Electron charge-transfer

A third method for generating ions in mass spectrometers that has been used extensively in physical chemistry is chemical ionization (Cl) [2]. Chemical ionization can involve the transfer of an electron (charge transfer), proton (or otlier positively charged ion) or hydride anion (or other anion). 

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

Figure 2. The average number of electron charges transferred from Zn atoms to Cu atoms in fee disordered alloys. The solid dots are calculated with the LSMS. The open circles are obtained using the CPA-LSMS. The squares are obtained using the SCF-KKR-CPA.

Fig. 20 Variation of the fraction <5 of an electronic charge transferred from B to XY on formation of B- XY with the ionisation energy 7b of B for the series XY = 02, BrO and IO. See text for the method of determination of Si from observed XY nuclear quadrupole coupling constants. The solid curves are the functions <5 = A exp(- al ) that best fit the points for each series B- XY. Data for B- -B are nearly coincident with those of B- BrO and have been excluded for the sake of clarity...

The present chapter will cover detailed studies of kinetic parameters of several reversible, quasi-reversible, and irreversible reactions accompanied by either single-electron charge transfer or multiple-electrons charge transfer. To evaluate the kinetic parameters for each step of electron charge transfer in any multistep reaction, the suitably developed and modified theory of faradaic rectification will be discussed. The results reported relate to the reactions at redox couple/metal, metal ion/metal, and metal ion/mercury interfaces in the audio and higher frequency ranges. The zero-point method has also been applied to some multiple-electron charge transfer reactions and, wheresoever possible, these results have been incorporated. Other related methods and applications will also be treated. 

Substituting ac + aa = 1, for single-electron charge transfer reactions, the above expression reduces to that of Delahay et al.n... 

If a two-electron charge transfer reaction takes place in two separate steps, each being accompanied by transfer of a single electron, the mathematical expression for the determination of kinetic parameters becomes more involved and complicated. 

It was therefore thought appropriate to suitably modify and develop the faradaic rectification theory for the study of multiple-electron charge transfer reactions. 

In any two-electron charge transfer reaction, the two steps can be represented as follows ... 

The value of a is taken to be the same in both of the steps of electron charge transfer and its value is assumed to be close to 0.5. 

If A co, is the rectification potential due to the first step of the reaction and AJECO 1 is the rectification potential contribution due to the second step, then the total rectified potential should be the sum of the rectified potentials for each individual step, i.e., AE + A oo . The combined faradaic rectification change for both the steps of electron charge transfer can be represented as38... 

Assuming that a practically remains constant in both the steps of electron charge transfer and putting AE + AE = A... 

In Appendix B, the mathematical derivation is given for obtaining experimentally the values of C°Rll, C°Rl, k , k , and k in the case of any three-electron charge transfer reaction. 

From the derivations in Appendix B, it is evident that the present faradaic rectification formulations for multiple-electron charge transfer not only enable the determination of kinetic parameters for each step of three-electron charge transfer processes but may also be extended to charge transfer processes involving a higher number of electrons. However, the calculations become highly involved and complicated. 

Very few references are available on the determination of the rate constant for each step of electron charge transfer in the reaction M2+ + 2e -> M(s), i.e., M2+ + e -> M+, M+ + c" -> M(s). Earlier studies are mostly related to two-electron charge transfer reactions either at M2+/Hg(dme), M2+/metal amalgam, or redox couple/Pt interfaces. Even in these studies, the kinetic parameters have been determined assuming that one of the two steps of the reaction is much slower and is in overall control of the rate of reaction in both... 

Some of the two-electron charge transfer reactions which have recently been studied are Cu(II)/Cu(s), Ni(II)/Ni(s), Cd(II)/Cd(s), and Zn(II)/Zn(s). Their kinetic parameters in different supporting electrolytes are given in Tables 1 and 2. 

Recently, the kinetic parameters for each step of this reaction in different supporting electrolytes have been obtained39,42 by applying the faradaic rectification theory as extended to multiple-electron charge transfer reactions. The kinetic parameters are listed in Table 1. 

By using Delahay s equation, assuming that both electron charge transfers occur in a single step. In this case, the rate constant obtained for the slowest reaction is of the order of 10-5 cm/s (Table 2). 

By applying the recently developed theory of faradaic rectification as applied to multiple-electron charge transfer reactions under the condition that k° and C°R = 1. Kinetic parameters are obtained for each step of the electron charge transfer. The value of /c° reported is of the order of 10 6 to 10-9 cm/s whereas that of fc is of the order of 10 3 cm/s in different supporting electrolytes.51... 

This reaction is found to be stable in sodium acetate and acetic acid buffer (pH 4.65), and so it has only been studied in this medium. The faradaic rectification theory becomes highly complicated when extended to three-electron charge transfer reactions due to the formation of the two intermediate species Al(II) and A1(I). In order to determine the three rate constants and the two unknown concentration terms, C°Rl and C°Ru, corresponding to the two intermediate species formed, it becomes necessary to carry out the experiment at five different concentrations of aluminum ion, each below 2.00 mM. 

Kinetic Parameters of Some Single-Electron Charge Transfer Redox Couples at a Platnium Interface ... 

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