Electron attachment table, rate constants

The initial kinetic energy of 0 ions produced by dissociative attachment in 02 at an electron energy of 6.9 e.v. may be determined from Equation 4 to be 1.64 e.v. using values of 1.465 e.v. (1) for A(0) and 5.09 e.v. (7) for D(O—O). The residence time for 0 ions calculated from Equation 1 is 6.0 X 10 7 sec. at 10 volts repeller potential. Rate constants for Reaction 6 determined from data at varying Vr are shown in Table I and are seen to increase sharply with increasing repeller potential, as expected for an endothermic process. 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

Dimerization is the characteristic reaction of radical-anions from activated alkenes. The rate constants for dimerization are high and the conjugate acids from such alkene radical-anions in many cases have low pKa values and. The data in Table 3.4 were obtained by following the changes in uv-absorbance after pule-radiolysis of the substrate in an aqueous buffer. Attachment of a solvated electron leads to the radical-anion. Changes in the initial absorbance with pH lead to determination of the pKg value, while the dimerization rate can be determined from changes in absorbance over a longer time scale. Radical-anions from esters and amides are pro-... 

The absolute rate constants were determined for a variety of reactions of the solvated electron in ethanol and methanol. Three categories of reaction were investigated (a) ion-electron combination, (b) electron attachment, and (c) dissociative electron attachment. These bimolecular rate constants (3, 27, 28) are listed in Table III. The rate constants of four of these reactions have also been obtained for the hydrated electron in water. These are also listed in the table so that a comparison may be made for the four rate constants in the solvents ethanol, methanol, and water. 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

TABLE 11.4 Thermodynamic Properties for Dissociative Electron Attachment to Alkyl Halides from Rate Constants at 298 K (in eV)... 

Electron Acceptance Reduction and Adduct Formation. Acceptance of electrons at specific sites on amino acids and peptides depends on their reactivities and produces different chemical consequences. Among the sites of particular importance are the terminal amino and carboxyl groups, the ring groups, the peptide carbonyl, and the sulfur bonds. Reactivities of these are reflected in the rate constants for reaction of solvated electrons with individual amino acids in aqueous solutions, as shown in Table I and as discussed by Simic (53). More detailed information, however, regarding the stepwise progression from attachment to specific radical formation has been obtained from low temperature studies. 

The formation reaction, 1, is very rapid. Absolute rate constants for electron attachment to four aromatic compounds in ethyl alcohol (4) are shown in Table I. These rate constants increase in order of increasing electron affinities of the aromatic molecule, but it should be noted that such a correlation may be fortuitous. The magnitude of the rate constants is equal to or near that of a diffusion controlled reaction, and a correlation with molecular size is also seen for this set of reactants. 

Table I. Rate Constants for the Formation of Radical Anions by Electron Attachment in Ethyl Alcohol at 25°C.

Table 7 Reaction Rate Constants, kg, for the Attachment of Electrons to Solutes in Various Solvents... 

An SECM feedback response to an electroactive polymer film can be controlled either by ET kinetics at the film-solution interface or by film conductivity. The contribution of lateral film conductivity to the effective ET rate measured by SECM was addressed in the recent study of polyaryl multilayers attached with ferrocenes [65] or ferrocene-terminated dendrimers [69] on unbiased carbon electrodes. In the latter study, the dependence of apparent ET rate constant, feei. on the generation of dendrimers (Table 6.2) was ascribed to the different efficiencies of electron transport inside and between dendrimers (Figure 6.22). Interestingly, the theory of the aforementioned triple potential step method predicts its potential to separately determine heterogeneous ET rate and lateral conductivity for a thin polymer film coated on an insulating surface [70]. 

The influence of the furazan moiety on rate and equilibrium constants for (j-adduct formation from 4-nitrobenzofurazan has been compared with that of the related S and Se analogs.229 Whether the adducts are formed by attachment at the 5- or 7-position (see Section I V,B,4), the sulfur and selenium 196, 197,198, and 199 adducts are formed much more slowly and are much less stable than the related furazan adducts 164 and 163 (Table XXXV). These effects have been interpreted as due to a combination of the aromatic character of the starting substrates, which increases in the order S > Se O, and of the electron-withdrawing effect of the heteroatom. 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

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