Reduction potential, Ered

Fig. 8.21 Relationship between the oxidation potential (E0x)> the reduction potential (ERed), the ionization potential (IP), the electron affinity (EA), and the solvation energies (AG°V+,...

The loss of an electron by M, M + + e, is the process of oxidation in electrochemistry. The electron is then accepted by an electrode of well defined potential, so that the oxidation potential Eox is the free energy of the reaction, as was seen in Figure 4.1. Similarly the reduction potential Ered is the energy of the reduction reaction, e.g. N + e - N. By definition the molecule, which is oxidized, is the donor (M in this case), and the molecule, which is reduced, is the acceptor. The electron transfer from M to N is therefore equivalent to the combined oxidation of the donor and reduction of the acceptor, so that the energy balance is... 

L is the compound with the unknown Ey and CH is a coreactant. A series of compounds A with reversible reduction potentials Ered(A) > 0X(C ) is added in stepwise experiments to the L/CH system where new reactions appear ... 

Compound Reduction potential, Ered Triplet energies,... 

Compound Reduction potential, Ered (V, relative to SCE) Singlet energies, (S,) (eV)... 

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

In such a case the one-electron reduction potential of A(Ered) may be shifted to the positive direction with an increase in the concentration of M according to Eq. 4, which is derived from the Nernst equation of the one-electron reduction potential in the presence of M [38] ... 

The fluorescence intensity of ZnP CONH Q is significantly quenched compared to the reference ZnP compound without Q due to efficient ET from the singlet excited state ( ZnP ) to Q in ZnP—CONH—Q (68). Such efficient ET results from the large driving force of electron transfer (—AGej = 0.91 eV in PhCN), which is determined from the one-electron oxidation potential of the ZnP moiety (Eox = 0-78V vs SCE), the one-electron reduction potential of the Q moiety (Ered = —0.36 V vs SCE), and the singlet excited-state energy of ZnP (2.05 eV). 

The photochemical activation of the phosphonium salt (Eq. 55) has its thermal counterpart in the facile (dark) conversion of the iodonium salt Ph2l+ Mn(CO)s . Owing to the greatly enhanced reduction potential of diphenyliodonium (Ered 0 V relative to the SCE [181]) as compared to tetraphenylphosphonium (Ered 2.3 V relative to the SCE [140]), electron transfer in the iodonium-metallate ion pair is now energetically feasible (AGet 0 eV). As a result, complete conversion of the charge-transfer salt to the electron-transfer products Mn2(CO)io and PhMn(CO)s is observed within minutes upon mixing of the two components [140]. 

The oxidation potential of carbanions, Eox, or the reduction potential of carbocations, Ered, could be a practical scale of stability as defined by (3). These potentials can be measured by voltammetry, although the scale is subject to assumptions regarding elimination of the diffusional potential and solvation effects. 

The linear relationships shown for E as a function of log i are frequently observed for only small deviations from equilibrium. It is shown subsequently that the linear relationship corresponds to an upset in the mechanism of transfer of the ions between the metal and the solution and is termed charge-transfer polarization. As the potential is changed progressively from E, the curves deviate from linearity (Fig. 3.2). Along the reduction branch, Ered M becomes more negative than the linear relationship would indicate. This additional deviation is due to removal of metal ions from the solution in the vicinity of the interface at a rate such that diffusion of the ions in the solution toward the inter-... 

The standard cell potential of a voltaic cell, depends on the particular cathode and anode half-cells. We could, in principle, tabulate the standard cell potentials for all possible cathode/anode combinations. However, it is not necessary to undertake this arduous task. Rather, we can assign a standard potential to each half-cell and then use these half-cell potentials to determine Etea- The cell potential is the difference between two half-cell potentials. By convention, the potential associated with each electrode is chosen to be the potential for reduction at that electrode. Thus, standard half-cell potentials are tabulated for reduction reactions, which means they are standard reduction potentials, denoted Ered- The standard cell potential, ceU> is the standard reduction potential of the cathode reaction, (cathode), minus the standard reduction potential of the anode reaction, (anode) ... 

The corresponding quantities in solution are Eqx° (D) and Ered° (A), the thermodynamic oxidation and reduction potentials of the donor and acceptor, respectively. 

Since the electron donating feature of an oxygen atom in a methoxyethyl group weakens the cation s positive charge, the electrostatic binding between the ammonium cation and anion weakens, and an ionic liquid forms. The limiting reduction and oxidation potentials (Ered and Eoxd) on platinum of the ionic liquids were measured by cyclic voltammetry at room temperature as shown in Fig. 3. 

The free energy changes of the outer shell upon reduction, AG° , are important, because the Nernst equation relates the redox potential to AG. Eree energy simulation methods are discussed in Chapter 9. Here, the free energy change of interest is for the reaction... 

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