Competition between electrons

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

Bonn M, Denzler DN, Eunk S, Wolf M. 2000. Ultrafast electron dynamics at metal surfaces Competition between electron-phonon coupling and hot-electron transport. Phys Rev B 61 1101-1105. 

Scheme 2 Competition between electron transfer (7 8) and water trapping (7 9+10) of sugar radical cation 7...

The results of Table 1 show clearly that electronation of nitrocumene (39) (Scheme 15) does compete with reaction with chemisorbed hydrogen, M(H), at some stage in the electrohydrogenation process. The simplest interpretation is a direct competition between electronation of the nitro compound (eq. [7]) and reaction of the adsorbed nitro compound with chemisorbed hydrogen, M(H) (eq. [13]). However, it is quite possible that the electronation of the adsorbed nitro compound (eq. [20]) could be faster than its reaction with M(H) (eq. [13]) and the competition would then be between the cleavage of the adsorbed radical anion (eq. [21]) and its reaction with M(H) (eq. [22]). 

It is noteworthy that the relative proportion of amine 44 and bicumyl (43) which reflects the ratio of the rate of electronation to the rate of reaction with M(H) (the competition between electronation and reaction with M(H)), varies with the Raney metal (compare entries 1 and 3 of Table 1, and entries 2 and 4) and with the electrode potential (compare entries 1 and 2). The more negative is the potential, the faster is the rate of electronation and the higher should be the proportion of bicumyl (43) as observed (entries 1 and 2). The less active the Raney metal as hydrogenation catalyst, the slower is the rate of reaction with M(H) (the lower is the amount of M(H) at the surface of the electrode) and the lower is the amount of aminocumene (44). RCu is the least active catalyst and the proportion of aminocumene (44) is indeed the lowest at the RCu cathode (entry 4). 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

In the electrochemical case, using, for example, cyclic voltammetry, one way of driving the potential toward more negative values is to increase the scan rate. This is true whether the linearization procedure or the convolution approach is followed. In the first case, equation (3.4) shows that the activation free energy at the peak, AG, is a decreasing function of the scan rate as a result of the kinetic competition between electron transfer and diffusion. The larger the scan rate, the faster the diffusion and thus the faster the electron transfer has to be in order to compete. This implies a smaller value AG, which is achieved by a shift of the peak potential toward more negative values. 

Under these conditions, as sketched on the left-hand side of Figure 4.16, the linear diffusion layer has become very thin, on the same order as the constrained diffusion layer. The response amounts therefore to the steady-state response of an assembly of nas independent disk microelectrodes. The shape of the S-wave and the location of the half-wave potential is a function of the last term in the denominator on the right-hand side of equation (4.18). The parameter that governs the kinetic competition between electron transfer and constrained diffusion is therefore... 

Fig. 5 Electrochemical stepwise electron-transfer-bond-breaking reactions. Competition between electron transfer, bond breaking and diffusion. E i2, Half-wave potential RX/RX- standard potential. The horizontal scale is given both in terms of X and k. The number on each curve is the value of A, and the value of log ky is given in parentheses. 5 is taken as 10 cm and D as 10" cm s" . (Adapted from Andrieux et al., 1978.)...

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation ... 

This competition between electrons and the heat carriers in the lattice (phonons) is the key factor in determining not only whether a material is a good heat conductor or not, but also the temperature dependence of thermal conductivity. In fact, Eq. (4.40) can be written for either thermal conduction via electrons, k, or thermal conduction via phonons, kp, where the mean free path corresponds to either electrons or phonons, respectively. For pure metals, kg/kp 30, so that electronic conduction dominates. This is because the mean free path for electrons is 10 to 100 times higher than that of phonons, which more than compensates for the fact that C <, is only 10% of the total heat capacity at normal temperatures. In disordered metallic mixtures, such as alloys, the disorder limits the mean free path of both the electrons and the phonons, such that the two modes of thermal conductivity are more similar, and kg/kp 3. Similarly, in semiconductors, the density of free electrons is so low that heat transport by phonon conduction dominates. 

From an extended study on the sequence selectivity of UV-induced cleavage of dsODNs (Table 12.12) it has been concluded that an ET to neighboring bases must occur [reaction (40)] followed by a subsequent competition between electron backdonation [reaction (41)], decay of the 5BrUra radical anion [reaction (42)] and hole transfer [reaction (43) Chen et al. 2000. 

In Section VIII.F the competition between electron transfer to the excited and ground states during recombination of electrochemically injected ions was studied in the framework of spinless IET. In fact, the primary products of such recombination are usually the low-energy triplet excitations. They are produced from triplet RIPs, while singlet RIPs recombine to the ground state ... 

The fluorescence spectrum is found to be markedly non-Boltz-mann and sharply peaked at the directly excited level throughout the laser pulse. This is due to two effects the competition between electronic quenching and rotational relaxation processes (4) and the short length of the laser pulse. Because the pulse is so short, steady state is not established throughout the upper rotational levels. The peaks of the fluorescence pulses from levels which are not directly excited by the laser lag the laser pulse peaks by one to four nanoseconds, depending on the energy gap between the given level and the directly excited level. 

The rate of recombination depends on the density of surface states and their occupation with electrons. In the case of an n-type semiconductor, the competition between electron transfer involving the reduced component of a redox couple and surface recombination can be described by the simplified reaction scheme... 

This section considers the competition between electron transfer and surface recombination under steady state conditions, and shows that no kinetic information can be obtained from the measured photocurrent. 

An example of the analysis of IMPS data based on the competition between electron transfer and recombination is provided by a detailed study of hydrogen evolution on illuminated p-InP in acid solution [29]. Since the photogenerated minority carriers are electrons in the case of p-InP, they are driven to the surface under depletion conditions, where they can reduce protons to hydrogen. Figure 8.9 is a set of experimental IMPS responses measured for p-InP in 1.0 mol dm-3 H2S04 at different potentials. The measurements were performed using a small ac modulation of the illumination intensity superimposed on a larger steady component. 

The competition between electron transfer at the electrode (146) and in solution (147) in turn depends upon the rate of cleavage of the radical anion Ar—X (148) since this reaction determines the thickness of the reaction... 

These carbocations can undergo three reactions (i) single-electron oxidation, (ii) hydride abstraction and (iii) electrophilic addition. Thus, these compounds behave as mild single-electron oxidants towards reductants which do not react by other pathways. Actually, what is most interesting in these carbocations is the competition between electron-transfer and the other reactions. For instance, a hydride transfer can occur either directly or via the electron-transfer pathway using a trityl salt. From a synthetic standpoint, a hydride transfer which cannot be achieved directly for steric reasons may be attempted by means of the electron-transfer pathway. 

It has also been demonstrated ( ), by studying the competition between electron loss and a unlmolecular decomposition of known activation energy, that sequential Infrared absorption continues to occur even after the anion has achieved enough total Internal energy to reach Its electron detachment threshold. This Implies that, near threshold, electron ejection must not be occurlng faster than the 10 -10 s photon absorption rate. These experiments do not, however, allow one to conclude with much certainty how far above... 

Quantitative evaluation of the number of radicals shows that, although the nature of the spectrum depends on the concentration of ethyl mercaptan, the total radical yield remains constant this suggests that the protective effect of the additive is due to a competition between electron scavenging by the additive and recombination with cations in the matrix, leading to scission of the polymer chain. The mechanism... 

IMPS Competition between Electron Transfer and Recombination. 106... 

Competition between electron transfer and radical abstraction, along with the greater ability of water and methanol to solvate the ions help complete the rationalization. Methanesulfmic acid 258 is thought to participate in further redox reactions and is not isolated. 

There has been a large number of theoretical studies since the inspiring work of Zusman devoted to dynamical solvent effects or, more generally, to effects associated with the competition between electronic transitions at strong coupling and various relaxation modes in the system [76-144]. Ovchinnikova [79] introduced an effective sink approximation to incorporate fast classical vibrational modes into the stochastic model of ET. She... 

The results for reaction of azide with several 4-methoxysty-rene radical cations provide an interesting example of the effect of solvent on the competition between electron transfer and addition. In TFE the 4-methoxystyrene, P-methyl-4-methoxystyrene, and p,P-dimethyI-4-methoxystyrene radical cations react with azide with rate constants of 7.0, 3.5, and 1.0 X 10 respectively. In all three cases the transient... 

Chemiluminescence and electrochemiluminescence efficiencies are often interpreted employing similar considerations. These efficiencies usually depend on the competition between electron transfer processes which form excited-state or ground-state (kg) products as in Scheme 1. In Scheme 1 (n - i) is an oxidation state index, the curly brackets are used to indicate the donor and acceptor species in close association (e.g., ion pairs or geminate radical pairs), and the asterisk indicates an electronically excited molecule. In a study of [Ru(bpy)3]", Bonafede et proposed that the efficiency of [( CT)Ru(bpy)3] chemiluminescence... 

Neglecting the back electron transfer step and the diffusion of redox active species to the liquid/liquid junction, the photocurrent signal can be expressed in terms of the surface concentration of porphyrins (see Equations 11.9 and 11.19) and the competition between electron transfer and excited state decay ". ... 

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