Electron affinities, standard potentials

It is clear that deuterium as a substituent has the electron-donating effect. In other words, it can decrease electron affinity of the whole molecule. Potentials of reversible one-electron reduction for naphthalene, anthracene, pyrene, perylene, and their perdeuteriated counterparts indicate that the counterparts exhibit slightly more negative potentials (Goodnow and Kaifer 1990, Morris and Smith 1991). For example, the measurable differences in the reduction potentials are equal to -13 mV for the pair of naphthalene-naphthalene-dj or -12 mV for the pair of anthracene-anthracene-djo. The possible experimental error does not exceed 2 mV (Morris and Smith 1991). In another example, in DMF with 0.1 M n-Bu4NPFg, the deuterated pyrenes were invariably found to be more difficult to reduce than pyrene itself. The largest difference observed, 12.4 mV, was between perdeuteriated pyrene and pyrene bearing no deuterium at all with standard deviations between 0.2 and 0.4 mV (Hammerich et al. 1996). 

This chapter gives a selected compilation of the standard and other characteristic (formal, half-wave) potentials, as well as a compilation of the constant of solubility and/or complex equilibria. Mostly, data obtained by electrochemical measurements are given. In the cases when reliable equilibrium potential values cannot be determined, the calculated values (calcd) for the most important reactions are presented. The data have been taken extensively from previous compilations [5-13] where the original reports can be found, as well as from handbooks [13-16], but only new research papers are cited. The constant of solubility and complex equilibria were taken from Refs 6-11,13,17-21. The oxidation states (OSs), ionization energies (IBs) (first, second, etc.), and electron affinities (EAs) of the elements and the... 

The outer-sphere one-electron reduction of CO2 leads to the formation of the 02 radical anion. In dry dimethylformamide, the C02/ C02 couple has been experimentally determined to be —2.21 V vs. standard calomel electrode (SCE) or approximately —2.6 V vs. the ferrocene/ferrocenium couple [21,22]. From pulse radiolysis experiments, the reduction potential of CO2 is —1.90 V vs. the SHE in water (—2.14 V vs. SCE) [23]. Theoretical calculations have been used to calculate the contributions of various factors to the reduction potential of CO2. These include the electron affinity of CO2,... 

When two conjugate redox pairs are together in solution, electron transfer from the electron donor of one pair to the electron acceptor of the other may proceed spontaneously. The tendency for such a reaction depends on the relative affinity of the electron acceptor of each redox pair for electrons. The standard reduction potential, E°, a measure (in volts) of this affinity, can be determined in an experiment such as that described in Figure 13-14. Electrochemists have chosen as a standard of reference the half-reaction... 

The property of the electronegativity of an atom in a molecule is different from the electrode potential of the element, which depends on the difference in free energjr of the element.in its standard state and in ionic solution, and it is different from the ionization potential of the atom, and from its electron affinity although it is related to these properties in a general way. 

Cyclic voltammetry studies showed the ionization potential and electron affinity of each component of the molecule in solution. The HOMO and LUMO energy levels were estimated from the equations Ehomo = E x + 4.4 eV and Elumo = T 14+4.4 eV, where E(r]x and E%, were oxidation and reduction potentials with respect to the standard hydrogen electrode (SHE) and the value of 4.4 is the ionization potential for hydrogen in eV [94,95], The HOMO and LUMO energy levels of the methine dye (compound 6) (Scheme 13) were determined to be -5.82 and -3.48 eV, respectively, with respect to the vacuum level from... 

The oxidation-reduction potential, E, (or redox potential) of a substance is a measure of its affinity for electrons. The standard redox potential (E0 ) is measured under standard conditions, at pH 7, and is expressed in volts. The standard free energy change of a reaction at pH 7, AG0, can be calculated from the change in redox potential AE0 of the substrates and products. A reaction with a positive AE0 has a negative AG0 (i.e. is exergonic). 

The lattice enthalpy U at 298.20 K is obtainable by use of the Born—Haber cycle or from theoretical calculations, and q is generally known from experiment. Data used for the derivation of the heat of hydration of pairs of alkali and halide ions using the Born—Haber procedure to obtain lattice enthalpies are shown in Table 3. The various thermochemical values at 298.2° K [standard heat of formation of the crystalline alkali halides AHf°, heat of atomization of halogens D, heat of atomization of alkali metals L, enthalpies of solution (infinite dilution) of the crystalline alkali halides q] were taken from the compilations of Rossini et al. (28) and of Pitzer and Brewer (29), with the exception of values of AHf° for LiF and NaF and q for LiF (31, 32, 33). The ionization potentials of the alkali metal atoms I were taken from Moore (34) and the electron affinities of the halogen atoms E are the results of Berry and Reimann (35)4. 

In one of the most extensive studies of metal chloride catalysts,1 twenty of them supported on carbon were investigated, and a correlation was proposed between their activity and the electron affinity of the metal cation divided by the metal valence. Since the correlation consisted of two straight lines, it cannot be used predictively. However, electron affinity is necessarily a one-electron process, whereas hydrochlorination is more likely to be a two-electron process, involving the 2ir electrons of ethyne. Because many of the cations investigated are divalent, standard electrode potential was suggested1 as a more suitable parameter for correlating with activity. 

A free energy of reduction AGred bears the relation, AGred = -nFE0, to the standard reduction potential <) > which is approximately identical to the first E /2- A combination of these relationships and Born s solvation model yields the following equation between the difference in electron affinity A a and the difference in the first half wave potential A i/2 for two species [33]. 

This equation permits the calculation of the lattice energy if we know F, the electron affinity of the halogen atom, S, the heat of sublimation of the alkali metal, /, the ionization potential of the metal and Z), the dissociation energy of the molecular halogen. These quantities are known, but to different orders of accuracy, and furthermore, the values should all refer to the same standard temperature, either absolute zero or room temperature, a condition which is not always fulfilled. However, the agreement between the calculated and observed values is sufficiently good to indicate that the theory developed for the lattice energy on the basis of ionic interaction is basically correct. 

Data from C.E. Moore, Ionization Potentials and Ionization Limits, National Standards Reference Data Series, U. S, National Bureau of Standards, Washington, DC, 1970, NSRDS-NBS 34) Electron affinity = At/ for... 

Sources Ionization energies cited in this chapter are from C. E. Moore, Ionization Potentials and Ionization Limits Derived from the Analyses of Optical Spectra, National Standard Reference Data Series, U.S. National Bureau of Standards, NSRDS-NBS 34, Washington, DC, 1970, unless noted otherwise. Electron affinity values listed in this chapter are from H. Hotop and W. C. Lineberger, J. Phys. Chem. Ref. Data, 1985,14,731. Standard electrode potentials listed in this chapter are from A. J, Bard, R. Parsons, and... 

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