Solvated electron species

The electrical transport properties of alkali metals dissolved in ammonia and primary amines in many ways resemble the properties of simple electrolytes except that the anionic species is apparently the solvated electron. The electrical conductance, the transference number, the temperature coefficient of conductance, and the thermoelectric effect all reflect the presence of the solvated electron species. Whenever possible the detailed nature of the interactions of the solvated electrons with solvent and solute species is interpreted by mass action expressions. 

The way in which this solvent modification occurs is suggested by the pattern of hyperfine constants for (which is one of the few solvated electron species sufficiently stable to obtain its NMR spectrum). The Knight shift of NMR lines is due to the contact Fermi (isotropic) hyperfine interaction of the excess electron with the magnetic nuclei (X) in the solvent molecules it is the measure of spin density, (0) in the r-type atomic orbitals centered on a given nucleus X ... 

Alkali metals in liquid ammonia can transfer an electron to the solvent, leading to so-called solvated electrons. These can add to the aromatic substrate 1 to give a reduced species, the radical anion 3 ... 

Persulfate (41) reacts with transition metal ions (e.g. Ag, Fe21, Ti31) according to Scheme 3.42. Various other reduetants have been described. These include halide ions, thiols (e.g. 2-mercaptoethanol, thioglycolic acid, cysteine, thiourea), bisulfite, thiosulfate, amines (triethanolamine, tetramethylethylenediamine, hydrazine hydrate), ascorbic acid, and solvated electrons (e.g. in radiolysis). The mechanisms and the initiating species produced have not been fully elucidated for... 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

The further fate of the solvated electrons depends on solution composition. When the solution contains no substances with which the solvated electrons could react quickly, they diffuse back and are recaptured by the electrode, since the electrochemical potenhal of electrons in the metal is markedly lower than that of solvated electrons in the solution. A steady state is attained after about 1 ns) at this time the rate of oxidahon has become equal to the rate of emission, and the original, transient photoemission current (the electric current in the galvaihc cell in which the illuminated electrode is the cathode) has fallen to zero. Also, in the case when solvated electrons react in the solution yielding oxidizable species (e.g., Zn " + Zn" ),... 

While aqueous solutions mean only dilute solutions, mixtures can be any proportion of HjO and DMSO. Cooper and coworkers found in the pulse radiolysis of H2O/DMSO mixtures two easily resolvable absorption bands at wavelengths >400nm. One band corresponds to the oxidizing species with a maximum at 600 nm and a relatively long half-life (1 to 4/is). The second band with higher wavelength (720-1500 nm) and shorter half-life is attributed to the solvated electrons. 

The first step is so fast that it can hardly be measured experimentally, while the second step is much slower (probably as a result of the repulsion of negatively charged species, R and R2-, in the negatively charged diffuse electric layer). The reduction of an isolated benzene ring is very difficult and can occur only indirectly with solvated electrons formed by emission from the electrode into solvents such as some amines (see Section 1.2.3). This is a completely different mechanism than the usual interaction of electrons from the electrode with an electroactive substance. 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

The alkali metals in liquid ammonia give deep coloured solutions which have been shown to contain solvated electrons. The unsaturated system takes up an electron to give an anion radical. There is evidence for this species from electron spin resonance studies. It accepts a proton from the solvent to give a radical which is reduced to a carbanion by another sodium atom. Finally the addition of a proton gives the reduced product. This proton is supplied by a protic solvent like enthanol and not from NH3. 

In the light of the success of the Birch conditions for reducing organic compounds it is not surprising that epoxides can be opened by solvated electrons [6-9]. The initially formed radical is then further reduced to give carbanionic species, which do not display the reactivity of radicals. This concept has been extended by Bartmann [10], Cohen et al. [11], Conrow [12], and Yus et al. [13,14] who employed aromatic radical anions as the reduc-... 

Recently, the radical cation of PBN has been characterized by matrix spectroscopy and its reactivity has been studied by fast spectroscopic methods (Zubarev and Brede, 1994), and found to conform to the behaviour deduced from the OsCU and TBPA + studies. y-Radiolysis of PBN in a glassy matrix of isobutyl chloride or Freon-113 (CF2C1CFC12) at 77 K produced an intensely green glass containing PBN +, the epr spectrum of which had an anisotropic nitrogen coupling constant Ay = 2.75 mT and gy = 2.0037. Tlie mechanism of the radiolysis reaction is well established (Neta, 1976) and involves the formation of solvated electrons (e ), which add to the matrix species and produce chloride ion, and positive holes (h+) which eventually come to rest at the matrix component of lowest Ip (Symons, 1997), in this case PBN (see reactions (30) and (31)). 

It seems reasonable to note that the micro-jet stream generated by the ultrasonic cavitation promotes mass transport. Such an effect was discussed for proton transport in aqueous solutions (Atobe et al. 1999). Understandably, a proton moves in the solution as a hydrated particle. Nevertheless, we should pay attention on the similarity between proton and electron, in the sense that both are essentially quantum particles. A solvated electron, therefore, can be considered as a species that is similar to a hydrated proton. Hence, the micro-jet stream can promote electron transfer. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Aromatic radical-cations are generated by pulse-radiolysis of benzene derivatives in aqueous solution. Radiolysis generates solvated electrons, protons and hydroxyl radicals. The electrons are converted by reaction with peroxydisulpbate ion to form sulphate radical-anion, which is an oxidising species, and sulphate. In another proceedure, electrons and protons react with dissolved nitrous oxide to form hydroxyl radicals and water, Hydroxyl radicals are then made to react with either thallium(i) or silver(i) to generate thallium(ii) or silver(ll) which are powerfully... 

Y is either a solvated electron (displaced electron formed during the radiolytic reaction) or the product of the electron having reacted with some compound in the reaction system [Allen et al., 1974 Hayashi et al., 1967 Kubota et al., 1978 Williams et al., 1967]. If Y is an electron, the propagating carbocation centers are converted to radical centers that subsequently undergo reaction with some species in the reaction system to form molecular species. The termination rate is given by... 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

In experiments done by Baxendale [24] and Baxendale and Sharpe [25] in mixed n-propanol-hexane systems, it was found that at low concentrations of alcohol, a solvated species was formed however, it was not the solvated electron. The existence of the solvated species was determined by the decrease in mobility of the electron in the solvent. The mobility never decreased to the level that would be expected for the solvated electron and no evidence of the solvated electron spectrum was detected. 

As demonstrated below, a primary charge viewed as a solvated electron or the molecular ion residing in an inert liquid does not account for experimental observations in many, if not in most, of the systems. While we cannot offer a specific, general model of these exceptional ions, we provide a general introduction to the known properties of such species. Furthermore, we argue that these species comprise the rule rather than the exception. The reader is invited to reach his or her own conclusions. 

It may appear from the above that only nonpolar liquids yield nonmolecular solvent anions upon the ionization. Perhaps this is misleading Most polar liquids studied by radiation chemists are aliphatic alcohols and water, and these liquids yield solvated electrons rather than radical ions. Although there has been sporadic interest in other polar liquids (e.g., neat acetone), the current state of knowledge of such systems does not allow one to reach any conclusion as to the nature of the reducing species observed therein (although, see Sec. 4). 

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