Experimental and computed absolute redox potentials of polycyclic aromatic hydrocarbons are highly linearly correlated over a wide range of structures and potentials

A more rigorous theoretical treatment of methods previously used to correlate computed energy values with experimental redox potentials, combined with the availability of well-developed computational solvation methods, results in a shift away from computing ionization potentials/electron affinities in favor of computing absolute reduction potentials. Seventy-nine literature redox potentials measured under comparable conditions from 51 alternant and nonalternant polycyclic aromatic hydrocarbons are linearly correlated with their absolute reduction potentials computed by density functional theory (B3LYP/6-31+G(d)) with SMD/IEF-PCM solvation. The resulting correlation is very strong (R(2) = 0.9981, MAD = 0.056 eV). When extrapolated to the x-intercept, the correlation results in an estimate of 5.17 ± 0.01 eV for the absolute potential of the ferrocene-ferrocenium redox couple in acetonitrile at 25 °C, indicating that this simple method can be used reliably for both calculating absolute redox potentials and for predicting relative redox potentials. When oxidation and reduction data are evaluated separately, the overall MAD value is improved by 50% to 0.028 eV, which improves relative potential predictions, but the computed values do not extrapolate to a reasonable estimate of the absolute potential of the ferrocene-ferrocenium ion reference.     

Synthesis of Ferrocene Derivatives Allowing Linear Free Energy Studies of Redox Potentials

A series of ferrocene derivatives, which have diverse redox potentials modulated by functional groups, have been synthesized as potential ‘multi‐potential’ probes. A Hammett constant analysis revealed a linear free energy correlation between the redox potentials and the electron density of the ferrocene derivatives as determined by the choice of functional group used to modify the ferrocene core.     

第一性原理方法预测水相核酸碱基及其代谢物的氧化还原电动势

氧化还原电动势是了解核酸中电荷/电子转移过程以及设计具有新型氧化还原活性的碱基类化合物的重要参数. 本文对82个芳香化合物的氧化还原电动势进行理论预测, 通过计算值和实验值的比较发现: 气相采用B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d)方法, 液相采用HF-COSMORS/UAHF方法, 对运用HF- CPCM/UAHF方法在水相重新优化的构型计算溶剂化能, 能有效预测芳香化合物水相氧化还原电动势, 该理论方法计算的绝对均方根误差(RMSD)为0.124 V. 运用该理论方法成功预测了属于芳香化合物的核酸碱基及其代谢物的水相氧化还原电动势. 根据预测结果, 讨论了核酸中电荷/电子转移过程以及结构改变对设计具有新型氧化还原活性的核酸碱基类化合物的影响. 本文为设计具有氧化还原活性的新型核酸碱基类化合物提供了一种理论方法.     

Dications of fluorenylidenes. Relationship between electrochemical oxidation potentials and antiaromaticity in diphenyl-substituted fluorenyl cations

The antiaromaticity of a series of dications of p-substituted diphenylmethylidene fluorenes was explored using three criteria attributed to aromaticity/antiaromaticity. The relative stability of the dications (energetic criterion) was measured via the redox potentials obtained by electrochemical oxidation under very fast sweep rates with microelectrodes. Comparison of redox potentials with those of a model system, p-substituted tetraphenylethylenes, shows relatively small destabilization of the potentially antiaromatic fluorenylidene dication. However, the amount of destabilization is comparable with the limited electrochemical data available for other antiaromatic systems. Nucleus independent chemical shifts (NICS) were calculated for these dications (magnetic criterion) and indicated their antiaromaticity. A good linear relationship between experimental and calculated (B3LYP/6-31G(d)) (1)H and (13)C NMR shifts for the three dications, 3c, 3e, and 3f, for which NMR data has been reported, validated the accuracy of the NICS values. Bond length alternation/elongation (structural criterion) was explored via the harmonic oscillator model of aromaticity (HOMA) using the geometries calculated with density functional theory, but there was insufficient variation to evaluate relative antiaromaticity. In addition, the presence of benzannulation appears to restrict bond length alternation to such an extent that the magnitude of the HOMA index is of little use in evaluating the antiaromaticity of many polycyclic hydrocarbons. Both NICS values and redox potentials for formation of the dication in these systems show a strong linear correlation with sigma(p)(+) values, with the more antiaromatic fluorenylidene dication possessing the more electron-withdrawing substituent. The correlation between NICS values and redox potentials is also good, as might be expected, suggesting a strong relationship between magnetic and energetic characteristics of antiaromaticity. However, magnetic characteristics appear to be a more sensitive probe than energetic characteristics evaluated through redox potentials or structural characteristics evaluated through HOMA calculations.     

Nitroxyl radicals: electrochemical redox behaviour and structure-activity relationships

Comparative study of electrochemical redox behaviour of five different nitroxyl radicals leads to the direct correlation between one-electron redox potentials and group electronegativity of the beta-substituent on the ring. Beta-substituents with an electron-donating effect caused a negative shift in the one-electron oxidation and one-electron reduction potentials of the nitroxyl radicals. In a similar aspect, beta-substituents with an electron-withdrawing effect behaved oppositely.     

Oxidation of nitroxyl radicals: electrochemical and computational studies

We have developed azaadamantane-type nitroxyl radicals (AZADOs) and azabicyclo-type nitroxyl radicals (ABNOs) as highly active alcohol oxidation catalysts. Herein, the electrochemical properties of these nitroxyl radicals were investigated by measuring their formal potentials using cyclic voltammetry (CV). The redox potentials were rationalized with the aid of density functional theory (DFT) calculations. A good correlation between the experimental redox potential and the DFT-computed energy differences (ΔE) between nitroxyl radicals and oxoammonium species was obtained, which shows the effectiveness of DFT in predicting the redox potentials of nitroxyl radicals. Redox potential appears to be an important factor of catalytic activity.     

Voltammetric monitoring of laccase-catalysed mediated reactions

Six different compounds capable of mediating laccase-catalysed reactions have been tested by cyclic voltammetry. They exhibited quasi-reversible electrodic behaviour with formal redox potentials ranging from 150 to 800 mV (E(0)' vs. SCE). The immersion of a laccase-coated glassy carbon electrode (GCE) in mediator solutions generated large cathodic catalytic currents easily recorded by cyclic voltammetry at low-potential scan rates. This current showed two well-defined pH profiles, which correlated with the variation of the mediator redox potentials at the pH range tested. The relevant effect of temperature on the activity of laccase has been assessed here. Likewise, it was shown that the current record varied with the substrate concentration. This trend fitted Michaelis-Menten kinetics, which allowed us to give an estimation of the affinity of the fungal laccase for the different mediators.     

Electronic properties of Si surfaces and side reactions during electrochemical grafting of phenyl layers

The electrochemical grafting process of 4-nitrobenzene and 4-methoxybenzene (anisole) from diazonium salt solutions has been investigated in situ by monitoring the current density, the band bending, and the nonradiative surface recombination during grafting at different potentials and different concentrations of the diazonium salt in the solution. Ex situ infrared spectroscopic ellipsometry has been used to inspect the Si surface species before and after the grafting process. The band bending decreases with either increasing concentration of diazonium salt or when the redox potential of the diazonium compound (anisole) is nearer to the competing H+/H2 couple. The surface recombination increases at more cathodic potentials if an electron donor group is present at the phenyl ring (nitrobenzene) and vice versa for the electron acceptor group (anisole). The influence of side reactions can be reduced by use of moderate concentration and moderate or strong cathodic potential, depending on the redox potential of the diazonium compound.     

Theoretical study on redox potentials of organic radicals in different solvents

The BMK density functional theory method has been used to examine the redox potentials of organic radicals in different solvents (DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; MeCN, acetonitrile). The polarizable continuum solvation model (PCM) was used to describe the solvation-free energies. The one-electron electrochemical standard potentials (E ⁰) of ca. 100 organic radicals in three solvents were calculated using a single, unified theoretical method whose reliability has been tested against almost all the available experimental data. It was found that the mean absolute deviation (MAD) between the theory and experiment was about 0.08 V. With the newly developed theoretical method in hand, more redox potentials of organic radicals in these three solvents were predicted by this single, unified method. The results showed that the redox potentials of organic radicals in different organic solvents including DMF and DMSO had good correlations with their redox potentials in MeCN.     

Singling out the electrochemistry of individual single-walled carbon nanotubes in solution

Bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs has given access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the determination of the redox properties of SWNTs as individual entities. Here we report an extensive voltammetric and vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs and determine the standard electrochemical potentials of reduction and oxidation as a function of the tube diameter of a large number of semiconducting SWNTs. We also establish the Fermi energy and the exciton binding energy for individual tubes in solution. The linear correlation found between the potentials and the optical transition energies is quantified in two simple equations that allow one to calculate the redox potentials of SWNTs that are insufficiently abundant or absent in the samples.     

Electrochemical behavior of α1/α2-[Fe(H2O)P2W17O61](7-) isomers in solution: experimental and DFT studies

The unusual redox behavior displayed by the two isomers of the Wells-Dawson phosphotungstate anion [Fe(H(2)O)P(2)W(17)O(61)](7-) is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK(a) of the electrolyte on POM-based redox potentials given the pK(a) of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers.     

Promethazine hydrochloride as a new redox indicator in vanadametry

Promethazine hydrochloride is proposed as a new redox indicator in vanadametry. It has been tested rigorously in the titration of iron(II), hydroquinone, uranium(IV) and antimony(III) with sodium vanadate. The indicator gives a very sharp reversible colour change from green to violet at the equivalence point. It has advantages over all the proposed redox indicators in vanadametry. Its redox and transition potentials are reported.     

Redox reactions on columns

In a study of redox reactions on columns, the work of Cerrai and Testa has been extended. It has been shown that insolubility of the redox compound is the most important factor and that adsorptive forces play little or no part. A useful redox column method for the determination of iron and vanadium, which compares favourably with that using a Jones reductor column, has been developed. The redox potentials of several substituted hydroquinones have been measured.     

The voltammetric behaviour of solid 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles

The electrochemical behaviour of 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles, attached to a graphite electrode and adjacent to an aqueous electrolyte solution, has been studied by cyclic voltammetry. DPPH exhibits one reversible redox couple with a formal potential of 0.340 V versus Ag|AgCl (pH=7.0). At more positive potentials, a redox couple appears with a formal potential E_f=0.733 V versus Ag|AgCl. The oxidation at this potential is followed by an irreversible chemical reaction generating a product which gives a redox couple with a formal potential at 0.177 V versus Ag|AgCl. The reduction process of this couple is followed by a slow chemical reaction in the course of which DPPH is reformed.     

Nonlinear optical properties of open-shell polychlorotriphenylmethyl radicals

The second-order nonlinear optical (SONLO) response of a series of the highly chemically and thermally stable polychlorotriphenylmethyl monoradicals 1–8 has been studied by Hyper-Rayleigh Scattering. These radicals exhibit relatively high nonlinear optical (NLO) responses with β values ranging from 118×10⁻³⁰ to 755×10⁻³⁰ esu, which may result from an enhancement of the SONLO activities according to their open-shell electronic state. Hyperpolarizability data for radicals 1–8 have also been correlated with their redox potentials, correlation that has turned out to be linearly proportional either with the oxidation or the reduction potentials. These data have been justified in terms of molecular orbital energies. Finally we have successfully explored the capacity of radicals 1–8 to participate in a molecular switching array involving two main steps, an acid/base reaction and a redox process. The presence of open-shell character for this family of compounds makes them attractive candidates to design new multifunctional materials that would simultaneously possess magnetism and NLO.     

Redox Titrations-II Location of inflection points of titration curves for homogeneous redox reactions

The relative positions of the inflection points and equivalence point of a homogeneous redox reaction have been studied by using the redox buffer capacity to derive an equation for the titration curve. The position of the inflection point corresponding to the maximum slope of the titration curve relative to the equivalence point depends on the electron transfers of the analyte and the titrant (the stoichiometric coefficients of the reaction equation) and on the difference between the formal potentials of the redox couples in a more complicated way than has been described previously.     

Theoretical studies on the redox potentials of Fe dinuclear complexes as models for hydrogenase

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.     

Synthesis of crowded triarylphosphines carrying functional sites

4,4′-Diphosphinobiaryls and 1,3- and 1,4-borylphosphinobenzenes carrying crowded triarylphosphine moieties were synthesized by reaction of the corresponding diarylchlorophosphine with an arylcopper(I) reagent. Intramolecular interaction of the phosphorus redox center with the other phosphorus or the boron redox center was investigated by cyclic voltammetry. 4,4′-Diphosphinobiaryls displayed two-step reversible redox waves with slight differences of the oxidation potentials due to weak interaction between two phosphorus redox centers across 4,4′-biarylene linkage. Borylphosphinobenzenes showed two step redox waves corresponding to oxidation at the phosphorus and reduction at the boron. Although significant interaction between the phosphorus and boron redox centers was not observed in the cyclic voltammograms due to large difference of the redox potentials between phosphorus and boron redox centers, an absorption due to weakly interacting phosphorus and boron was observed in the UV-Vis spectrum of the 1,4-borylphosphinobenzene.     

Synthesis and in situ FTIRS characterization of conducting polymers obtained from aminobenzoic acid isomers at platinum electrodes

The electrochemical homo-polymerization of o-, m- and p-aminobenzoic acids has been performed on Pt electrodes in perchloric acid aqueous medium by cyclic scanning of the potential. Different limit potentials were employed to obtain thin polymeric films. When the switching potential was extended beyond the respective monomer oxidation peak, a rather degraded material was obtained. In situ FTIR spectroscopy has been used to characterize the redox response of films synthesized at the lower potential limits. Characteristic absorption features related with benzenoid and quinoid rings and different types of C-N bonds suggest the presence of redox processes similar to those undergone by the parent compound polyaniline. In addition, the existence of a chemical interaction between -COOH and -NH- groups in the reduced state of the three homopolymers studied can be suggested. Carbon dioxide has been detected at potentials higher than 1.0 V (RHE) irrespective of the polymeric material, thus indicating its degradation.     

The formal potentials and electrode kinetics of the proton/hydrogen couple in various room temperature ionic liquids

The Hydrogen evolution reaction has been quantitatively investigated at a Pt electrode in series of room temperature ionic liquids vs. Ag/Ag(+) redox couple. The measured formal potentials of the H(2)/H(+) (HNTf(2)) redox couple in each RTIL reveals a dependence on the nature of anion, suggesting significant interaction between proton and anion.