Potentials, Oxidation, and Reduction

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

Table 12.1. Oxidation and Reduction Potentials for Some Aromatic Hydrocarbons"...

Figures 12-12 and 12-13 document that trap-free SCL-conduction can, in fact, also be observed in the case of electron transport. Data in Figure 12-12 were obtained for a single layer of polystyrene with a CF -substituted vinylquateiphenyl chain copolymer, sandwiched between an ITO anode and a calcium cathode and given that oxidation and reduction potentials of the material majority curriers can only be electrons. Data analysis in terms of Eq. (12.5) yields an electron mobility of 8xl0 ycm2 V 1 s . The rather low value is due to the dilution of the charge carrying moiety. The obvious reason why in this case no trap-limited SCL conduction is observed is that the ClVquatciphenyl. substituent is not susceptible to chemical oxidation.

Fig. 7. Schematic diagram showing the oxidation and reduction potentials of conducting polymers relative to oxygen reduction and water oxidation.

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

Only very scarce knowledge on this subject is available. Oxidation and reduction potentials of seven benzoylsilanes and three benzoylgermanes were measured102. The values are... 

All of the zirconium pz and porphyrin sandwich complexes have up to two reversible ring oxidations and three reversible ring reductions. The zirconium porphyrazines are harder to oxidize by -400 mV and easier to reduce by at least 400 mV than the analogous porphyrins, making them better oxidants and worse reductants. The heteroleptic pz-porphyrin sandwich complex has oxidation and reduction potentials between those measured for the porphyrin and the pz sandwiches, as expected (Table VII). 

Bp- + DMA+ ip of 9.7kcaI/mol, which is derived from the difference in energies of 3Bp (69kcal/mol) and 3 Bp I DMA1 (59.3 kcal/mol), obtained from oxidation and reduction potentials [24]. From both a theoretical and an experimental perspective, when AG for electron transfer is no more negative than about 10 kcal/mol in a polar solvent such as acetonitrile, the electron... 

Table 16 Oxidation and reduction potentials for 21-chalcogenaporphyrins 123-125 and 21,23-dichalcogenaporphyrins 126 -129...

Excited states are both better oxidising and better reducing agents than their ground states. To a first approximation the oxidation and reduction potentials can be calculated as follows ... 

In the liquid phase, the equivalents of IPs and EAs are the electrochemical oxidation and reduction potentials and analogous thermochemical cycles have been used in the literature to calculate pK values. However, oxidation and reduction potentials of RsSi radicals are not yet established experimentally and, therefore, the solution thermochemical cycles suffer from these limitations. 

To get support for the postulation of ion-radical nature of the rate-determining step, one can plot the reaction rate constant values against the oxidation and reduction potentials of the reaction partners. When both plots occur to be linear, it will support the postulation of the ion-radical route. 

Korzhinskii, D.S. 1963. Correlation between activity of oxygen, oxidity and reduction potential in endogenic mineral formation. Izvestiya of Academy of Sciences of USSR. Geological seria. No. 3. 54-61. (In Russian)... 

Energetic considerations based on the separation of solvated ions at the encounter distance a show that solvated ion-pair formation from 1M is sufficiently exothermic in polar solvents to effectively prevent the production of excited singlet states 1M by the reverse process. Table XVIII lists values for free energies AGIM of ion-pair formation in acetonitrile estimated24 from the oxidation and reduction potentials, D/D+ and EA-tA, of donor and acceptor using the relationship... 

Table 8.11). This series of M3N Cgg does not display as many differences in their oxidation and reduction potentials as the M3N Cgo series. This is presumably due to...

For the monocyclic complexes 1-7, oxidation to Ni(III) occurs at +0.90-+0.93 V and reduction to Ni(I) at -1.46--1.55 V vs SCE. However, in the macropolycyclic ligand complexes 9-14, oxidation and reduction occur at + 1.25-+1.60 V and at -0.94-—1.40 V vs SCE, respectively. That is, electrochemical oxidation of Ni(II) complex to Ni(III) species is easier for the monocyclic complexes, whereas electrochemical reduction to Ni(I) is easier for the macropolycyclic complexes. The anodic shifts in both oxidation and reduction potentials for Ni(II) macropolycyclic complexes in part may be attributed to the tertiary nitrogen donors of the ligands. 

The introduction of -alkyl substituents to the secondary amine donors of the macrocycle results in anodic shifts in both oxidation and reduction potentials of the complexes relative to the parent ligand systems (Table II). The extent of anodic shifts depends on the number of alkyl groups introduced to the ligand (47,55a). That is, -alkylation makes the attainment of the Ni(I) state easier and the Ni(III) state more difficult. The stabilization of Ni(I) species by -alkylation is ascribed to solvation and stereochemical effects (55b, 60). -Ethyl groups have greater inductive effects than -methyl groups and yield less anodic shift in both oxidation and reduction potentials (47). This anodic shift of redox potentials may be attributed to weaker Ni-N interactions in the -alkylated complexes. The weaker Ni-N interaction for the tertiary amine results in the stabilization of antibonding o--orbitals of the Ni(II) complex, which makes it more favorable to add an electron, but less favorable to remove an electron. 

The configuration of the macrocyclic ligand affects the electrochemical properties of Ni(II) complexes (Table I) (56a, 54). For example, the oxidation and reduction potentials of CR,S,R,S)-[Ni(14)]2+ are shifted by +0.14and +0.13 V, respectively, compared with those of the Rfi,S,S isomer. Similar trends are also observed for a series of R,Sfi,S and Rfi,S,S isomers of -methylated cyclam derivatives (61a, 61b). The anodic shift of the redox potentials for the i ,S ,S-Ni(II) complex indicates that the complex is more difficult to oxidize to Ni(III) but easier to reduce to Ni(I), compared with the RJl,S,S complex. This may be related to the reduced ligand field strength of the R,Sfi,S complex, which stabilizes the antibonding -orbitals and thus makes addition of an electron more favorable while removal of an electron is less favorable. 

Oxidation and reduction potentials in solution are also computed via reference to particular thermodynamic cycles as illustrated in Figure 11.10. In this case, however, the... 

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. 

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