Walther Jaenicke

Kinetics of electron exchange in N,N,N′,N′-tetramethyl-p-phenylenediamine in aprotic solvents and the semiclassical theory of electron transfer

The homogeneous electron exchange of the N,N,N′N′-tetramethyl-p-phenylenediamine radical (S˙+) with the reduced (R) and the totally oxidized (T2+) forms has been measured in aprotic solvents by means of e.s.r. line-broadening. Solvent polarity and temperature were varied. For each temperature the logarithm of the rate constant k1(S˙+⇌ R) is a linear function of the solvent parameter γ= 1/n2–1/Iµ(where n is the refractive index and Iµ the dielectric constant) and at constant γ(0.27 ⩽γ⩽ 0.53) a linear function of 1/T. The outer reorganization energy can be calculated using an ellipsoidal model of the reactants and a reaction distance equal to the molecular distance in the radical dimer. The inner reorganization energy is calculated from i.r. and crystallographic data. Nuclear tunnelling is considered since the intramolecular frequencies are high. The experimental function ln k1(γ, T–1) cannot be reproduced by the theory unless additional interaction energies such as ion association S˙+⋯ClO–4 and especially ion-molecule interactions S˙+⋯R are allowed for. The preexponential factors of the theoretical expressions are considerably greater than the experimental values, especially if a precursor equilibrium is assumed. The rate constant k2(S˙+⇌ T2+) is small compared with k1. The difference is mainly caused by the Coulomb repulsion term.

Comparison between heterogeneous and homogeneous electron transfer in p-phenylenediamine systems

The one-electron oxidation of selected p-phenylenediamines to form the corresponding radical cations has been studied at a Pt electrode in dimethylformamide and acetonitrile solutions containing 0.1 mol dm–3 NaClO4. The standard redox potentials, the diffusion coefficients and the standard rate constants have been evaluated in the range between room temperature and the melting point of the respective solvent using cyclic voltammetry. It has been shown that the dynamics of solvent reorientation affects the heterogeneous electron-transfer rate, as opposed to the homogeneous process. The free energies of activation are much greater in the heterogeneous than in the homogeneous case. Both results are explained by the different shape of the activated complexes, consisting of one or two ellipsoidal molecules.

ESR-spectroscopic investigation of the parallel electron and proton exchange between quinones and their radicals: Part I. Measurements at 298 K

Abstract Using the alternating line width effect of ESR spectroscopy, the rate constants of the independent fast proton and slower electron transfer between some quinones and their protonated radicals were measured. H atoms are not exchanged. The radicals HQ · are therefore stable in solvents like isopropanol in which no previous dissociation is possible.

Rate constants, activation free enthalpies and activation entropies of the electrochemical reduction of 1,4-diazines in DMF

Abstract The standard exchange current densities of nine 1,4-diazines at gold electrodes were measured in DMF in the temperature range of +25 up to −59°C. From the resulting free enthalphy of activation of pyrazine as a function of temperature an activation entropy of −1.5 k is calculated. It is compared with the activation entropy of −1.1 k following from the theory of Marcus. The relative change of the free enthalpy of activation with temperature is only a property of the temperature dependences of refractive index and dielectric constant of the solvent. It is used to obtain the free enthalpies of activation for the standard temperature of 298 K. The experimental values for the compounds are compared with theoretical values, calculated from the inner and outer reorganization energies λ i and λ 0 . For λ i the bond lengths and force constants were obtained from Huckel calculations, for λ o different approximations according to Marcus and Peover were used. The calculated and the measured values coincide within less than 10%. The relation between the free solvation enthalpy and the outer reorganization energy is discussed.

Kinetik und mechanismus der elektrochemischen reduktion von 2,2′-dipyridyl I. Primärprozesse: Vorgelagerte isomerisierung eines komplexes☆

Abstract Polarography and chronopotentiometry are used to study the primary processes of the reduction of 2,2′-bipyridine at mercury electrodes in aqueous solutions (12 cis - and trans -isomer. The cis -form is more stable, forming a chelate, the trans -form however is more easily reducible. The kinetic and thermodynamic data of the isomerization are determined. The first reduction product reacts irreversibly giving a substance which can be reoxidized to bipyridine via a radical intermediate.

Heterogeneous Electron Exchange of Quinones in Aprotic Solvents

A measuring cell with low probe resistance is described in which electron transfer rates can be investigated even in solvents of low polarity (e.g. in chloroform). A planar Au working electrode is used. The probe distance can be optimized and is between 0.04 and 0.08 mm, depending on the solvent. The method of cell optimization and the computerized performance of the experiments and the evaluation of the results is described. The standard potential and the standard rate constant k° of the heterogeneous electron transfer between p4xnzoquinone and its radical anion is measured in several aprotic solvents with polarity parameters y between 0.27 and 0.53 (> = 1/n2

Some Basic Concepts of Photographic Development

AbstractIn contact with a redox system like a developer, silver bromide shows n-type semiconductivity. From this, the rate of electron injection during the development is calculated. It is shown that only surface centres can be reduced. Therefore the electrode theory of development is valid for the primary stages of development too. The development centre as a surface state and its stability against a redox system is discussed in view of the theory of semiconductor electrodes. The effect of developer adsorption at the nucleus upon the kinetics of development is doubtful, with the exception of the phenomenon of superadditivity. Space charges within the grain influence the situation of the latent image in the grain, but their effect upon development seems to be small. The electrochemical kinetics of physical development are derived, especially for the case of diffusion controlled reactions. In chemical development etching and reduction occur at the same place of the grain unless inhibition effects are signi...

Heterogeneous electron exchange of quinones in aprotic solvents: Part IV. Influence of molecular structure and size on the exchange rate constants both reductions steps

The standard rate constants of the electron transfer between 13 substituted quinones and their radical anions (k°1) as well as those between the radical anions and the dianions (k°2) were measured by means of cyclic voltammetry at a plane gold electrode in dimthylflormamide with 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte. The observed Gibbs energies of activation ΔG*2 of the second reduction step were much higher than those of the first reduction step (ΔG*1), but they are linearly related to each other. Theoretical values ΔG* corresponding to Marcuss theory were calculated by means of bond lengths and force constants for the inner reorganization energy and with a spherical model and a three-centre model for the outer reorganization energy. THe experimental ΔG*1 values are generally better explained by the three-centre model but are somewhat higher. It is shown that using non-spherical molecules and the observed dependence of k on the supporting electrolyte, the Marcus activation Gibbs energies for electrochemical (ΔG*1(het)) and homogeneous electron transfer (ΔG*1(hom)) should be approximately equal. This was confirmed by comparison with published data of the homogeneous exchange of some quinones in ethanol. However, a constant additional negative term in ΔG*1(hom) had to be considered which was explained by hydrogen bonding in the protic solvent.

Kinetics and mechanism of the electrochemical reduction of 2,2′-bipyridine: Part II. Consecutive isomerization processes of dihydrobipyridine

Abstract 2,2′-Bipyridine is reduced by electrolysis at constant potential at a Hg electrode in the pH range 12 e − -, 2 H + -step was found. The first product Y is transformed in a fast irreversible reaction into a substance A, which undergoes slow consecutive reactions according to the scheme A agB agC. The mechanism was simulated by a computer program and the rate constants optimized. B is further reducible in a 2 e − -, 2 H + -step, forming D. Therefore at long times of electrolysis only D remains. The substances Y, A, B, C are isomeric dihydrobipyridines. D is a tetrahydrobipyridine. The substances A, B, C and D could be isolated in solution, and partially identified by u.v.-spectroscopy, but polymerized if they were concentrated. The stable substance C is 2,5-dihydrobipy, Y probably N,N′-dihydrobipy. A reaction mechanism is proposed and it is shown that only two reaction paths remain possible, A being either the 1,4- or the 1,6-dihydrobipy.

Model experiments of adsorption and adsorption displacement in photographic development—I. Tensammetric measurements of the system hydroquinone-sulfite with additives at a gold electrode

Abstract By means of alternating current polarography (tensammetry) the adsorption isotherms of hydroquinone, some N -substituted p -phenylenediamines, 1-phenyl-pyrazolidin-3-one, sulfite, and iodide were measured at pH = 10.5 at a stationary gold electrode. The results can be represented by Langmuir isotherms. Adsorbed hydroquinone is displaced by sulfite, the other substances are able to displace sulfite from the electrode surface. The displacement is irreversible with the exception of iodide. It was used to study the isotherms of adsorption displacement by means of the decrease of the sulfite peak with increasing amounts of the displacing substances. The constants of adsorption displacement were measured. The results are in agreement with observations upon catalytic effects of some additives in photographic development.