Kinetics Of Electron Transfer Reactions Research Articles

Analysis of the kinetics of electron transfer reactions of hemoglobin and myoglobin with inorganic complexes

The kinetics of methemoglobin reduction by Fe(EDTA) 2− have been studied and found to follow a second order rate law with k = 29.0 M −1 s −1 [25°C, μ = 0.2 M , pH 7.0 (phosphate)], ΔH ‡ = 5.5 ± 0.7 kcal/mol , and ΔS †= −33 ± 2 e.u. . The electrostatics-corrected self-exchange rate constant (k 11 corr) for hemoglobin based on the Fe(EDTA) 2− cross-reaction is 2.79×10 −3 M −1 s −1. This rate constant is compared with others reported for a water-soluble iron porphyrin and calculated from published data for the reactions of myoglobin and hemoglobin with Fe(EDTA) 2− and Fe(CDTA) 2−/−. The k 11 corr values for these systems range over ten orders of magnitude with heme ⪢ myoglobin > hemoglobin.

Electron transfer reactions at film covered metal electrodes

Experimental results concerning the kinetics of electron-transfer reactions at metal electrodes covered by anodic oxide films are discussed and compared to predictions from the theory. The rate equations used in the comparison are presented assuming a n-type semiconducting film. Semi-quantitative agreement between experiment and theory is observed. At electrodes covered by very thin films, direct electron exchange between the metal and the electrolyte predominates. The exchange cd drops exponentially with film thickness. For thicknesses greater than a few nm, electron exchange preferentially occurs with the oxide film even for oxides having a large band gap. The transfer coefficients then depend on electrode potential in an often non-monotonous way, and the rate is independent of thickness. The examples of oxide covered niobium and titanium are discussed in detail considering the influences of donor density and structure of the oxide, of the potential distribution at the semiconductor—electrolyte interface, and of the reorganization energy of the redox system.

Kinetics of electron transfer reaction between manganate and permanganate ions. Calculation of rate constant and activation energy

The rate constant and activation energy of the electron transfer reaction in a polar medium, between MnO–4 and MnO2–4 ions, are calculated in terms of the theory of nonadiabatic transitions. The dependence of the matrix element Vif, determining the probability of the electron transition in the intersystem crossing point, on the distance between the ions is investigated. The exponential factor of the rate constant involves the contributions due to the solvent reorganization, the deformation of metal–ligand bonds and the work term. The change of the rate constant dependant on the distance between the ions is considered. Agreement between the calculated and experimental activation energy becomes better if the dependence of the temperature coefficient of the static permittivity on the ionic strength is taken into account.

Polyelectrolyte acceleration of electron transfer reactions

The kinetics of electron-transfer reactions of some Co(III) complexes with Ru(NH 3) 6 2+ (outer sphere), Cr(H 2O) 6 2+ (inner sphere), and V(H 2O) 6 2+ (inner sphere) have been studied in the presence of varying concentrations of the sodium salt of poly(methacryloxyethylsulfonic acid), (PMES), using the stopped-flow technique. A large acceleration of reaction rates was observed with Ru(NH 3) 6 2+ and V(H 2O) 6 2+, with acceleration factors up to 20,000. The accelerated rates of electron transfer show a nonlinear dependence on polyelectrolyte concentrations: first, increasing with increasing polyelectrolyte concentration then reaching a maximum and subsequently decreasing. The magnitude of acceleration depends strongly on the charge of oxidants, being much larger for dipositive than for unipositive ions. Acceleration specificity is shown by the reduction of cis-Co(en) 2Cl 2 + by Ru(NH 3) 6 2+, in which case the acceleration factor is much higher in the presence of PMES than in the case of the corresponding trans isomer. The acceleration of the reaction of cis-Co(en) 2PyCl 2+ with Ru(NH 3) 6 2+ by the polyelectrolyte is practically equal to the acceleration of the reduction of this complex by V(H 2O) 6 2+, whereas the reduction of cis-Co(en) 2NH 2Cl 2+ by Ru(NH 3) 6 2+ is more strongly accelerated than the reduction by V(H 2O) 6 2+. Activation energies have been determined for some of the reactions and have been interpreted in terms of a multiple equilibrium reaction scheme. At a given concentration of reactants and polyelectrolyte, the acceleration factor increases with decreasing temperature and, further, the concentration of polyelectrolyte at which the maximum acceleration occurs decreases with decreasing temperature. The reaction rates of inner sphere reduction using Cr(H 2O) 6 2+ exhibit an entirely different behavior in the presence of PMES; no acceleration effect is observed. Two distinct reaction paths can be identified, with evidence for the formation and decay of one or more binuclear intermediates. The electron-transfer rate corresponding to the decay of the intermediates is much slower in the presence of PMES.

The Kinetics of Electron-transfer Reactions between Oxalato-Cobalt(III) Complexes and Iron(II) in Acid Aqueous Solutions

Abstract An investigation of the Fe2+ reductions of Co(ox)(NH3)4+, Co(ox)(en)2+, Co(ox)(trien)+, Co(ox)(dip)2+, Co-(ox)2(en)−, and cis-Co(ox)2(NH3)2− has been carried out spectrophotometrically. The rate constants of Co(ox)N4+-type complexes increase in this order: Co(ox)(phen)2+>Co(ox)(dip)2+>Co(ox)(NH3)4+>Co(ox)(en)2+>Co(ox)-(trien)+, and these of Co(ox)2N2−-type complexes, cis-Co(ox)2(NH3)2−>Co(ox)2(en)−. The variation in reactivity is mainly ascribed to a change in the enthalpy of activation. The rate constants of Co(ox)(NH3)4+, Co(ox)(en)2+, cis-Co(ox)2(NH3)2−, and Co(ox)33− increase with the values of the hal-wave potential of the polarographic reduction (CoIII→CoII) and with the decrease in the wave number of the maxima of the absorption spectra of the complexes.

Kinetics of Electron-transfer Reactions Involving Ethylenediaminetetra- acetatochromate(II) Ions as Reductants. I. Reactions with Some Isothiocyanatoammine Complexes of Chromium(III)

Abstract Kinetics of electron-transfer reactions of ethylenediaminetetraacetatochromate(II) ions with some isothiocyanatoammine complexes of chromium (III), Cr(NCS)n(NH3)6−n(3−n)+(n=0, 1,4 and 6) have been studied. The former ions are present in the forms of both CrII(H2O)edta2− and CrII(H2O)Hedta−, but the reaction of CrII(H2O)Hedta− with Cr(NCS)n(NH3)6−n(3−n)+ is not significant in the pH range from 3.6 to 5.3. Second-order rate constants (l mol−1sec−1) of the reactions between CrII(H2O)edta2− and Cr(NCS)n(NH3)6−n(3−n)+ at ionic strength 1.0 and 25°C are <10−2 Cr(NH3)63+, 0.2 for CrNCS(NH3)52+, 2.3 for Cr(NCS)4(NH3)2−, and 28.2 for Cr(NCS)63−.