Properties of the hydrated electron

It has already been seen that the hydrogen atom and the hydrated electron are interconvertible. Thus they form a conjugate acid—base pair. 

It is thus possible to calculate the standard free energy change for the reaction 

Since AG° = —F , this corresponds to a standard electrode potential of —2..11 V. The hydrated electron is thus an extremely potent reducing agent, much more powerful than the hydrogen atom, for which is —2.10 V. The free energy and heat of hydration are —37.4 and —38.1 Real mole , respectively. The entropy of hydration is —1.90 cal mole °K [31]. 

The diffusion coefficient for the hydrated electron has been calculated from conductivity measurements [32] to be (4.9 0.25) x 10 cm sec . The equivalent conductance is much higher than that of all other ions except OH and but considerably lower than the ammoniated 

The radius of charge distribution of the hydrated electron has been estimated by several methods. The values obtained from hydration energies, encounter radius, i.e. the radius required to account for experimental diffusion controlled rate coefficients, and Jortner s cavity-continuum model are all in the range 0.25—0.30 nm [33]. This clearly indicates that the electron is not associated with a single water molecule only, but rather that the charge of the electron is smeared out over 3—4 water molecules. 

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This chapter is devoted to the important relationship between electrode potentials and the changes in Gibbs energy (AO ) for half-reactions and overall reactions. In discussions of the properties of ions in aqueous solution it is frequently more convenient to represent changes in Gibbs energy, quoted with units of k.I mol-1, in terms of electrode potentials, quoted with units of volts (V). The electrochemical series is introduced. The properties of the hydrated electron are described. 

Other Properties of e aq. Table I lists various properties of the hydrated electron. 

Table 3.2. The physical properties of water in the water pool (WP) in reverse micelles of sodium bis-2-ethylhexyl sulfosuccinate (Aerosol OT or AOT) in isooctane. The spectroscopic properties of the hydrated electrons (e"aq) in the micellar water pool (Wo)...

Further Predictions of Thermodynamic Properties of the Hydrated Electron... 

With respect to the dynamical properties of the hydrated electron in cluster systems, the first principle dynamics using ab initio molecular dynamics and so on have been extensively applied. [135, 180, 371, 408, 446] They revealed information about the structure and relative stabilities of the isomer clusters. Nonadiabatic dynamics of a solvated electron in various photochemical processes has also been studied experimentally. [62, 123, 294, 329] Rossky and co-workers [327, 468] also studied the relaxation dynamics of excess electrons using quantum molecular dynamics simulation techniques. Here the nonadiabatic interactions were taken into account basically within the scheme of surface hopping technique. [444]... 

We have already noticed that the electrons in the redox phenomena do not exactly play the same part as that played by the protons in the acid-base phenomena. This is due in part to the brief lifespan of electrons in water. It is interesting, however, to know some properties of the hydrated electron. 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

Among other reactions of the hydrated electron may be mentioned the reaction with methylene blue. Methylene blue may be regarded as the prototype of easily reducible biological substances such as NAD, cytochrome-c, etc. The preliminary value for the rate constant for reaction with the hydrated electron (5) has now been shown to be too high, and the more reasonable value of 2.5 X 1010 M-1 sec.-1 has been obtained (19). It is thus no longer necessary to attribute special properties to methylene blue. 

To summarize, there is still a need for carefully determining more rate constants for various substances of biological interest in their various charged forms. This phase of the subject will be complete when critically chosen values have passed into the Tables and when theoretical correlations have been sufficiently developed to enable rate constants for unexamined substances to be reliably predicted. There is also still a need to correlate the reactivity of the hydrated electron with the reactivity of free radicals such as H, OH, organic radicals, peroxy radicals, etc., so as to be able to predict the reactivity of unexamined free radicals. Another need is to establish the influence of conditions on the rate constants. The influence of ionic strength is now well known, but other factors, such as the dielectric properties of the medium, have been shown to have an effect in some cases (2, 20). Also, the effect of temperature has been investigated in only a few cases (9). 

In the few years that have elapsed since these observations, a great deal of information has accumulated on the properties and rate coefficients of the hydrated electron. Some of its characteristic properties are shown in Table 8. The subscript f refers to formation from a hypothetical standard state for electrons having zero entropy, enthalpy and free energy and in a vacuum at the bulk electrostatic po-... 

EJ Hart and M Anbar have detailed the characteristics and the chemistry of the solvated electron in water, otherwise known as the hydrated electron and denoted by e] y or e. A number of reviews on the solvated electron are also available.In this article, we will recall briefly the main steps of the discovery and the principal properties of the solvated electron. We will then depict its reactivity and focus on recent results concerning the effect of metal cations pairing with the solvated electron. At last, we will present results on the solvation dynamics of electron. Due to the development of ultrashort laser pulses, great strides have been made towards the understanding of the solvation and short-time reactivity of the electron, mainly in water but also in polar solvents. However, due to the vast and still increasing literature on the solvated electron, we do not pretend for this review to be exhaustive. 

See reviews a) Edwards P.R, The electronic properties of metal solutions in liquid ammonia and related solvents, Adv. Inorg. Chem. Radiochem., i982,25, 135-185. b) Boag J.W., Pulse radiolysis a historical account of the discovery of the optical absorption spectrum of the hydrated electron, in Early developments in radiation chemistry", KrohJ. (Ed.), Royal Society ofChemistry, Cambridge, 1989, 7-20. 

More recently, assisted by photochemistry, spectroscopy in its varied forms, chromatography, computers, and applied electronics, radiation chemistry is assaulting many of the problems associated with the properties of transient species at an unprecedented rate. Commonplace, already, is the study of intermediates lasting only milli- and micro-seconds. A rapidly developing subdivision is on the horizon—that of nanosecond and picosecond chemistry. Knowledge of the nature and rates of these reactions has been of inestimable aid in untangling reaction mechanisms in chemistry and biology. For example, the discovery of the hydrated electron and the determination of its rate constants has aided the interpretation of reactions in aqueous media. Recent studies on solvated and... 

The free energy, enthalpy, entropy, and volume of the hydrated electron are measurable in principle from the temperature and pressure dependencies of the forward and reverse rates of the unimolecular reaction of this species with water to form hydrogen atom and hydroxide ion. Data presently available determine values only for free energies of activation in both directions and for enthalpy and entropy of activation in one direction. Values for the other properties can be predicted if it is assumed that the enthalpy, entropy, and volume of the hydrated electron can be calculated by extrapolating measurements on halide ions to the radius (2.98 A.) necessary to fit the free energy data. The predictions for enthalpy and entropy are thought to be reasonably accurate, but the value for volume change is less reliable. 

The question whether or not radical ions are formed upon irradiation of liquids and stabilized enough for detection or engagement in bimolecular chemical reactions has moved radiation chemists ever since the early days of this research field. This was particularly exciting with respect to low polarity solvents, but even for aqueous solutions conclusions had to rely mainly on indirect evidence. A real breakthrough came with the experimental discovery of the hydrated electron and other powerful one-electron reductants (e.g., a-hydroxyalkane radicals such as (CH3)2C OH). Applying these new tools a large number of organic radical anions were detected and characterized with respect to their optical and chemical properties, particularly by pulse radiolysis. 

Aromatic ketones. The photophysics and photochemistry of benzophenone have been widely studied as a model for aromatic ketones. Earlier results on the spectroscopic properties of the transient radicals and their reactivities have been reviewed by Kavarnos and Turro, and questions related to the photoionization of benzophenone have been summarized by Corner et TR EPR studies of benzophenone in micellar solutions have been published by Murai. The photoionization of benzophenone and some of its derivates (monosubstituted carboxylic acids, tetracarboxyhc add) were investigated in our laboratory using FT EPR. The spin polarization (TM CIDEP) of the hydrated electron and the successor radicals of the benzophenone radical-cation indicates that the biphotonic mechanism of the photoionization of all benzophenone derivatives occurs via the triplet state of the benzophenones. 

The coefficients Co, nnd C2 (denoted as mq, ai, and aj in Ref. 33) are influenced by various molecular properties of the solvent and an ion, including their electron-donating or accepting abilities. Hence, these coefficients are specific to the ion. Nevertheless, they may be considered as common to a family of ions such as the polyanions whose surface atoms, directly interacting with solvents, are oxygens. This is the case for hydrated cations or anions whose surfaces are composed of some water molecules that interact with outer water molecules in the W phase or with organic solvents in the O phase. 

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

Iron (III) is a stronger EPA than iron (II) so that the electron pair availability at the N atoms of the cyano groups is lower in the oxidized ion. In addition, iron(II) is a stronger n EPD than iron(III) which increases the electron-pair donor properties of the reduced form. Thus, outer-sphere hydration is considerably stronger for the ferrocyanide ion than for the ferricyanide ion. 

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