Solvated electron reactions

In this scheme DMSO is to be regarded as a solvent anion formed by an electron attachment or solvent decomposition from free or solvated electrons. Reaction 13 can be... 

We should remember (1) that the activation energy of eh reactions is nearly constant at 3.5 0.5 Kcal/mole, although the rate of reaction varies by more than ten orders of magnitude and (2) that all eh reactions are exothermic. To some extent, other solvated electron reactions behave similarly. The theory of solvated electron reaction usually follows that of ETR in solution with some modifications. We will first describe these theories briefly. This will be followed by a critique by Hart and Anbar (1970), who favor a tunneling mechanism. Here we are only concerned with fe, the effect of diffusion having been eliminated by applying Eq. (6.18). Second, we only consider simple ETRs where no bonds are created or destroyed. However, the comparison of theory and experiment in this respect is appropriate, as one usually measures the rate of disappearance of es rather than the rate of formation of a product. 

Schindewolf and WUnschel [112] have studied solvated electron reactions in liquid ammonia and water with several univalent anions and divalent cations. Ions such as NO3, N02, and BrO in water showed diffusion-limited behaviour and the ions Cd2+, Ni2+, Co2+, and Zn2+ in water displayed diffusion-limited behaviour or faster. Schindewolf and WUnschel considered that reactions of none of these ions were quite diffusion-limited in liquid ammonia. Applying the hydrodynamic correction suggests that the anionic reaction with solvated electrons may just be diffusion-limited, but the cations reaction with solvated electrons remains slower than diffusion-limited. 

Hart and Anbar [17] pointed out that effective rate coefficients of solvated electron reactions with many strong oxidants were larger than those implied by the encounter distances for the solvated electron and oxidant by a factor of 1.5—2.0 times. Some of these effective encounter distances are listed in Table 5, together with others from recent work. For... 

It is clear that the study of solvated electron reaction rates, using metal-amine solutions as the source of electrons, is currently in the early stages of development, and the conclusions drawn must be very limited in scope. If the species present in metal solutions in ethylenediamine and other amines can be firmly identified, the techniques described in this paper should prove useful for studying reactions of solvated electrons with... 

A solvated electron may be regarded as the simplest radical anion. The chemistry of solvated electron reactions is qualitatively similar to radical anion reactions but the physical properties of electron solutions are very complicated [167]. They are not suitable as a model of radical anions. 

The studies into the electrochemical kinetics of solvated electrons were to some extent stimulated by the hypothesis put forward in the second half of 6O s (see Sect. 8) for explaining the role of solvated electrons as intermediate products of electrode reactions, and also by the development made at that time in organic synthesis involving the participation of solvated electrons (see Sect. 9). Undoubtedly, knowledge of the mechanism of electrode generation of solvated electrons is of fundamental importance. Electrochemistry is the chemistry of the electron , Professor A. N. Frumkin once said. In fact, electron reactions at the interface of electronic and ionic conductors are inevitably associated with the electron addition or detachment process. In a solvated electron reaction no heavy particle (atom or molecule) acts as electron acceptor, or donor. In this sense, the electrode reactions of solvated electrons are the most simple electrode processes. Therefore, an insight into the solvated electron reaction mechanism is necessary for electrochemical kinetics as a whole. 

Using pulse radiolysis, Dye et al. [99] have studied the formation of Na" from and sodium in ethylenediamine. The reaction of caesium with water in ethylenediamine again shows that the solvated electron reaction with water is slow [95, 100]. 

Solvated Electron Reaction Rates. Purines and Pyrimidines. The reaction rates of e aq with purines and pyrimidines at neutral pH are shown in Table II. All are very reactive, the reaction rates being close to diffusion controlled. However, the pyrimidine cytosine, which has an amino group at the C-4 position, is somewhat less reactive than thymine and uracil which have carbonyl groups at this position. Adenine, which also has an amino group in this position, has a very high reactivity, but this is probably because of the presence of the positively charged imidazole ring. 

It is obvious that during separation the organic phase is under the effect of intensive y, P, and a radiation fields. Therefore, radiation resistance is a very important issue. TBP decomposes to products dibutyl phosphate (G 0.2 pmol J ), monobutyl phosphate (0.02 pmol J ), and gaseous hydrogen (0.15 pmol J ). In addition to these main components some minor products, butane, butene, C1-C3 hydrocarbons, butanol, and phosphoric esters are also observed. Dibutyl phosphate is suggested to form in solvated electron reaction... 

The reaction of all polynuclear aromatic compounds with the solvated electron (reaction 1) is very rapid (2,3), >10 m" s , and in most cases it is diffusion controlled. Under the experimental conditions used, this process was complete long before the protonation took place. 

Evidence for the solvated electron e (aq) can be obtained reaction of sodium vapour with ice in the complete absence of air at 273 K gives a blue colour (cf. the reaction of sodium with liquid ammonia, p. 126). Magnesium, zinc and iron react with steam at elevated temperatures to yield hydrogen, and a few metals, in the presence of air, form a surface layer of oxide or hydroxide, for example iron, lead and aluminium. These reactions are more fully considered under the respective metals. Water is not easily oxidised but fluorine and chlorine are both capable of liberating oxygen ... 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

These reactions proceed to equilibrium when the potential of the solvated-electron electrode equals that of the alkali metal L13] ... 

OCV conditions, by a newly formed SEI is expected to be a slow process. The SEI is necessary in PE systems in order to prevent the entry of solvated electrons to the electrolyte and to minimize the direct reaction between the lithium anode and the electrolyte. SEI-free Li/PE batteries are not practical. The SEI cannot be a pure polymer, but must consist of thermodynamically stable inorganic reduction products of... 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

S-N bond cleavage 159 S-O bond lengths 543 Solvated electrons 897, 905 Solvent effects 672 in elimination reactions 772 S-O stretching frequencies 543, 545, 546, 552-555, 560-562 Spiroconjugation 390 Stereoselectivity 779, 789 of cylcoaddition reactions 799 of sulphones 761 Steroids... 

This is called the SrnI mechanism," and many other examples are known (see 13-3, 13-4,13-6,13-12). The lUPAC designation is T+Dn+An." Note that the last step of the mechanism produces ArT radical ions, so the process is a chain mechanism (see p. 895)." An electron donor is required to initiate the reaction. In the case above it was solvated electrons from KNH2 in NH3. Evidence was that the addition of potassium metal (a good producer of solvated electrons in ammonia) completely suppressed the cine substitution. Further evidence for the SrnI mechanism was that addition of radical scavengers (which would suppress a free-radical mechanism) led to 8 9 ratios much closer to 1.46 1. Numerous other observations of SrnI mechanisms that were stimulated by solvated electrons and inhibited by radical scavengers have also been recorded." Further evidence for the SrnI mechanism in the case above was that some 1,2,4-trimethylbenzene was found among the products. This could easily be formed by abstraction by Ar- of Ft from the solvent NH3. Besides initiation by solvated electrons," " SrnI reactions have been initiated photochemically," electrochemically," and even thermally." ... 

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