Activation parameters for electron transfer

Table II. Summary of Rate and Activation Parameters for Electron Transfer According to the Reaction Co(NH3)4(NH2R)X< " , Fe(CN)6 " Co + 4NH3 + NH2R + X - + Fe(CN)6 "...

Table 6 Rate and activation parameters for electron transfer reactions involving electronically excited metal complexes. For conventions used, see Table 3... 

Table 6 Rate constants and activation parameters for electron-transfer reactions involving polynuclear complexes. The expression in the column headed Rate is defined so that for a reaction of stoicheiometry a At + bBj +. .. —> products, the rate is given by R = —(lla)d[Ai]ldt, etc. For other conventions, see Table 2a... 

The kinetics of oxidation of Pseudomonas aeruginosa azurin, bean plastocyanin, and Rhus v. stellacyanin by the tris-cobalt(iii) complexes of phen and three of its derivatives have been reported (Table 15) the reactivity order for Co(phen)3 + as the oxidant (stellacyanin > plastocyanin > azurin) matches that found previously for the Fe(edta) reduction of the proteins (and for which ionic strength and pH effects have now been reported). It is suggested that the activation parameters for electron transfer from reduced plastocyanin and azurin may be accounted for in terms of oxidant-induced protein structural changes which expose active sites that are, by comparison with stellacyanin, inaccessible to reagent attack. Segal and Sykes have extended the work with plastocyanin and Co(phen)3 + to higher concentrations (up to 4.0 X 10 mol of oxidant and have observed a deviation from linearity in the... 

Each of these free energy relationships employs the intrinsic barrier AGo+ as the disposable parameter. [The intrinsic barrier represents the activation energy for electron transfer when the driving force is zero, i.e., AG = AGo at AG = 0 or the equili-... 

B. Activation Parameters for Coupled Electron Transfer and Spin Change... 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

Figure 9. Compensation plot of activation parameters for the electron transfer rate constant k2 taken from Table I...

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

Verdazyls (111) can also transfer an electron to o -quinones to give the verdazylium cation (113) and a semiquinone anion (114) (80IZV2785), or to tetranitromethane to give the cation (113) and the tetranitromethane anion radical (115) (74MI22100). Rate constants and activation parameters for the electron transfer from triphenylverdazyl to tetracyanoethylene have been determined by Soviet chemists (79ZOR2344). 

There has been keen interest in determination of activation parameters for electrode reactions. The enthalpy of activation for a heterogeneous electron transfer reaction, AH X, is the quantity usually sought [3,4]. It is determined by measuring the temperature dependence of the rate constant for electron transfer at the formal potential, that is, the standard heterogeneous electron transfer rate constant, ks. The activation enthalpy is then computed by Equation 16.7 ... 

Kinetic data for electron transfer between two metalloproteins are presented in Table V. The rate constants and activation parameters for the Ps(II)-Ps(III) and Az(I)-Az(II) exchange reactions were calculated from the kinetic data for the first three reactions (for which K 1, AH° 0, AS° 0 in addition, the rate constant for the Hh(II)-Ps(III) reaction is independent of ionic strength (31)). The calculated exchange data were then used to predict the kinetic parameters for the Ps(II)-Az(II) reaction. As is evident from Table V, the agreement of the observed and predicted parameters is satisfactory, particularly since the Ps( II )-Az( II) reaction has a relatively complex mechanism (57) involving conformational changes on both Ps(III) and Az(I). 

The measurement of ket for single electron-transfer reactions is of particular fundamental interest since it provides direct information on the energetics of the elementary electron-transfer step (Sect. 3.1). As for solution reactants, standard rate constants, k t, can be defined as those measured at the standard potential, E, for the adsorbed redox couple. The free energy of activation, AG, at E°a is equal to the intrinsic barrier, AG t, since no correction for work terms is required [contrast eqn. (7) for solution reactants] [3]. Similarly, activation parameters for surface-attached reactants are related directly to the enthalpic and entropic barriers for the elementary electron-transfer step [3],... 

The activation parameters for the exchange reactions of 17 and 18 were determined by a combination of variable-temperature ll NMR lineshape analysis16 and spin saturation transfer experiments.17 Rate data for 17 were measured over a temperature range of 100 "C. Rates for compound 18 were measured over a 65 °C range. The enthalpy of activation was found to be considerably smaller in the case of 17 (12.2(2) kcal/mol) relative to 18 (17.6(3) kcal/mol). Ion pair dissociation is therefore facilitated by the presence of a lone pair of electrons on the boron substituent. The entropy of activation for 17 is -2.3(6) eu, while that of 18 is 8(1) eu. The more positive entropy of activation measured for 18 may be interpreted as the creation of two independent particles from a closely associated ion pair. 

Electrochemical rate constants and activation parameters for inorganic electrochemical reactions span wide ranges e.g., standard rate constants vary from ca. 10 to > 10 cm s. For transition-metal redox couples, values of AH vary - from 0 to 80 kJ mol". As for homogeneous redox processes, it is highly desirable to elucidate the underlying reasons that are responsible for the observed rate parameters. In the following sections, the theory of electrochemical electron-transfer kinetics is outlined, followed by a discussion of some pertinent experimental results. 

The rate constant and activation parameters for the electron-transfer reactions are given in Table I. The reaction rate of the Cu/3 complex via the inner-sphere was smaller than that of the Cu/ethylimidazole complex. The coordination of the substrate to Cu(II) ion was enthalpically unfavored as compared to the homogeneous Cu complex. On the other hand, the outer-sphere reaction with Fe(II)(phenanthroline)3 proceeded faster for the Cu/3 system than for the homogeneous Cu/ethylimidazole complex. 3 made a significant favorable entropic contribution to the outer-sphere electron-transfer reaction. 

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