Homogeneous self-exchange

Experimental values of AG and the pre-exponential factor were obtained from a plot of In k,. vs 1/T under the assumption that the slope is — AG /R, and the hidden assumption that AG is temperature independent (AS is zero). Comparison between the calculated and observed pre-exponential factor was used to infer significant non-adiabaticity, but one may wonder whether inclusion of a nonzero AS would alter this conclusion. From an alternative perspective, reasonable agreement was noted for the values of ke and the homogeneous self-exchange rate constant after a standard Marcus-type correction was made for the differing reaction types. 

The significance of these quantities is analogous to that for the activation parameters for homogeneous self-exchange reactions. Thus, AH equals the activation enthalpy for conditions... 

Relationships having the same form as eq 14 can also be written for the enthalpic and entropic contributions to the intrinsic free energy barriers (10). Provided that the reactions are adiabatic and the conventional collision model applies, eq 14 can be written in the familiar form relating the rate constants of electrochemical exchange and homogeneous self-exchange reactions (13) ... 

Also, the observed rates probably refer to outer-sphere pathways, and the rate constants for the corresponding homogeneous self-exchange reactions are available or can be estimated from rate data for closely related cross reactions (15). These h ex... 

Rate Constants and Thermodynamic Parameters for Selected Electrochemical Exchange and Homogeneous Self-Exchange Reactions at 25°C. 

Rate constant for homogeneous self exchange, corrected for electrostatic work terms using Debye-Huckel-Bronsted model. Data taken from sources quoted in ref. 15 unless otherwise stated. 

Rate constant for homogeneous self exchange, calculated from corresponding value of k using... 

Intrinsic free energy of activation for homogeneous self-exchange, obtained from values of corr giVen in Table 1 using AGih = "RT ln(kcorr/Zh) where 2 x 1()11, -1 ... 

Intrinsic enthalpy of activation for homogeneous self-exchange. Values for v + + and... 

The homogeneous self-exchange ET process is the case where Red2=Redi and Ox2 = Ox in Eq. (4.10). 

For the relation between the rate constant for homogeneous self-exchange ET process (kex) and the standard rate constant of the corresponding electrode reaction (kg), see 4) in Chapter 9. 

Ey2 reversible half-wave potential, k, vxl> experimental standard rate constant, a transfer coefficient, (outer-Helmholtz plane, L ron standard rate constant after correction for the double-layer effect (see 4)), lcex rate constant for the homogeneous self-exchange electron-transfer (see 4) in Chapter 9) obtained with a HMDE in DMF-0.5 M BU4NCIO4 at 22 2°C, except the last two obtained with a DME in DMF-0.1 M Bu4NI at 30°C. 

Another interesting result obtained by Shalev and Evans [55] is that the rate constant for the homogeneous self-exchange electron transfer, log fcex, between various Q and Q in DMF decreases linearly with the increase in AGgV(Q/Q -). This is because log k is mainly governed by the solvent reorganiza-... 

Given the importance and great variety of electron-transfer reactions of polypyridine complexes, systematic kinetic studies are surprisingly scarce and only few kinetic data are available. Some representative rate constant values for homogeneous self-exchange redox reactions were reported ... 

The coefficient 2 in Eq. (h) arises because the free-energy profile for the homogeneous self-exchange reaction (f) will then equal the sum of those of the constituent forward and reverse electrochemical reactions (g). 

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