Inner-sphere Electron-transfer Reactions Research Articles

Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant.

Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting. Incomplete consideration of these points has led to discrepancies in interpretation of data from the literature. Our reanalysis of these data suggests that proton-transfer elementary reaction steps have a nonzero intrinsic free energy barrier, implying, in the parlance of Marcus theory, that there is non-negligible nuclear reorganization. Outcomes from our analyses are generalizable to inner-sphere electron-transfer reactions such as those involved in (photo)electrochemical fuel-forming reactions.

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions.

We report investigations of the deposition of nanometer-thick Ln(OH)3 films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)3 films are deposited from aqueous solutions of LaCl3 onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH-, which complexes with Ln3+, resulting in the precipitation of Ln(OH)3. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH3)63+ reduction, FcMeOH oxidation, and Fe(CN)64-/3- oxidation/reduction) were investigated at Ln(OH)3-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)3 films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H2O and O2. We further demonstrate that the thickness of La(OH)3 films can be controlled by anodic dissolution. Additionally, we show that La3+ lowers the supersaturation of dissolved H2 required to nucleate a stable nanobubble.

Voltammetric Mapping of Hydrogen Evolution Reaction on Pt Locally via Scanning Electrochemical Cell Microscopy.

The advancement in nanoscale electrochemical tools has offered the opportunity to better understand heterogeneity at electrochemical interfaces. Scanning electrochemical cell microscopy (SECCM) has been increasingly used for revealing local kinetics and the distribution of active sites in electrocatalysis. Constant-contact scanning and hopping scanning are the two commonly used modes. The former is intrinsically faster, whereas the latter enables full voltammetry at individual locations. Herein, we revisit a less used mode that combines the advantages of hopping and constant-contact scan, resulting in a faster voltammetric mapping. In this mode, the nanodroplet cell in SECCM maintains contact with the surface during the scanning and makes intermittent pauses for local voltammetry. The elimination of frequent retraction and approach greatly increases the speed of mapping. In addition, iR correction can be readily applied to the voltammetry, resulting in more accurate measurements of the electrode kinetics. This scanning mode is demonstrated in the oxidation of a ferrocene derivative on HOPG and hydrogen evolution reaction (HER) on polycrystalline Pt, serving as model systems for outer-sphere and inner-sphere electron transfer reactions, respectively. While the kinetics of the inner-sphere reaction is consistent spatially, heterogeneity is observed for the kinetics of HER, which is correlated with the crystal orientation of Pt.

Driving force dependence of inner-sphere electron transfer for the reduction of CO2 on a gold electrode.

The kinetics of the inner-sphere electron transfer reaction between a gold electrode and CO2 was measured as a function of the applied potential in an aqueous environment. Extraction of the electron transfer rate constant requires deconvolution of the current associated with CO2 reduction from the competing hydrogen evolution reaction and mass transport. Analysis of the inner-sphere electron transfer reaction reveals a driving force dependence of the rate constant that has similar characteristics to that of a Marcus-Hush-Levich outer-sphere electron transfer model. Consideration of simple assumptions for CO2 adsorption on the electrode surface allows for the evaluation of a CO2,ads/CO2 •- ads standard potential of ∼-0.75 ± 0.05 V vs Standard Hydrogen Electrode (SHE) and a reorganization energy on the order of 0.75 ± 0.10 eV. This standard potential is considerably lower than that observed for CO2 reduction on planar metal electrodes (∼>-1.4 V vs SHE for >10 mA/cm2), thus indicating that CO2 reduction occurs at a significant overpotential and thus provides an imperative for the design of better CO2 reduction electrocatalysts.

An insight into the outer- and inner-sphere electrochemistry of oxygenated single-walled carbon nanohorns (o-SWCNHs)

The electrocatalysis/interference of single-walled carbon nanohorns (o-SWCNHs) in relation to outer-sphere and inner-sphere electron transfer reactions.

Investigating the Control by Quantum Confinement and Surface Ligand Coating of Photocatalytic Efficiency in Chalcopyrite Copper Indium Diselenide Nanocrystals

In the past few years, there has been immense interest in the preparation of sustainable photocatalysts composed of semiconductor nanocrystals (NCs) as one of their components. We report here, for the first time, the effects of structural parameters of copper indium diselenide (CuInSe2) NCs on visible light-driven photocatalytic degradation of pollutants under homogeneous conditions. Ligand exchange reactions were performed replacing insulating, oleylamine capping with poly(ethylene glycol) thiols to prepare PEG-thiolate-capped, 1.8–5.3 nm diameter CuInSe2 NCs to enhance their solubility in water. This unique solubility property caused inner-sphere electron transfer reactions (O2 to O2•−) to occur at the NC surface, allowing for sustainable photocatalytic reactions. Electrochemical characterization of our dissolved CuInSe2 NCs showed that the thermodynamic driving force (−ΔG) for oxygen reduction, which increased with decreased NC size, was the dominant contributor to the overall process when compared to ...

An experimental and ab initio study on the abiotic reduction of uranyl by ferrous iron

It is important to understand the mechanisms controlling the removal of uranyl from solution from an environmental standpoint, particularly whether soluble Fe(II) is capable of reducing soluble U(VI) to insoluble U(IV). Experiments were performed to shed light into discrepancies of recent studies about precipitation of U-containing solids without changing oxidation states versus precipitation/reduction reactions, especially with respect to the kinetics of these reactions. To understand the atomistic mechanisms, thermodynamics, and kinetics of these redox processes, ab initio electron transfer (ET) calculations, using Marcus theory, were applied to study the reduction of U(VI)aq to U(V)aq by Fe(II)aq (the first rate-limiting ET-step). Outer-sphere (OS) and inner-sphere (IS) Fe–U complexes were modeled to represent simple species within a homogeneous environment through which ET could occur.Experiments on the chemical reduction were performed by reacting 1mM Fe(II)aq at pH 7.2 with high (i.e., 0.16mM) and lower (i.e., 0.02mM) concentrations of U(VI)aq. At higher U concentration, a rapid decrease in U(VI)aq was observed within the first hour of reaction. XRD and XPS analyses of the precipitates confirmed the presence of (meta)schoepite phases, where up to ∼25% of the original U was reduced to U4+ and/or U5+-containing phases. In contrast, at 0.02mM U, the U(VI)aq concentration remained fairly constant for the first 3h of reaction and only then began to decrease due to slower precipitation kinetics. XPS spectra confirm the partial chemical reduction U associated with the precipitate (up to ∼30%). Thermodynamic calculations support that the reduction of U(VI)aq to U(IV)aq by Fe(II)aq is energetically unfavorable. The batch experiments in this study show U(VI) is removed from solution by precipitation and that transitioning to a heterogeneous system in turn enables the solid U phase to be partially reduced.Ab initio ET calculations revealed that OS ET is strongly kinetically inhibited in all cases modeled. OS ET as a concerted proton-coupled ET reaction (ferrimagnetic spin configuration) is thermodynamically favorable (−35kJ/mol), but kinetically inhibited by concurrent proton-transfer (10−19s−1). OS ET as a sequential proton-coupled ET reaction is thermodynamically unfavorable (+102kJ/mol) as well as kinetically inhibited, where ET is the rate-limiting step (10−12s−1). In contrast, the reduction of U(VI)aq to U(V)aq by Fe(II)aq as an IS ET reaction is both thermodynamically favorable (−16kJ/mol) and kinetically rapid (108s−1); the IS ET rate is several orders of magnitude faster than the OS ET rate. Thus, reduction of U(VI)aq to U(V)aq by Fe(II)aq in a homogenous system could occur if an IS Fe–U complex can be achieved. However, the formation of IS Fe–U complexes in an homogeneous solution is predicted to be low; considerable thermodynamic and kinetic barriers exist to proceed from an OS ET reaction to an IS ET reaction, a process that needs to overcome dehydration of the first solvation shell (+96kJ/mol) and hydrolysis of Fe(II)aq. The computational results complement and further substantiate experimental results where the reduction of U(VI)aq by Fe(II)aq does not occur.

Mechanistic Study of Electrooxidation of Ethanol on PtSn Nanoparticles in Alkaline and Acid Media

In this study, we report on the structure-to-property relationship for PtSn ethanol electrooxidation catalysts in alkaline and acid environment. Relationship between surface chemistry, as determined by XPS and electrochemical performance in both acid and alkaline media, is facilitated by multivariate analysis. Upon transitioning from acidic to an alkaline environment, changes in the material structure and electrochemical reaction mechanisms are observed. Electrocatalysts containing larger particles with larger relative amounts of metallic Pt and Sn perform better in acid indicating inner-sphere electron transfer reaction on active PtSn alloy phase. PtSn electrocatalysts containing larger amounts of oxidized Pt and Sn perform better in alkaline indicating that hydroxyl species that are natively present on oxidized Pt and Sn are promoting an outer-sphere electron transfer.

Isoniazid metal complex reactivity and insights for a novel anti-tuberculosis drug design

For over a decade, tuberculosis (TB) has been the leading cause of death among infectious diseases. Since the 1950s, isoniazid has been used as a front-line drug in the treatment of TB; however, resistant TB strains have limited its use. The major route of isoniazid resistance relies on KatG enzyme disruption, which does not promote an electron transfer reaction. Here, we investigated the reactivity of isoniazid metal complexes as prototypes for novel self-activating metallodrugs against TB with the aim to overcome resistance. Reactivity studies were conducted with hydrogen peroxide, hexacyanoferrate(III), and aquopentacyanoferrate(III). The latter species showed a preference for the inner-sphere electron transfer reaction pathway. Additionally, electron transfer reaction performed with either free isoniazid or (isoniazid)pentacyanoferrate(II) complex resulted in similar oxidized isoniazid derivatives as observed when the KatG enzyme was used. However, upon metal coordination, a significant enhancement in the formation of isonicotinic acid was observed compared with that of isonicotinamide. These results suggest that the pathway of a carbonyl-centered radical might be favored upon coordination to the Fe(II) owing to the π-back-bonding effect promoted by this metal center; therefore, the isoniazid metal complex could serve as a potential metallodrug. Enzymatic inhibition assays conducted with InhA showed that the cyanoferrate moiety is not the major player involved in this inhibition but the presence of isoniazid is required in this process. Other isoniazid metal complexes, [Ru(CN)(5)(izd)](3-) and [Ru(NH(3))(5)(izd)](2+) (where izd is isoniazid), were also unable to inhibit InhA, supporting our proposed self-activating mechanism of action. We propose that isoniazid reactivity can be rationally modulated by metal coordination chemistry, leading to the development of novel anti-TB metallodrugs.

Volume Profile Analysis for the Reversible Binding of Superoxide to Form Iron(II)-Superoxo/Iron(III)-Peroxo Porphyrin Complexes

The one-electron reduced iron(II)-dioxygen adduct, {Fe(II)-O(2)}(-), is known to be an important intermediate in the catalytic cycle of heme (mono)oxygenases. The same type of species, considered as Fe(III)-peroxo, can be formed in a direct reaction between a Fe(II) center and superoxide. In a unique high-pressure study of the reaction between superoxide and the Fe(II) complex of a crown ether porphyrin conjugate in dimethylsulfoxide (DMSO), the overall Fe(II)-superoxide interaction mechanism could be visualized and the nature of all species that occur along the reaction coordinate could be clarified. The equilibrium between the low-spin and high-spin forms of the starting Fe(II) complex was quantified, which turns out to be the actual activation step toward substitution and subsequent inner-sphere electron transfer reactions. The constructed reaction volume profile demonstrates that the reaction product consists of Fe(III)-peroxo and Fe(II)-superoxo species that exist in equilibrium, which can better account for the versatile reactivity of {Fe(II)-O(2)}(-) adducts toward different substrates.

Reactions of aqueous iron–DFOB (desferrioxamine B) complexes with flavin mononucleotide in the absence of strong iron(II) chelators

The mechanisms controlling microbial uptake of Fe III–siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the Fe III center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of Fe III–DFOB by various reducing agents in solutions containing Fe II-chelating colorimetric agents for monitoring reaction progress, but the importance of these findings is unclear because the colorimetric agents themselves stabilize and enhance the reactions being monitored. This study examines the reduction of Fe III complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMN HQ) in the absence of strong Fe II-chelating agents, and Fe redox cycling in solutions containing DFOB and oxidized and reduced FMN species. Experimental results demonstrate that the rate and extent of Fe III–DFOB reduction is strongly dependent on pH and FMN HQ concentration. At pH ⩾ 5, incomplete Fe III reduction is observed due to two processes that re-oxidize Fe II, namely, the autodecomposition of Fe II–DFOB complexes (Fe II oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of Fe II–DFOB complexes with the fully oxidized flavin mononucleotide product (FMN OX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse Fe II oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state Fe II concentrations observed over a range of pH conditions in the presence of excess FMN HQ and FMN OX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of Fe III/Fe II and FMN OX/FMN HQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated Fe III–DFOB and Fe II–DFOB complexes, where protonation of one hydroxamate group opens up two Fe coordination positions. This finding suggests that ternary complex formation (FMN–Fe–DFOB) facilitates inner-sphere electron transfer reactions between the flavin and Fe center.

Ligand and Substrate Effects in Gas‐Phase Reactions of NiX+/RH Couples (X=F, Cl, Br, I; R=CH3, C2H5, nC3H7, nC4H9)

The reactions of small saturated hydrocarbons by gaseous nickel cations NiX+ (X=F, Cl, Br, I) are investigated by means of electrospray ionization mass spectrometry. The halide cations are obtained from solutions of the corresponding Ni(II) salts in water or methanol as solvents. NiF+ is the only Ni(II) halide complex that brings about thermal activation of methane. The branching ratios of the observed reactions with C2H6, C3H8, and nC4H10 are shifted systematically by changing the nature of both the ligand X and the substrate RH. In the elimination of HX (X=F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going from NiF+ to NiI+. A reversed trend is observed in the losses of small closed-shell neutral molecules, that is, H2, CH4 and C2H6, which dominate the gas-phase ion chemistry of NiI+/RH couples. Additionally, inner-sphere electron-transfer reactions take place for a few systems, that is, the delivery of hydride or methanide ions from the hydrocarbon to NiX+ in the course of which the hydrocarbon is converted to a carbenium ion and the cationic metal complex gives rise to a neutral RNiX molecule (R=H, CH3). This process gains importance with decreasing atomic number of the halides and with increasing the size of the alkane. Thus, it constitutes the major pathway in the reactions of NiF+ with propane and n-butane, whereas it is not observed for any of the NiI+/RH couples investigated. Concerning the regioselectivity of the reactions with propane and n-butane, heterolytic cleavage of secondary carbon--hydrogen bonds is clearly preferred compared to that of primary ones, as revealed by deuterium labeling studies. For the NiF+/C3H8 couple, the selectivity of the hydride transfer is as large as 360 in favor of the secondary positions. Though smaller, large preferences for the activation of secondary C--H bonds are also operative in homolytic bond activation of RH (R=nC3H7, nC4H9).

Mechanistic Analysis of Reductive Nitrosylation on Water-Soluble Cobalt(III)-Porphyrins

The reactions of NO and/or NO2- with three water-soluble cobalt porphyrins [Co(III)(P)(H2O)2]n, where P = TPPS, TCPP, and TMPyP, were studied in detail. At pH < 3, the reaction with NO proceeds through a single reaction step. From the kinetic data and activation parameters, the [Co(III)(P)(NO)(H2O)]n complex is proposed to be the primary product of the reaction with NO. This complex reacts further with a second NO molecule through an inner-sphere electron-transfer reaction to generate the final product, [Co(III)(P)(NO-)](n-1). At pH > 3, although a single reaction step is also observed, a systematic study as a function of the NO and NO2- concentrations revealed that two reaction steps are operative. In the first, NO2- and NO compete to substitute coordinated water in [Co(III)(P)(H2O)2]n to yield [Co(III)(P)(NO)(H2O)]n and [Co(III)(P)(NO2-)(H2O)](n-1) as the primary reaction products. Only the nitrite complex could be detected and no final product formation was observed during the reaction. It is proposed that [Co(III)(P)(NO)(H2O)]n rapidly reacts with NO2- to form the nitrite complex, which in the second reaction step reacts with another NO molecule to generate the final product through an inner-sphere electron-transfer reaction. The reported results are relevant for the interaction of vitamin B(12a) with NO and NO2-.

Structural investigation of the bridged activated complex in the reaction between hexachloroiridate(iv) and pentacyanocobaltate(ii)

Time resolved energy dispersive X-ray absorption spectroscopy has been used to follow the structural evolution of the inner-sphere electron transfer reaction between [IrCl6]2- and [Co(CN)5]3-, and to characterise the local structure of the iridium metal centre in the bridged activated complex formed during the reaction.

Chlorine Dioxide Reduction by Aqueous Iron(II) through Outer-Sphere and Inner-Sphere Electron-Transfer Pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).

Correlation between the energy and electron density representations of reactivity: mPW1K study of the asymmetric SN2 reactions at the saturated nitrogen

Abstract Gas phase inner-sphere electron transfer (ET) reactions of SN2, Y− + NH2X → X− + NH2Y (Y=H, CCH, CN, SH, OCH3, OH, NC, F, Cl, Br, and X=F, Cl, Br), have been investigated at the mPW1K/6-311++G(d,p) level. The correlation between energy (from potential energy surface) and electron density (from the theory of the atoms in molecules) representations of reactivity is presented. We conclude that the reactivity parameters of energy, electron density and geometry correlate well with one another within each group of Ys, which is gathered according to a similar electron density on the critical point of H–Y bonds. Moreover, within each group, Leffler–Hammond’s postulate is well confirmed. And the more looseness and the larger asymmetric of the TS, the lower the central barrier heights, hence the less electron density transferred is accomplished in going from the reactant to the TS. Also, in this type of ET reactions, ET takes place through the covalent bond.

Mechanistic studies on the interaction of reduced cobalamin (vitamin B12r) with nitroprusside.

The electron-transfer reaction between reduced cobalamin (Cbl(II)) and sodium pentacyanonitrosylferrate(II) (sodium nitroprusside, NP), as well as the subsequent processes following the electron-transfer step, were investigated by spectroscopic (UV-vis, (1)H NMR, EPR), electrochemical (CV, DPV) and kinetic (stopped-flow) techniques. In an effort to clarify the complex reaction pattern observed at physiological pH, systematic spectroscopic and kinetic studies were undertaken as a function of pH (1.8-9) and NP concentration (0.0001 - 0.09 M). The kinetics of the electron-transfer reaction was studied under pseudo-first-order conditions with respect to NP. The reaction occurs in two parallel paths of different order, viz. pseudo-first and pseudo-second order with respect to the NP concentration, respectively. The contribution of each path depends on pH and the [NP]/[Cbl(II)] ratio. At low pH and total NP concentration (pH < 3, [NP]/[Cbl(II)] approximately 1), the cyano-bridged successor complex [Cbl(III)-(mu-NC)-Fe(I)(CN)(3)(NO(+))](-) (1(s)()) is the final reaction product formed in an inner-sphere electron transfer reaction that is coupled to the release of cyanide from coordinated nitroprusside. At higher pH, subsequent reactions were observed which involve the attack of cyanide released in the electron transfer step on the initially formed cyano-bridged species, and lead to the formation of Cbl(III)CN and [Fe(I)(CN)(4)(NO(+))](2)(-). The strong dependence of the rate and mechanism of the subsequent reactions on pH is attributed to the large variation in the effective nucleophilicity of the cyanide ligand in the studied pH range. An alternative electron-transfer pathway observed in the presence of excess NP involves the reaction of the precursor complex [Cbl(II)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](2)(-) (1(p)()) with NP to give [Cbl(III)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](-) (2) and reduced nitroprusside, [Fe(CN)(5)NO](3)(-), as the initial reaction products. Analysis of the kinetic data allowed elucidation of the rate constants for the inner- and outer-sphere electron-transfer pathways. The main factors which influence the kinetics and thermodynamics of the observed electron-transfer steps are discussed on the basis of the spectroscopic, kinetic and electrochemical results. A general picture of the reaction pathways that occur on a short (s) and long (min to h) time scale as a function of pH and relative reactant concentrations is derived from the experimental data. In addition, the release of NO resulting from the one-electron reduction of NP by Cbl(II) was monitored with the use of a sensitive NO electrode. The results obtained in the present study are discussed in reference to the possible influence of cobalamin on the pharmacological action of nitroprusside.

DFT study of inner-sphere electron transfer in a gas-phase SN2 reaction at the saturated carbon

The SN2 reaction is investigated from the point of view of an inner-sphere electron transfer reaction. Density functional theory (DFT) calculations are applied to investigate the intrinsic reaction pathway (IRP) of the symmetrical SN2 (inner-sphere electron transfer) reactions, Xa− + CH3Xb → XaCH3 + Xb− (X = F, Cl, Br, I). The geometries of the stationary points in the potential energy surface are compared with available experimental data and with calculations at the MP2 level. The changes in the geometrical parameters, the projected frequencies, charge distribution and dipole moment along the reaction coordinate are discussed. Some relationships between the amount of transferred electron density and the changes in geometrical parameters, vibrational frequencies and energies are given: the greater the amount of transferred electron density, the smaller the decrease in the frequency of the C–Xb stretching mode (from reactant to TS), and the lower the activation energy and the complexation energy. Moreover, the amount of transferred electron density is related to the electronegativity of the halogen atom. Two parameters in the electron transfer theory, the coupling interaction and the reorganization energy, which affect the reactivity are discussed. We conclude that the stronger the CH3–Xb bond, the worse the acceptor.

The effect of triphenylmethyl cations on the electroreduction of O2 in nitrobenzene or acetonitrile

Abstract The electroreduction of O 2 in two aprotic solvents (nitrobenzene and acetonitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, φ 3 C + (φ=phenyl). The addition of φ 3 C + causes the electroreduction of O 2 to change from the one-electron reduction to O 2 − into the two-electron reduction to φ 3 CO 2 Cφ 3 at a potential that is 1 V more positive than that where O 2 is reduced to O 2 − . This large positive shift in potential is used to estimate a value of the equilibrium constant for the association between O 2 2− and two φ 3 C + cations ( K =3×10 26 M −2 ). The mechanism of the reduction of O 2 to φ 3 CO 2 Cφ 3 can be regarded as an inner-sphere electron-transfer reaction between φ 3 C and O 2 or, equivalently, as a radical addition to O 2 .

The ring expansion of a dimethyl-substituted vinylcyclobutene derivative by metal complexes that can undergo an inner-sphere electron transfer

The ring expansion of a dimethyl-substituted vinylcyclobutene derivative to the corresponding dimethyl-substituted cyclohexadiene or aromatic compound was studied. It was found that metal complexes of Ni(I), Ti(III), Sm(II), and Fe(II), which can undergo an inner-sphere electron-transfer reaction, allow the rearrangement to occur at room temperature. Other oxidation states of these metals and complexes that can only undergo an outer-sphere electron-transfer process do not promote this ring expansion reaction.