Reactions solvated electrons, fast

Pulse radiolysis is widely used for the study of fast free radical reactions and reactions of solvated electrons [223]. 

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)). 

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

Many of the fast chemical reactions discussed in the preceding sections involve at least one reactant which is of low symmetry. The reactions of the solvated electron with nitrate, naphthalene or pyrene are instances where the oxidant has a mirror plane (in the molecular plane) in the accepting orbital. Hence, reaction of the solvated electron with such a scavenger when both are contained in this plane should be slower than in other configurations. Similarly, the contact quenching of fluorescence from naphthalene or 1,2-benzanthracene by carbon tetrabromide [7], or... 

The rotational relaxation times of these nitrocompounds have not been measured. Comparison with the studies of perylene by Klein and Haar [253] suggests that most of these nitrocompounds have rotational times 10—20 ps in cyclohexane. For rotational effects to modify chemical reaction rates, significant reaction must occur during 10ps. This requires that electron oxidant separations should be <(6 x 10-7x 10-11)J/2 2 nm. Admittedly, with the electron—dipole interaction, both the rotational relaxation and translational diffusion will be enhanced, but to approximately comparable degrees. If electrons and oxidant have to be separated by < 2 nm, this requires a concentration of > 0.1 mol dm-3 of the nitrocompound. With rate coefficients 5 x 1012 dm3 mol-1 s 1, this implies solvated electron decay times of a few picoseconds. Certainly, rotational effects could be important on chemical reaction rates, but extremely fast resolution would be required and only mode-locked lasers currently provide < 10 ps resolution. Alternatively, careful selection of a much more viscous solvent could enable reactions to show both translational and rotational diffusion sufficiently to allow the use of more conventional techniques. 

The Chemical Reactivity of e aq. The chemical behavior of solvated electrons should be different from that of free thermalized electrons in the same medium. Secondary electrons produced under radio-lytic conditions will thermalize within 10 13 sec., whereas they will not undergo solvation before 10 n sec. (106). Thus, any reaction with electrons of half-life shorter than 10 n sec. will take place with nonsolvated electrons (75). Such a fast reaction will obviously not be affected by the ultimate solvation of the products, since the latter process will be slower than the interaction of the reactant with the thermalized electron. This situation may result in a higher activation energy for these processes compared with a reaction with a solvated electron. No definite experimental evidence has been produced to date for reactions of thermalized nonsolvated electrons, although systems have been investigated under conditions where electrons may be eliminated before solvation (15). 

Jhe discovery by radiation chemists of solvated electrons in a variety of solvents (5, 16, 20, 22, 23) has renewed interest in stable solutions of solvated electrons produced by dissolving active metals in ammonia, amines, ethers, etc. The use of pulsed radiolysis has permitted workers to study the kinetics of fast reactions of solvated electrons with rate constants up to the diffusion-controlled limit (21). The study of slow reactions frequently is made difficult because the necessarily low concentrations of electrons magnify the problems caused by impurities, while higher concentrations frequently introduce complicating second-order processes (9). The upper time limit in such studies is set by the reaction with the solvent itself. 

For solvents in which the lifetime of the solvated electron is short, it cannot be observed in this way. For instance in water, the hydrated electron may be formed by dissolving alkali metals. But the metal dissolution timescale is much longer (a hundred of milliseconds) than the lifetime of the electron (a few microseconds) and, as soon as solvated electrons are produced, a very fast reaction occurs between two solvated electrons producing molecular hydrogen, leading to the explosive combustion in air that accounts for the hazardous contact of alkali metal and water. 

In radiolysis, one of the most important reactions of solvated electrons is recombination with positive ions and radicals that are simultaneously produced in close proximity inside small volumes called spurs. These spurs are formed through further ionization and excitation of the solvent molecules. Thus, in competition with diffusion into the bulk, leading to a homogeneous solution, the solvated electron may react within the spurs. Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically, ° and also in a few other solvents. " Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the... 

One example of fast reaction rates measured with a picosecond system, involving intramolecular electron transfer, has already been described in Section 3.5.5. Another example is the measurement of rate constants of solvated electrons in... 

The use of hexamethylphophoramide (HMPA) in the electrolytic reduction of substrates, such as benzene derivatives, aliphatic ketones, amides, and olefins, involves the intermediate formation of solvated electrons, since very negative potentials must be reached. The substrates are normally difficult to reduce. However, it is important not to add so much proton donor to the solvent that the rate of its reaction with the solvated electrons becomes too high. Generally, ethanol can be used as a cosolvent [311], and the best stability of the intermediate solvated electron combined with an optimum protonating efficiency was found to be the mxiture of HMPA-ethanol (33-66 mol moP ) the relatively high concentration of EtOH does not protonate ens because HMPA appears to be selectively adsorbed on the cathode interface and the reaction of Cys with the substrated is fast. With such a ratio of ethanol, a maximum current efficiency was found [312]. 

The position of the heteroatom may also strongly affect the global reactivity of the molecule. Thus, ethers, esters, thioethers, and even amines, that are normally weakly reactive toward the electron, can afford cleavage at the condition to be in an activated position (benzylic or allylic) to the X group (a fast cleavage of the radical anion results in a large shift to less cathodic potentials when the global bielectronic step is considered). In a different way, the reduction of ethers and amines could be achieved only under the conditions of the Birch reaction (i.e. reduction by Li metal in amines or by a solvated electron). These conditions can be electrochemically... 

Figure 5 depicts the decay of the solvated electron due to spur reactions in two different solvents, water and tetrahydrofuran. In both liquids, the solvent relaxation is very fast (less than 1 ps), therefore, the absorption signals on the picosecond time scale are due to the fully solvated electron. As the dielectric constant of tetrahydrofuran is low (e = 7.6 compared to 80 for water), the electrostatic attraction is not screened by the solvent and geminate recombination between the solvated electron and the cation can occur over long separation distances in contrast to water. Moreover, the mobility of ej in THF is roughly three times higher than that in water. That explains why the decay ofthe solvated electron is more important in tetrahydrofuran compared to water [19]. 

Reaction of Stable Solvated Electrons with Water. One of the most promising ways of generating a homogeneous solution of hydrated electrons has been pursued by Dewald, Dye, Eigen, and DeMaeyer (26), who mixed a solution of electrons solvated in ethylenediamine with water. These authors took a solution of Cs in ethylenediamine, a solvent in which solvated electrons are stable, and combined it in a fast-flow mixing cell with a solution of water in ethylenediamine. They then followed the rate of decay of the near infrared absorption band of e ed as a function of water concentration. More recently other active metals have been used and the kinetics fully analyzed (32). The second-order rate constant (20M 1 sec. 1) obtained is attributed to Reaction 16 and compared with... 

We conclude this article on a note of optimistic speculation. Clearly the above results on solvated electrons establish the potential of ultrashort laser pulses to probe the fundamental details of the dynamics of electron transfer reactions, which will be the cornerstone for the development of microscopic theories of electron dynamics in the condensed phase. Electrons are ubiquitous species, and the practical reflection of this appears in research areas such as photosynthesis, dielectric breakdown, fast optical... 

Accordingly, a concept was developed, that involves one of the organic compounds, e.g., toluene as a fullerene-dissolving medium (12,13). The systematic fullerene reduction was then obtained via addition of adequate co-solvents, namely, acetone and 2-propanol. Acetone was chosen as an efficient electron scavenger to hinder a reaction between solvated electrons and toluene. Followed by a fast protonation a radical species with a reducing character is formed. In addition, the (CH3)2 COH species is identical with the main product of the radiolysis of the second co-solvent, 2-propanol. 

The ease with which solvated electrons can be produced by high-energy radiolysis-induced ionisation of solvent molecules probably explains the fact that the technique has been used to study fast reactions between ionic species. Since the nature of the solvated electron is not too well defined, this necessarily clouds the interpretation of studies of ionic reactions (Hunt [109], Vannikov [110], Kenney-Wallace [111]). 

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