rapid method has been developed to extract higher actinides and separate them from similar elements by cocrystallization with NaCl from aqueous ethanolic solutions.

Energetics of the Sequential Electroreduction and Electrooxidation Steps of Benzenoid Hydrocarbons

Abstract The first and second reduction (E1⁄2.1red, E1⁄2.2red) and the first oxidation (E1⁄2.1oxd) standard potentials of benzenoid alternant hydrocarbons (BAH) were experimentally determined by means of cyclic voltammetry (CV) in nonaqueous solvents. The second standard oxidation potential (E1⁄2.2oxd) was however, estimated by checking the scan rate dependence of the irreversible CV curve. The equations pertinent to these potentials and their mutual relations were formulated from the points of view of the Born–Haber-type thermodynamic energy cycle and SCFMO calculations. Of the SCFMO calculations, the MO-paring property established in the PPP-type π-electron theory was very successful in the discussion of the equations given above. Under the acceptable assumptions that the solvation energies due to mono- and dications are put equal to those of the mono- and dianions respectively, and using the MO-pairing property, the equation of (E1⁄2.1oxd+E1⁄2.1red)=(E1⁄2.2oxd+E1⁄2.2red) was derived. The experimental results were well described by this equation. The solvationenergy values were evaluated by applying the experimentally determined reduction or oxidation potentials to the theoretical equations. An examination of the solvation energies has shown that these values can be interpreted by means of the Born-type equation.

“In situ” investigation by simultaneous voltammetry and UV-visible reflectance spectroscopy of some viologen radicals absorbed on a platinum electrode

The quaternary salts of N, N′-dimethyl-4,4′-dipyridinium dication (methyl viologen, MV 2+) and of two of its derivatives, an o-xylene bridge dimer, DV 4+, and a long-chain monosubstituted viologen, C 14MV 2+, were investigated by coupled voltammetry and UV-visible reflectance spectroscopy. The absorption spectra of submonolayers of radicals produced “in situ” were obtained from reflectograms recorded at convenient wavelengths and compared to absorption spectra of bulk species. The persistence of adsorbed radicals at the surface of the electrode for potentials more positive than their standard oxidation potentials and the shifts in the position of the absorption peaks, are ascribed to interactions with the metallic sites or with the neighbouring species.

Ionization potentials and electron affinities in aqueous solution

By combining gas-phase proton affinities and aqueous pK/sub a/ values, it is possible to obtain free energies of hydration for a large number of cations and anions. With some corrections these numbers can be used to convert gas-phase ionization potentials, I, and electron affinities, A, into the corresponding aqueous solution values, I' and A'. An assumption must be made about the free energy of hydration of H/sup +/, taken as -259.5 kcal/mol. The absolute potential of the hydrogen electrode is then 4.50 V, and values of I' and A' can be converted to standard oxidation or reduction potentials. The absolute electronegativity, (I' + A')/2, in solution is estimated and found to be little different from the gas-phase value, (I + A)/2. The absolute hardness, (I'-A')/2, is much smaller than the gas-phase value, but it is shown that the solution value has little utility. The gas-phase value should be used even in solution.

Correlation between oxidation potentials of the lanthanides in aqueous solutions and chloride melts

Based on the literature and experimental data obtained by the authors, it appears that the differences in the oxidation potentials of the M 3+/ M 2+ couples for two f-elements in chloride melts and in some other solvents coincide with the differences in the standard oxidation potentials of the M 3+/M 2+ couples for the same elements in aqueous solutions. Also, a dependence between the solubility of the metals of f-elements, in their molten trichlorides and the standard oxidation potential values of the M 3+/M 2+ and M 3+/M 0 couples for aqueous solutions has been established. The correlation determined enabled us to plot a single scale of the standard oxidation potentials of the M 3+/M 2+ couples for f-elements as well as calculate the metal solubilities in their molten trichlorides.

Electron transfer from anthracenes. Comparison of photoionization, charge-transfer excitation and electrochemical oxidation

Quantitative measures of electron detachment from anthracene and various meso-substituted derivatives (D) are examined in the gas phase and in solution. The photoelectron spectra show well-resolved bands for E1 corresponding to the first ionization [Formula: see text] from a π-molecular orbital (HOMO) with maximum electron density at the 9,10-positions. The same series of anthracenes form 1:1 electron donor–acceptor complexes with tetracyanoethylene [D,TCNE], which exhibit distinctive colors associated with charge transfer transitions hvCT to the ion pair[Formula: see text]. These measures of the gas phase energetics of electron transfer from the anthracenes are compared with their standard oxidation potentials [Formula: see text] determined from the reversible cyclic voltammograms in methylene chloride and trifluoroacetic acid solutions. Although there are direct, linear free energy relationships between E1and hvCT as well as E1and [Formula: see text], the best correlation is observed between [Formula: see text] and hvCT. The validity of the latter is discussed in terms of offsetting effects arising from differences in solvation energy and steric effects attendant upon 9,10-substitution in anthracenes.

Electron transfer from aromatic hydrocarbons and their .pi.-complexes with metals. Comparison of the standard oxidation potentials and vertical ionization potentials

Les parametres energetiques du transfert d'electrons pour une serie d'alkylbenzenes substitues sont mesures en solution et en phase gazeuse. Les potentiels d'oxydation standards E Ar 0 sont obtenus a partir de voltammogrammes cycliques reversibles dans l'acide fluorhydrique en presence d'electrodes microvoltammetriques. Les valeurs obtenues sont en bonne correlation avec les potentiels d'ionisation des hydrocarbures aromatiques en phase gazeuse. Cas aussi, des polyalkylbenzenes. Donnees, de plus, sur les hydrocarbures aromatiques complexes π au chrome tricarbonyle

A Simple Deviation Function to Summarize Data In Solvent Mixtures. The Standard Oxidation Potential of the Silver Chloride Electrode in Water-Ethylene Glycol Mixtures

A Simple Deviation Function to Summarize Data In Solvent Mixtures. The Standard Oxidation Potential of the Silver Chloride Electrode in Water-Ethylene Glycol Mixtures

Cyclic Voltammetry of Rh(I) Complexes and the Oligomers. A Correlation between the Anodic Peak Potentials and the Rate Constants for the Electron Transfer Reactions with Inorganic Oxidants

Abstract The cyclic voltammogram of a CH3CN solution of [Rh(p-MeOC6H4NC)4]+ exhibits three anodic current peaks which correspond to the oxidations of the monomer, the dimer, and the trimer, with no cathodic wave on the reverse scan. The anodic peak potentials Eoxp in such irreversible cyclic voltammograms were determined for various Rh(I) monomers and the oligomers such as [Rh(RNC)4]+, [Rh2(RNC)8]2+, [Rh2(dppm)2(RNC)4]2+, [Rh2(dicp)4]2+, [Rh3(RNC)12]3+, and [Rh4(dicp)8]4+ (R=p-MeOC6H4 and Ph, etc; dppm=bis(diphenylphosphino)methane, dicp=1,3-diisocyanopropane). The anodic peak potentials Eoxp vary mainly with the degree of oligomerization of the Rh(I) complexes, decreasing in the order monomer>dimer>trimer>tetramer, in parallel with the energies of the highest occupied molecular orbitals EHOMO. It has been found that the Eoxp values are linearly correlated with logarithm of the rate constants for the electron transfer reactions with inorganic oxidants such as [Fe(bpy)3]3+ and [Co(bpy)3]3+ (bpy=2,2′-bipyridine) in the context of the Marcus theory as expected when the standard oxidation potentials Eoxo would be used. It is thus suggested that the anodic peak potentials of the Rh(I) complexes in the irreversible system can be used as the standard oxidation potentials as far as the relative values are concerned.

Thermochemical parameters for organic radicals and radical ions. Part 1. The estimation of the pKa of radical cations based on thermochemical calculations

Three methods for estimating the pKa of a radical cation using thermochemical cycles are discussed. These methods enable the calculations of the pKa of radical cations which have not been determined experimentally. Radical cation acids are classified according to the nature of their respective conjugate bases. The application of these methods is illustrated for a σ-acid (the benzene radical cation) and π-acid (the toluene radical cation). The potential of these methods to estimate relative and absolute solvation energies is also discussed. The use of thermochemical cycles in estimating standard oxidation potentials of organic compounds is outlined.Calculations show that the toluene radical cation is an extremely strong acid and the benzene radical cation a moderately strong acid in acetonitrile solution. Difficulties in directly determining the pKa value of aromatic hydrocarbon radical cations are also discussed.

Reduction of plutonium to the bivalent state

The existence of the bivalent state of plutonium and the standard oxidation potential of the Pu3+/Pu2+ couple, equal to −2.59 V, have been established for the first time.

Comparaison Des Valeurs Du Potentiel Normal Du Couple Cu(II)/Cu(I) Dans Les Chlorures Alcalins Fondus, Obtenus Par Spectrophotométrie D'Absorption Et Par Polarographie

AbstractThe standard oxidation potentials of Cu(II)/Cu(I) in three alkali chloride melts at 450°C have been determined by absorption spectrophotometry and by pulse polarography; the values obtained are discussed and compared with those found in the literature.

The standard oxidation potentials of the (II–III) and (III–IV) redox systems of the lanthanides and the actinides in non-aqueous solvents

The standard oxidation potentials of the (II–III) and (III–IV) redox systems of the lanthanides and the actinides in non-aqueous solvents

ChemInform Abstract: OXIDATION OF CARBON MONOXIDE OVER TRANSITION METAL ION‐ZEOLITE CATALYSTS

Adsorption of oxygen and oxidation of carbon monoxide was investigated on several X and Y type zeolites, their Na+ being replaced by transition metal ions. Oxygen adsorption was observed on Cr(II), Cu(I), and Fe(II) ions whose oxidation potentials are high, giving an oxygen atom to metal ion ratio of 0.43–0.62 or nearly 0.5. A small amount of oxygen was adsorbed on Tl(I), Co(II), and Mn(II), as expected from their standard oxidation potentials. No oxygen was adsorbed on Cu(II) and Ni(II), their valences being difficult to increase. The activities of these transition metal ions for oxidation of carbon monoxide increased exponentially with the increase of their standard oxidation potential. Kinetic studies on CO oxidation over Fe(II)–X and –Y zeolites have been made, and a Rideal mechanism is suggested where CO molecules in the gas phase attack the oxygen atoms dissociatively adsorbed in the bridged form of Fe3+–O2–Fe3+. The difference in the activities of Fe(II)–X and –Y catalysts is discussed.