Self-exchange Constants Research Articles

Kinetics of Electron-Transfer Reactions at Nanoelectrodes

The kinetics of several fast heterogeneous electron-transfer reactions were investigated by steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM). The disk-type, polished Pt nanoelectrodes (3.7-400-nm radius) were characterized by a combination of voltammetry, scanning electron microscopy, and SECM. A number of experimental curves were obtained at the same nanoelectrode to attain the accuracy and reproducibility similar to those reported previously for micrometer-sized probes. A new analytical approximation was developed and used for analysis of steady-state tip voltammograms. The self-consistent kinetic parameter values with the uncertainty margin of approximately 10% were obtained for electrodes of different radii and for a wide range of the SECM tip/substrate separation distances. The determined standard rate constants are compared to those previously measured at the electrodes of different dimensions, and the correlation between the heterogeneous and self-exchange rate constants is discussed.

Probing Aspergillus niger glucose oxidase with pentacyanoferrate(III) aza- and thia-complexes

Complexes of pentacyanoferrate(III) and biologically relevant ligands, such as pyridine, pyrazole, imidazole, histidine, and other aza- and thia-heterocycles, were synthesized. Their spectral, electrochemical properties, electron exchange constants, electronic structure parameters, and reactivity with glucose oxidase from Aspergillus niger were determined. The formation of the complexes following ammonia replacement by the ligands was associated with the appearance of a new band of absorbance in the visible spectrum. The constants of the complexes formation calculated at a ligand–pentacyanoferrate(III) concentrations ratio of 10:1, were 7.5 × 10 −5, 7.7 × 10 −5, and 1.8 × 10 −3 s −1 for benzotriazole, benzimidazole, and aminothiazole ligands, respectively. The complexes showed quasi-reversible redox conversion at a glassy carbon electrode. The redox potential of the complexes spanned the potential range from 70 to 240 mV vs. saturated calomel electrode (SCE) at pH 7.2. For most of the complexes self-exchange constants ( k 11) were similar to or larger than that of hexacyanoferrate(III) (ferricyanide). The complexes containing pyridine derivatives and thia-heterocyclic ligands held a lower value of k 11 than that of ferricyanide. All complexes reacted with reduced glucose oxidase at pH 7.2. The reactivity of the complex containing pyrazole was the largest in comparison to the rest of the complexes. Correlations between the complexes’ reactivity and both the free energy of reaction and k 11 shows that the reactivity of pentacyanoferrates obeys the principles of Marcus’s electron transfer theory. The obtained data suggest that large negative charges of the complexes decrease their reactivity.

Self-exchange electron transfer in high oxidation state non-oxo metal complexes: amavadin

The electron transfer self-exchange rate constant between the oxidized and reduced forms of amavadin equals approximately 1 x 10(5) dm3 mol(-1) s(-1) at 25 degrees C and represents the first unambiguous example for a vanadium(IV/V) couple.

Physical Parameters and Electron-Transfer Kinetics of the Copper(II/I) Complex with the Macrocyclic Sexadentate Ligand [18]aneS6

The electron-transfer kinetics of the complex formed by copper(II/I) with the sexadentate macrocyclic ligand 1,4,7,10,13,16-hexathiacyclooctadecane ([18]aneS6) have been measured in acetonitrile with a series of three oxidizing agents and three reducing agents. These studies have been supplemented by determinations of the redox potential and the stability constants of the Cu(I)- and Cu(II)([18]aneS6) complexes in both acetonitrile and aqueous solution. The Marcus cross relationship has been applied to the cross-reaction rate constants for the six reactions studied to resolve the electron self-exchange rate constant for the Cu(II/I)([18]aneS6) complex. An average value of k11 = 3 x 10(3) M(-1) s(-1) was obtained at 25 degrees C, mu = 0.10 M in acetonitrile. This value is approximately 2 orders of magnitude smaller than the values reported previously for the corresponding Cu(II/I) complexes with the quadridentate and quinquedentate homoleptic homologues having all ethylene bridges, namely, 1,4,7,10-tetrathiacyclododecane ([12]aneS4) and 1,4,7,10,13-pentathiacyclopentadecane ([15]aneS5). This significant difference in reactivity is attributed to the greater rearrangement in the geometry of the inner-coordination sphere that accompanies electron transfer in the Cu(II/I)([18]aneS6) system, wherein two Cu-S bonds are ruptured upon reduction. In contrast to other Cu(II/I) complexes with macrocyclic polythiaethers that have self-exchange rate constants within the same range, no evidence for conformationally gated electron transfer was observed, even in the case of the most rapid oxidation reaction studied.

Ion Atmosphere Relaxation Controlled Electron Transfers in Cobaltocenium Polyether Molten Salts

A room-temperature redox molten salt for the study of electron transfers in semisolid media, based on combining bis(cyclopentadienyl)cobalt with oligomeric polyether counterions, [Cp2Co](MePEG350SO3), is reported. The transport properties of the new molten salt can be varied (plasticized) by varying the polyether content. The charge transport rate during voltammetric reduction of the ionically conductive [Cp2Co](MePEG350SO3) molten salt exceeds the actual physical diffusivity of [Cp2Co]+ because of rapid [Cp2Co](+/0) electron self-exchanges. The measured [Cp2Co](+/0) electron self-exchange rate constants (k(EX)) are proportional to the diffusion coefficients (D(CION)) of the counterions in the melt. The electron-transfer activation barrier energies are also close to those of ionic diffusion but are larger than those derived from optical intervalent charge-transfer results. Additionally, the [Cp2Co](+/0) rate constant results are close to those of dissimilar redox moieties in molten salts where D(CION) values are similar. All of these characteristics are consistent with the rates of electron transfers of [Cp2Co](+/0) (and the other donor-acceptor pairs) being controlled not by the intrinsic electron-transfer rates but by the rate of relaxation of the ion atmosphere around the reacting pair. In the low driving force regime of mixed-valent concentration gradients, the ion atmosphere relaxation is competitive with electron transfer. The results support the generality of the recently proposed model of ionic atmosphere relaxation control of electron transfers in ionically conductive, semisolid materials.

Electron transfer reaction of Cu(II) porphyrin complex: Effect of structural deformation on the electron self-exchange rate

Homogeneous one-electron oxidation reaction of the sterically constrained nonplanar Cu(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin complex ([Cu(OETPP)]) by Cu 2+ giving the π-cation radical of [Cu(OETPP)] was investigated spectrophotometrically in acetonitrile. The electron self-exchange rate constant of the [Cu(OETPP)] + /0 couple was determined to be 9.3 × 10 3 M −1 s −1 at T = 25.0 °C using the Marcus cross relation for the outer-sphere electron transfer reaction, and the relatively slow self-exchange reaction as compared with those for the planar Cu(II) porphyrin complexes was ascribed to the significant deformation of the complex accompanying the oxidation reaction.

MARY spectroscopy: magnetic field effects on fluorescence intensities used for measuring electron transfer rates

Unprecedented ab initio simulations of exchange-affected magnetic field on reaction yield (MARY) spectra of real radical pair (RP) systems are presented which provide a novel, reliable method for the determination of electron self-exchange rate constants by MARY spectroscopy. The MARY technique is an interesting alternative to EPR in the study of short-lived and unstable radicals. A RP lifetime of a few nanoseconds is sufficient compared to at least 100 ms necessary for the CW EPR line broadening technique.

Kinetic studies of the reduction of hexaaquacobalt(III) perchlorate by a cobaloxime, [Co(dmgBF 2) 2(H 2O) 2

The reaction of Co ( OH 2 ) 6 3 + with Co(dmgBF 2) 2(H 2O) 2 in 1.0 M HClO 4/LiClO 4 was found to be first-order in both reactants and the [H +] dependence of the second-order rate constant is given by k 2obs = b/[H +], b at 25 °C is 9.23 ± 0.14 × 10 2 s −1. The [H +] dependence at lower temperatures shows some saturation effect that allowed an estimate of the hydrolysis constant for Co ( OH 2 ) 6 3 + as K a = 9.5 × 10 −3 M at 10 and 15 °C. Marcus theory and the known self-exchange rate constant for Co(OH 2) 5OH 2+/+ were used to estimate an electron self-exchange rate constant of k 22 = 1.7 × 10 −4 M −1 s −1 for Co ( dmgBF 2 ) 2 ( H 2 O ) 2 + / 0 .

Blue Copper Model Complexes with Distorted Tetragonal Geometry Acting as Effective Electron-Transfer Mediators in Dye-Sensitized Solar Cells

The electron self-exchange rate constants of blue copper model complexes, [(-)-sparteine-N,N'](maleonitriledithiolato-S,S')copper ([Cu(SP)(mmt)])(0/)(-), bis(2,9-dimethy-1,10-phenanthroline)copper ([Cu(dmp)(2)](2+/+)), and bis(1,10-phenanthroline)copper ([Cu(phen)(2)](2+/+)) have been determined from the rate constants of electron transfer from a homologous series of ferrocene derivatives to the copper(II) complexes in light of the Marcus theory of electron transfer. The resulting electron self-exchange rate constant increases in the order: [Cu(phen)(2)](2+/+) < [Cu(SP)(mmt)](0/)(-) < [Cu(dmp)(2)](2+/+), in agreement with the order of the smaller structural change between the copper(II) and copper(I) complexes due to the distorted tetragonal geometry. The dye-sensitized solar cells (DSSC) were constructed using the copper complexes as redox couples to compare the photoelectrochemical responses with those using the conventional I(3)(-)/I(-) couple. The light energy conversion efficiency (eta) values under illumination of simulated solar light irradiation (100 mW/cm(2)) of DSSCs using [Cu(phen)(2)](2+/+), [Cu(dmp)(2)](2+/+), and [Cu(SP)(mmt)](0/)(-) were recorded as 0.1%, 1.4%, and 1.3%, respectively. The maximum eta value (2.2%) was obtained for a DSSC using the [Cu(dmp)(2)](2+/+) redox couple under the light irradiation of 20 mW/cm(2) intensity, where a higher open-circuit voltage of the cell was attained as compared to that of the conventional I(3)(-)/I(-) couple.

Measurement of the Free-Energy Dependence of Interfacial Charge-Transfer Rate Constants using ZnO/H2O Semiconductor/Liquid Contacts

The dependence of electron-transfer rate constants on the driving force for interfacial charge transfer has been investigated using n-type ZnO electrodes in aqueous solutions. Differential capacitance versus potential and current density versus potential measurements were used to determine the energetics and kinetics, respectively, of the interfacial electron-transfer processes. A series of nonadsorbing, one-electron, outer-sphere redox couples with formal reduction potentials that spanned approximately 900 mV allowed evaluation of both the normal and Marcus inverted regions of interfacial electron-transfer processes. All rate processes were observed to be kinetically first-order in the concentration of surface electrons and first-order in the concentration of dissolved redox acceptors. The band-edge positions of the ZnO were essentially independent of the Nernstian potential of the solution over the range 0.106-1.001 V vs SCE. The rate constant at optimal exoergicity was observed to be approximately 10(-)(16) cm(4) s(-)(1). The rate constant versus driving force dependence at n-type ZnO electrodes exhibited both normal and inverted regions, and the data were well-fit by a parabola generated using classical Marcus theory with a reorganization energy of 0.67 eV. NMR line broadening measurements of the self-exchange rate constants indicated that the redox couples had reorganization energies of 0.64-0.69 eV. The agreement between the reorganization energy of the ions in solution and the reorganization energy for the interfacial electron-transfer processes indicated that the reorganization energy was dominated by the redox species in the electrolyte, as expected from an application of Marcus theory to semiconductor electrodes.

Kinetic studies of tris(2,2′-bipyridine)iron(III) perchlorate with cobaloxime, [Co(dmgBF 2) 2(H 2O) 2

The kinetics of the reduction of Fe ( bipy ) 3 3 + by Co(dmgBF 2) 2(H 2O) 2 in 0.041 M HNO 3/NaNO 3 was found to be first-order in both the oxidizing and reducing agents and the second-order rate constant is given by k obs = k 1 + k 2 K[Cl −], with k 1=1.59 × 10 6 M −1 s −1and k 2 K = 1.83 × 10 8 M −2 s −1, at 25 °C. The term that is first-order in [Cl −] is attributed to the formation of an ion-pair between Fe ( bipy ) 3 3 + and Cl −. For k 1, the activation parameters Δ H* and Δ S* are 2.22 ± 0.02 kcal mol −1 and −22.7 ± 0.8 cal mol −1 K −1, respectively. The self-exchange rate constant of k 22 ≈ 8.7 × 10 −3 M −1 s −1 for Co ( dmgBF 2 ) 2 ( H 2 O ) 2 + / 0 was estimated using Marcus theory and the known self-exchange rate constant for Fe ( bipy ) 3 3 + / 2 + .

Electron transfer reactions of tris(polypyridine)cobalt(iii) complexes, [Co(N–N)3]3+, with verdazyl radicals in acetonitrile solution

The kinetics and mechanisms of the reactions of 3-(4-X)-phenyl-1,5-diphenyl-verdazyl radicals where X = Cl, H, CH3 and CH3O with [Co(N-N)3]3+, N-N = 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen) and 4,7-dimethyl-1,10-phenanthroline (4,7-Me2phen), have been investigated in acetonitrile at 25 degrees C and ionic strength 0.05 mol dm(-3)(nC4H9)4NPF6 using stopped flow spectrophotometry. In all cases, transfer of one electron from the radical takes place resulting in the production of a Co(II) species and a verdazylium cation. The electron transfer occurs by an outer-sphere mechanism and the reactions appear to be consistent with Marcus theory. The self-exchange rate constants for the verdazyl-verdazylium cation have been estimated and are of the order of 3.4(+/-1.9) x 10(7) dm(3) mol(-1) s(-1). This rate constant is consistent with the fact that the reactions of [Ru(bpy)3]3+ with verdazyl radicals are too rapid to be investigated by stopped flow spectrophotometry.

Kinetics and mechanisms of the electron transfer reactions of oxo-centred carboxylate bridged complexes, [Fe3(µ3-O)(O2CR)6L3]ClO4, with verdazyl radicals in acetonitrile solution

A range of oxo-centred, carboxylate bridged tri-iron complexes of general formula [Fe3(mu3-O)(O2CR)6L3]ClO4(R=CH2CN, CH2F, CH2Cl, CH2Br, p-NO2C6H4; L=pyridine, 3-methylpyridine, 4-methylpyridine, 3,5-dimethylpyridine, 3-cyanopyridine and 3-fluoropyridine) have been prepared and characterised. The choice of R and L was dictated by the requirement that the complexes undergo a one-electron reduction when reacted with verdazyl radicals. All except the complexes where L=pyridine and R=CH2CN, CH2Cl and p-NO2C6H4 have not been previously reported. The redox behaviour of these compounds has been investigated using cyclic voltammetry in acetonitrile in the absence and in the presence of free L. In general, all complexes exhibited reversible one-electron reductions. Electrochemical behaviour improved in the presence of an excess of L. The kinetics of the electron transfer reaction observed when acetonitrile solutions of the complexes were reacted with a range of verdazyl radicals were monitored using stopped-flow spectrophotometry. Under the experimental conditions, the reactions were quite rapid and were monitored under second-order conditions. Marcus linear free energy plots indicated that the outer-sphere electron transfer reactions were non-adiabatic in nature. Nevertheless, application of the self-exchange rate constants of the verdazyl radicals, k11, and the tri-iron complexes, k22, to the Marcus cross-relation resulted in calculated values of the cross-reaction rate constant, k12, that were within a factor of five of the experimentally determined value.

Comparison of Thermal and Optical Electron-Transfer Barriers in Ruthenium Redox Polyether Melts

Combining redox active moieties with polyether oligomers produces amorphous, highly viscous, room-temperature molten materials notable for their high concentrations of redox sites that in mixed valent form exhibit facile electron-transfer chemistry. This paper describes experimental determinations and comparison of thermal and optical energy barriers for electron transfers in a specific redox polyether melt, namely the metal complex Ru(bpy350)2(CN)2 where bpy350 is 4,4‘-dimethyl-2,2‘-dipyridyl alkyl-bonded to 350 MW polyethylene glycol monomethyl ether. Electrochemical measurements provide apparent electron self-exchange rate constants (kEX) within the Ru(III/II) form of the melt and physical diffusion rates of perchlorate ion (DClO4), along with the associated thermal activation barrier energies. Plasticization by CO2 induced swelling of the melt results in enhanced mass and charge transport rates and decreasing energy barriers for both. Optical spectra of the same, electrochemically generated, mixed valent melt display an absorbance interpreted as an intermolecular Ru(II→III) intervalence charge transfer band. The energy barrier results show that the observed thermal electron transfer energy barrier is only an apparent one, being instead controlled by ion atmosphere relaxation processes, whereas the optical energy barrier corresponds to that of the actual electron-transfer process itself.

Loop-Contraction Mutagenesis of Type 1 Copper Sites

The shortest known type 1 copper binding loop (that of amicyanin, Ami) has been introduced into three different cupredoxin beta-barrel scaffolds. All of the loop-contraction variants possess copper centers with authentic type 1 properties and are redox active. The Cu(II) and Co(II) sites experience only small structural alterations upon loop contraction with the largest changes in the azurin variant (AzAmi), which can be ascribed to the removal of a hydrogen bond to the coordinating thiolate sulfur of the Cys ligand. In all cases, loop contraction leads to an increase in the pK(a) of the His ligand found on the loop in the reduced proteins, and in the pseudoazurin (Paz) and plastocyanin (Pc) variants the values are almost identical to that of Ami ( approximately 6.7). Thus, in Paz, Pc, and Ami, the length of this loop tunes the pK(a) of the His ligand. In the AzAmi variant, the pK(a) is 5.5, which is considerably higher than the estimated value for Az (<2), and other controlling factors, along with loop length, are involved. The reduction potentials of the loop-contraction variants are all lower than those of the wild-type proteins by approximately 30-60 mV, and thus this property of a type 1 copper site is fine-tuned by the C-terminal loop. The electron self-exchange rate constant of Paz is significantly diminished by the introduction of a shorter loop. However, in PcAmi only a 2-fold decrease is observed and in AzAmi there is no effect, and thus in these two cupredoxins loop contraction does not significantly influence electron-transfer reactivity. Loop contraction provides an active site environment in all of the cupredoxins which is preferable for Cu(II), whereas previous loop elongation experiments always favored the cuprous site. Thus, the ligand-containing loop plays an important role in tuning the entatic nature of a type 1 copper center.

Synthetic models for active sites of reduced blue copper proteins: minimal geometric change between two oxidation states for fast self-exchange rate constants

We have prepared two thiolato copper redox pairs ligated by HB ( 3 , 5 - i Prpz ) 3 - as models for blue copper proteins. The k ex value of the coordination number invariant Cu(II/I) couples is 51 times faster than that of the coordination number variant Cu(II/I) couples at −20 °C. We also discuss the electron-transfer process.

Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation.

The transfer of a hydrogen atom from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)], [FeII(H2bip)3]2+, to the stable nitroxide, TEMPO, was studied by stopped-flow UV-vis spectrophotometry. The products are the deprotonated iron(III) complex [FeIII(H2bip)2(Hbip)]2+ and the hydroxylamine, TEMPO-H. This reaction can also be referred to as proton-coupled electron transfer (PCET). The equilibrium constant for the reaction is close to 1; thus, the reaction can be driven in either direction. The rate constants for the forward and reverse reactions at 298 K are k1 = 260 +/- 30 M-1 s-1 and k-1 = 150 +/- 20 M-1 s-1. Interestingly, the rate constant for the forward reaction decreases as reaction temperature is increased, implying a negative activation enthalpy: DeltaH1 = -2.7 +/- 0.4 kcal mol-1, DeltaS1 = -57 +/- 8 cal mol-1 K-1. Marcus theory predicts this unusual temperature dependence on the basis of independently measured self-exchange rate constants and equilibrium constants: DeltaHcalcd = -3.5 +/- 0.5 kcal mol-1, DeltaScalcd = -42 +/- 10 cal mol-1 K-1. This result illustrates the value of the Marcus approach for these types of reactions. The dominant contributor to the negative activation enthalpy is the favorable enthalpy of reaction, DeltaH1 degrees = -9.4 +/- 0.6 kcal mol-1, rather than the small negative activation enthalpy for the H-atom self-exchange between the iron complexes.

Electrocatalysis in Nucleic Acid Molten Salts

This paper describes redox chemistry in semisolid molten salts ionic liquids of DNA in which the counterions of the phosphates are redox-active metal complexes with bipyridine ligands labeled with MW 350 poly(ethylene glycol) (PEG) “tails”, e.g., M(bpy350)3DNA (where M = Co, Ni, and bpy350 = 4,4‘-(CH3(OCH2CH2)7OCO)2-2,2‘-bipyridine). Other redox-active metal complexes are added to the M(bpy350)3DNA melt: (a) the PEG-tailed metal bipyridine complexes Fe(bpy350)3(ClO4)2 and Ru(bpy350)3(ClO4)2 and (b) the nontailed complexes Os(bpy)3Cl2 (bpy = 2,2‘-bipyridine) and Os(bpy)2dppzCl2 (dppz = dipyridophenazine). In example a, electrogeneration of the powerful oxidizers [Fe(bpy350)3]3+ and [Ru(bpy350)3]3+ gives microelectrode voltammetry indicative of electrocatalytic oxidation of DNA base sites. Since physical diffusion of the metal complexes is slow in the viscous semisolids (and that of DNA is nil), the rate of electron hopping between the base sites of the DNA becomes a significant contributor to the overall charge transport rate, as deduced from analysis of the voltammetry. DNA base site self-exchange rate constants of 1.1 × 106 and 1.8 × 106 s-1 are estimated from measurements using Fe(bpy350)33+ and Ru(bpy350)33+ oxidants, respectively. In example b, a complex known to be a DNA intercalator in aqueous solutions is found to not be an intercalator in the DNA molten salt environment, as deduced from measurements showing the physical diffusion coefficients of aqueous nonintercalator Os(bpy)3Cl2 and aqueous intercalator Os(bpy)2dppzCl2 to be indistinguishable in the M(bpy350)3DNA melt.

Electron self-exchange kinetics determined by MARY spectroscopy: theory and experiment.

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.

Reaction of hydroquinone with hematite: II. Calculated electron-transfer rates and comparison to the reductive dissolution rate

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate. Here, a reductive dissolution rate was measured and compared to electron-transfer rates calculated using Marcus theory. An experimental rate constant was measured at 9.5×10 −6 s −1 and the reaction order with respect to the hematite concentration was found to be 1.1. Both the dissolution rate and the reaction order of hematite concentration compare well with previous measurements. Of the Marcus theory calculations, the inner-sphere part of the reorganization energy and the electronic coupling matrix element for hydroquinone self-exchange electron transfer are calculated using ab initio methods. The second order self-exchange rate constant was calculated to be 1.3×10 7 M −1 s −1, which compares well with experimental measurements. Using previously published data calculated for hexaquairon(III)/(II), the calculated electron-transfer rate for the cross reaction with hydroquinone also compares well to experimental measurements. A hypothetical reductive dissolution rate is calculated using the first-order electron-transfer rate constant and the concentration of total adsorbed quinone. Three different models of the hematite surface are used as well as multiple estimates for the reduction potential, the surface charge, and the adsorption density of hydroquinone. No calculated dissolution rate is less than five orders of magnitude faster than the experimentally measured one.