Standard Oxidation Potentials Research Articles

Theoretical Study of One-Electron Reduction And Oxidation Potentials of N-Heterocyclic Compounds

Computational protocols that successfully predict standard reduction potentials of N-heterocyclic compounds in dimethyl formamide and their standard oxidation potentials in acetonitrile were developed. Different solvation models were verified in conjunction with the MPWB1K/6-31 + G(d) level of density functional theory. For reduction potentials calculations, the PCM(UA0) and SMD(Bondi) models were used to compute solvation energies of neutral forms and anion-radical forms, respectively. For oxidation potential calculations, the best results were obtained by a combination of SMD(UAHF) and PCM(Bondi) models to compute solvation energies of neutral forms and cation-radical forms, respectively. The mean absolute deviations (MAD) and root mean square errors (RMSE) of the current theoretical models for reduction potentials were found to be 0.09 V and 0.10, respectively, and for oxidation potentials MAD = 0.12 V and RMSE = 0.16.

Chain extended vinyl aniline modified cyclohexanone formaldehyde resin

PurposeThe purpose of this paper was as follows. The in situ chemical oxidation (ICO) of vinyl aniline modified cyclohexanone formaldehyde resin (CFVAnR) in the presence of potassium persulphate salt (K2S2O8) was accomplished in dimethyl sulphoxide (DMSO) solutions at 35°C and 70°C. The chemical composition and structure of the oxidized CFVAnR were characterised by nuclear magnetic resonance (1H‐NMR) spectroscopy, Fourier transform infrared (FT‐IR) spectroscopy, DSC, TGA and SEM.Design/methodology/approachThe reactive vinyl and amine groups of the cyclohexanone formaldehyde resin were reacted in the presence of potassium persulphate. CFVAn resin was dissolved in DMSO inside a round bottom flask immersed in a water bath thermostated at reaction temperature of 35°C and 70°C, respectively. Solution of K2S2O8 was prepared in DMSO and this was added into the resin solution.FindingsIt was shown that the vinyl groups in the resin were reacted easily with persulphate salt to obtain chain extended CFVAnR (CECFVAnR). The oxidised 4‐vinyl aniline modified cyclohexanone‐formaldehyde resin has solublity in organic solvents.Research limitations/implicationsThe reaction mixture must be stirred continuously. Temperature should be controlled to prevent the thermal polymerisation of vinyl group and higher branching of amino groups. The persulphate ion (S2O82−) has a standard oxidation potential, but when activated by heat (40‐60°C). Furthermore, sulphate radical can react with water or hydroxide to produce hydroxyl radical (−OH). When persulphate is used for in situ chemical oxidation (ISCO) application at relatively low temperatures (e.g. <20°C), the oxidation reactions are usually less aggressive due to a slow generation rate of SO4−.Practical implicationsThis study provides technical information for the synthesis of chain extended resins. The modified resins contain vinyl groups. The chemical radical system has been used to polymerise these vinyl groups and resins with much higher molecular weight might be produced. The resins may also promote the adhesive strength of a coating and corrosion inhibition to metal surfaces of a coating.Social implicationsThis resin will be used for the preparation of AB‐ and ABA‐type block copolymers. These block copolymers may exhibit different properties due to incorporation of ketonic resins into the block copolymer structure.Originality/valueChain extended CFVAnR (CECFVAnR) was synthesised by one step reduction‐oxidation reaction, at 35°C (CECFVAnR1) and 70°C (CECFVAnR2) in the presence of potassium persulphate salt. These soluble resins may overcome difficulties in the applications of polymers and open new application areas. Therefore, the chain extended vinyl aniline modified resin may find a number of new application areas as well as existing UV curable resin and polymer applications.

Theoretical study of ionization and one‐electron oxidation potentials of N‐heterocyclic compounds

A number of density functionals was utilized to predict gas-phase adiabatic ionization potentials (IPs) for nitrogen-rich heterocyclic compounds. Various solvation models were applied to the calculation of difference in free energies of solvation of oxidized and reduced forms of heterocyclic compounds in acetonitrile (AN) for correct reproduction of their standard oxidation potentials. We developed generally applicable protocols that could successfully predict the gas-phase adiabatic ionization potentials of nitrogen-rich heterocyclic compounds and their standard oxidation potentials in AN. This approach is supported by a MPW1K/6-31+G(d) level of theory which uses SMD(UA0) approximation for estimation of solvation energy of neutral molecules and PCM(UA0) model for ionized ones. The mean absolute derivation (MAD) and root mean square error (RMSE) of the current theoretical models for IP are equal to 0.22 V and 0.26, respectively, and for oxidation potentials MAD = 0.13 V and RMSE = 0.17.

Functionalization of glassy carbon surface by means of aliphatic and aromatic amino acids. An experimental and theoretical integrated approach

Glassy carbon (GC) electrode surfaces are functionalized through electrochemical assisted grafting, in oxidation regime, of six amino acids (AA): β-alanine (β-Ala), l-aspartic acid (Asp), 11-aminoundecanoic acid (UA), 4-aminobenzoic acid (PABA), 4-(4-amino-phenyl)-butyric acid (PFB), 3-(4-amino-phenyl)-propionic acid (PFP). Thus, a GC/AA interface is produced featuring carboxylic groups facing the solution. Electrochemical (cyclic voltammetry and electrochemical impedance spectroscopy) and XPS techniques are used to experimentally characterize the grafting process and the surface state. The theoretical results are compared with the experimental evidence to determine, at a molecular level, the overall grafting mechanism. Ionization potentials, standard oxidation potentials, HOMO and electron spin distributions are calculated at the CCD/6-31G* level of the theory. The comparison of experimental and theoretical data suggests that the main electroactive species is the “zwitterionic” form for the three aliphatic amino acids, while the amino acids featuring the amino group bound to the phenyl aromatic moiety show a different behaviour. The comparison between experimental and theoretical results suggests that both the neutral and the zwitterionic forms are present in the acetonitrile solution in the case of 4-(4-amino-phenyl)-butyric acid (PFB) and 3-(4-amino-phenyl)-propionic acid.

Topochemical Oxidation of Transition Metals in Layered Double Hydroxides by Anthraquinone-2-sulfonate

colored solid precipitates for the Co-Fe-LDH and dark yellow for the Ni-Fe-LDH, respectively, as shown in the insets of Figure 1. The color of the starting CoCl2-FeCl2 solution was pink due to colorless ferrous ions dissolved. After refluxing with the AQS2, the pink color of the solution changed to a brownish one, suggesting the existence of ferric ions, and finally the brown solid precipitates were yielded. Without the AQS2, pink colored precipitates of brucite-like Co 2+ -Fe 2+ hydroxides were obtained as mentioned in a previous report. 5 The results strongly suggest that the redox reactions between Fe 2+ and AQS2 have been involved in the coprecipitation by the HMT hydrolysis. The oxidation potential of AQS2 2− /AQS2 was measured as −0.60 V (vs Ag/AgCl), 6 which is probably capable of oxidizing the Fe 2+ ions in Fe(OH)2 phase because of +0.58 V of standard oxidation potential in Fe(OH)3/Fe(OH)2. 7 According to our previous report for hydrotalcite-like α-cobalt hydroxide containing AQS2, 8 the AQS2 did not oxidize the Co 2+ ions under similar reaction conditions. In the present study, Fe 2+ was oxidized into Fe 3+ , resulting in the Co 2+ -Fe 3+ hydroxide layers forming the LDH structures. Figure 1 shows typical SEM images of the Co-Fe-LDH and the Ni-Fe-LDH samples, consisting of uniform platelets with an average lateral size of 0.5 μm and a thickness of ~70 nm for both samples. Wellcrystallized LDH platelets in SEM inspections further revealed little impurity such as gel-like Fe(OH)3 phase, often observed in alkaline solution syntheses in the presence of oxygen. 4

Studies on synthesis of 5,5'-distyryl-2,2'-bithiophene derivatives and their optical and electrochemical properties

Pi-Conjugated bithiophene derivatives are expected to show remarkable optical and electrochemical properties owing to the characteristic electronic state. Recently, we reported that Pd-catalyzed C-H homocoupling of thiophene derivatives smoothly occurred in the presence of AgN03 and KF, and the corresponding bithiophene derivatives were obtained in high yields. In this study, the re-conjugated bithiophene derivatives having aryl groups symmetrically through carbon-carbon double bonds were synthesized by the C-H homocoupling reaction and Wittig reaction. In order to clarify the influence of terminal functional groups to physical properties, energy gaps were estimated from the onset of absorption in the UV -vis spectrum and quantum yields of fluorescence were calculated on the basis of fluorescence spectra. Furthermore, standard oxidation potentials were determined by cyclic voltammetry and the electron properties were calculated by density functional theory method.

Crown Ether Vinylogous Tetrathiafulvalene Receptors: Complexation Interference on the Molecular Movements Triggered by Electron Transfer

The synthesis of a series of crown ether substituted vinylogous tetrathiafulvalenes (TTFVs) has been carried out through oxidative coupling of bisdithiafulvenes. These new receptors have been fully characterized using electrochemical, spectroelectrochemical, and molecular modeling experiments. These studies show that, upon oxidation, either a clip movement (TTFVs 3a,b) or a stretch movement (TTFV 3c) occurs, depending on the length of the crown ether chains. Preliminary electrochemical studies, undertaken on TTFV 3c in dichloromethane, show a little shift of the first standard oxidation potential toward more positive values upon addition of the Pb(2+) ion, but a considerable variation of the electron-transfer kinetics. This result introduces an interesting concept for the preparation of sensors not based on thermodynamic variations but on kinetic modifications of the electron transfer.

Mechanistic investigation of N-thioamidoimidates: Oxidation by heterogeneous and homogeneous electron transfers

Electrochemical study of N-thioamidoimidates 1 is carried out in non-aqueous solvent at platinum electrodes by means of cyclic voltammetry at different scan rates. A mechanistic investigation shows that the oxidation of the NH bond of these substrates leads, during macroscale electrolysis, to oxadiazole-3-thione 2 with satisfactory yields according to an ECE mechanism. On the other hand, the rate of deprotonation k H and the standard oxidation potential of the N-thioamidoimidate cation radical/ N-thioamidoimidate couple were derived directly and indirectly by the use of the homogenous redox catalysis. The perfect agreement of the predicted kinetic with the experiment data both under homogeneous and heterogeneous electron transfer provides further support for the ECE mechanism.

Evaluation of antioxidants using oxidation reaction rate constants

An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of the antioxidants were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed pseudo zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34 ± 0.03) × 10−3 at pH 6.8, (1.20 ± 0.02) × 10−3 at pH 4.0 and (6.58 ± 0.02) × 10−3 mol·L−1·h−1 at pH 9.2, respectively.

Calculations of Oxidation Potentials of Redox Shuttle Additives for Li-Ion Cells

The oxidation potentials of seventeen molecules used as candidate shuttle additives in Li-ion cells were calculated using density functional theory and compared with experiment. The root-mean-square deviation between the calculated and measured oxidation potentials of these seventeen molecules is with the maximum deviation being , indicating that the ab initio calculation is in good agreement with the experiment. Neglecting thermal contributions in the calculation of standard oxidation potentials at ambient conditions does not lead to significant errors. An empirical relation between the oxidation potentials and the orbital energies of these molecules in solution is presented. The oxidation potential of a molecule could be estimated based on the orbital energy of the molecule’s highest occupied molecular orbital or its cation’s lowest unoccupied molecular orbital in solution with an error less than for most molecules reported.

Theoretical Determination of Standard Oxidation and Reduction Potentials of Chlorophyll-ain Acetonitrile

QM/MM calculations were performed on ethyl chlorophyllide-a and its radical cation and anion, by using the density functional (DF) B3LYP method to determine the molecular characteristics, and a molecular mechanics (MM) method to simulate the solvating medium. The presence of the solvent was accounted for during the optimization of the geometry of the 85-atom chlorophyll-a system by using an ONIOM methodology. A total of 24 solvent molecules were explicitly considered during the optimization process, and these were treated by the universal force field (UFF) method. Initially, the split-valence 3-21G basis set was used for optimizing the geometry of the 85-atom species, neutral, cation and anion. Electronic energies were then determined for the optimized species by making use of the polarized 6-31G(d) basis set. The ionization energy calculated (6.0 eV) is in very good agreement with the observed one (6.1 eV). The MM+ force field was used to investigate the dynamics of the acetonitrile molecules around the neutral species as well as the radical ions of chlorophyll. The required atomic charges on all the atoms were obtained from calculations on all involved molecules at the DFT/6-31G(d) level. Randomly sampled configurations were used to determine the first solvation layer contribution to the free energy of solvation of various species. A truncated 46-atom model of ethyl chlorophyllide-a was used to evaluate the thermal energies of neutral chlorophyll molecule relative to its two radical ions in the gas phase. Born energy, Onsager energy, and the Debye-Huckel energy of the chlorophyll-solvent aggregate were added as perturbative corrections to the free energy of solvation that was initially obtained through molecular dynamics method for the same complex. These calculations yield the oxidation potential as 0.75 +/- 0.32 V and the reduction potential -1.18 +/- 0.31 V at 298.15 K. The calculated values are in good agreement with the experimental midpoint potentials of +0.76 and -1.04 V, respectively.

Oxidation by heterogeneous and homogeneous electron transfer of N-amidoimidates: mechanistic investigation

Anodic oxidation of N-amidoimidates 1 was investigated in acetonitrile at a platinum electrode by means of cyclic voltammetry at low and high scan rates. The electron transfer at the electrode or in solution by homogeneous redox catalysis using organic catalysts yielded the cation radical, which decays by deprotonation of the NH bond to give a radical. The rate of deprotonation k H and the standard oxidation potential of the N-amidoimidate cation radical/N-amidoimidate couple were derived from both techniques. The agreement of the predicted kinetics with the experiment data both under homogeneous and heterogeneous electron transfer conditions provides further support for the ECE mechanism.

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates.

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.

Potentiometric, spectrophotometric and density functional study of the interaction of N-hydroxyacetamide with oxovanadium(IV): the influence of ligand to the V(IV)/V(V) oxi-reduction reaction

The interaction of N-hydroxyacetamide (acetohydroxamic acid, HL) with V(IV) in aqueous solution has been investigated using potentiometric and spectrophotometric experiments. Density functional method (DFT) has been used aiming to understand the ligand chelation at a molecular level. Stability constants have been estimated for species with the metal/ligand ratios 1:1 and 1:2 from spectrophotometric and potentiometric measurements. The stability of these V(IV) species toward oxidation has been investigated. Experiments carried out in an oxygen atmosphere led to the displacement of the titration curves with respect to the one obtained under inert atmosphere. Spectrophotometric evidence of the V(IV)/V(V) oxidation in the presence of N-hydroxyacetamide is presented. It has been shown that V(IV)/V(V) oxidation in the presence of N-hydroxyacetamide by the oxygen can be simulated using the standard programs for simulating the equilibrium in a multiligand/multimetal system. In this approach, the oxygen is considered a ligand and the log β estimated from the standard oxidation potential. The structure and respective tautomers of the species have been optimized from DFT calculations. Geometrical and thermodynamical properties have been estimated for the most stable complexes. The VOL→VOL 2 equilibrium constant has been theoretically estimated with a less than 1.5 logarithmic unit of error with respect to the experimental estimate. The oxidation process has also been investigated and it is adequately described by the equation: 4[VOL 2]+2H 2O+O 2→4[VO(OH)L 2]. The calculated value of Δ G for this reaction is about −46.2 kcal mol −1, in excellent agreement with the experimental estimates.

Field assessment of nanoscale bimetallic particles for groundwater treatment.

A field demonstration was performed in which nanoscale bimetallic (Fe/Pd) particles were gravity-fed into groundwater contaminated bytrichloroethene and other chlorinated aliphatic hydrocarbons at a manufacturing site. With diameters on the order of 100-200 nm, the nanoparticles are uniquely suited to rapidly degrade redox-amenable contaminants and for optimal subsurface delivery and dispersion. Approximately 1.7 kg of the nanoparticles was fed into the test area over a 2-day period, resulting in minimal clogging of the injection well. The test area was located within a well-characterized region of the contaminant plume and included an injection well and three piezometer couplets spaced 1.5 m apart. Despite the low nanoparticle dosage, trichloroethene reduction efficiencies of up to 96% were observed over a 4-week monitoring period with the highest values observed at the injection well and adjacent piezometers. Data from the field assessment were consistent with the results of pre-injection laboratory studies, which showed rapid dechlorination of target chlorinated compounds accompanied by a sharp decrease of standard oxidation potential and an increase in pH.

An estimation of the ionization potentials of actinides from a simple dependence of the aqueous standard potentials on the ionization potentials of elements including lanthanides

It was found recently that the ionization potentials and electron affinities of elements in the gaseous state are linked to the standard potentials of their principal oxidation states in aqueous solutions by simple linear relations. These relations are characteristic for each group of elements of the periodic table. In the case of the actinides, in the absence of adequate data on their ionization potentials, the linear relation obtained for the group IIIB elements (including lanthanides) has been found useful for estimating them using the available data on their standard oxidation potentials (obtained in most cases by radio-polarography).

Destruction of aniline by mediated electrochemical oxidation with Ce(IV) and Co(III) as mediators

Mediated electrochemical oxidation has been employed to test the feasibility of treating soluble organic wastes. We report Ce(IV)- and Co(III)-mediated electrochemical oxidation of aniline at various electrodes in acidic media as an example of organic waste. Aniline was oxidized by an electrogenerated electron transfer mediator, Ce4+ or Co3+, in the anolyte and carbon dioxide was produced as a final oxidation product. Carbon dioxide was collected by bubbling through a barium hydroxide solution. When a powerful oxidizing agent, Ce(IV) or Co(III), was used as an electron shuttling mediator, parameters affecting the coulombic efficiency for aniline oxidation were the standard oxidation potentials of the mediators, their concentrations and the reaction temperature. Intermediate species produced during the oxidation of aniline were identified by cyclic voltammetric and absorption spectroscopic measurements.

Electropolymerization mechanism of poly(dibenzylidene)tetrathiapentalene

The electropolymerization mechanism of (2,5-dibenzylidene)-1,3,4,6-tetrathiapentalene (3) has been investigated by cyclic voltammetry. Standard oxidation potential and lifetime of radical-cation intermediate (RC) have been determinated by fast scan voltammetry. The initial steps ot the polymerization involve coupling of two RC to the corresponding dimeric dication, as evidenced by the variation of peak potentials with the scan rate.

Uranium in geologic fluids: Estimates of standard partial molal properties, oxidation potentials, and hydrolysis constants at high temperatures and pressures

Theoretical methods are used with the available experimental data to provide estimates of parameters for the revised-HKF equations of state for aqueous uranium species. These parameters are used with standard state thermodynamic data at 25°C and 1 bar to calculate equilibrium constants for redox reactions among the four most common oxidation states of uranium (U(III), U(IV), U(V), and U(VI)), and their hydrolysis reactions at temperatures to 1000°C and pressures to 5 kb. A total of nineteen aqueous uranium species are included. The predicted equilibrium constants are used to construct oxidation potential-pH diagrams at elevated temperatures and pressures and to calculate the solubilities of uraninite as functions of temperature and pH, which are compared to experimental data. Oxidation potential-pH diagrams illustrate the relative stabilities of aqueous uranium species and indicate that U (IV) and U(VI) species predominate in aqueous solution in the U-O-H system. Increasing temperature stabilizes U (VI) and U (III) species relative to U (IV) species, but U (IV) species dominate at oxidation states consistent with mineral-buffer assemblages and near-neutral pH. At low pH, U(VI) is stabilized relative to U (IV) suggesting that uranium transport in hydrothermal systems requires either acidic solutions or potent complexes of U(IV).

Photophysics of Supercomplexes. Adduct between Ru(bpy)(CN)(4)(2-) and the [32]ane-N(8)H(8)(8+) Polyaza Macrocycle.

The formation of a supercomplex between the Ru(bpy)(CN)(4)(2-) (bpy = 2,2'-bipyridine) complex and the [32]ane-N(8)H(8)(8+) macrocycle (1) has been studied in water and in acetonitrile. In acetonitrile, supercomplex formation is accompanied by (i) large hypsochromic shifts in the absorption spectrum (color changes from deep violet to yellow) and in the emission spectrum, (ii) large anodic shifts in standard oxidation (0.73 V) and reduction (0.37 V) potentials, (iii) typical shifts of (1)H-NMR signals for the macrocycle N-bound protons and the complex bipyridine protons, and (iv) a large increase in the MLCT excited-state lifetime of the complex. In water, the spectral shifts and the changes in standard potential are much less pronounced, but supercomplex formation is evidenced by (13)C-NMR (and (1)H-NMR) and by emission lifetime changes. In both solvents, supercomplex formation is complete in 1:1, 1.0 x 10(-4) M solutions, indicating very large stability constant values. A structure of the supercomplex with the macrocycle bound in a "boat" conformation to the four cyanide ligands of the complex, plausible in terms of molecular models, is consistent with all the experimental data. In water, the supercomplex further associates with added negative species containing carboxylate functions, as shown by partial reversal of the lifetime changes. When the added species is also a potential electron transfer quencher (such as, e.g., Rh(dcb)(3)(3-), dcb = 4,4'-dicarboxy-2,2'-bipyridine), however, association is not accompanied by quenching. This behavior is attributed to the structure of the supercomplex-quencher adduct, in which the macrocycle acts as an insulating spacer between the excited complex and the quencher.