Properties of hydrated electrons

Hydrated electron probe inverse micelles. Hydrated electrons (e aq) are expected to be a very good probe to test the water pool of reverse micelles. The physical properties of hydrated electrons obtained by pulse radiolysis in AOT reverse micelles were experimentally determined (Calvo-Perezet al., 1981 Pileni,... 

A symposium on solvated electrons (41) and a number of recent reviews (15, 42, 57, 58, 73) have highlighted the discovery of hydrated electrons (e aq), their chemical reactivity and correlations to other solvated electrons. Some of the physical properties of hydrated electrons,... 

Table I. Properties of Hydrated Electrons Determined Experimentally...

Of the three models that have been proposed to explain the properties of excess electrons in liquid helium, two are considered in detail (1) The electron is localized in a cavity in the liquid (2) The electron is a quasi-free particle. The pseudopotential method is helpful in studying both of these models. The most useful treatment of electron binding in polar solvents is based on a model with the solution as a continuous dielectric medium in which the additional electron induces a polarization field. This model can be used for studies with the hydrated electron. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

The reaction of hydrated electrons formed by radiolysis with peroxydisulfate yields the sulfate radical anion SO4 which is a strong chemical oxidant (Eqx = 2.4 V/NHE) [50, 58]. The oxidation of both purine and pyrimidine nucleotides by S04 occurs with rate constants near the diffusion-controlled limit (2.1-4.1 x 10 M s ). Candeias and Steenken [58a] employed absorption spectroscopy to investigate acid-base properties of the guanosine cation radical formed by this technique. The cation radical has a pKa of 3.9, and is rapidly deprotonated at neutral pH to yield the neutral G(-H) . Both G+ and G(-H) have broad featureless absorption spectra with extinction coefffcients <2000 at wavelengths longer than 350 nm. This has hampered the use of transient absorption spectra to study their formation and decay. Candeias and Steenken [58b] have also studied the oxidation of di(deoxy)nucleoside phosphates which contain guanine and one of the other three nucleobases by SO4 , and observe only the formation of G+ under acidic conditions and G(-H) under neutral conditions. 

It would be superfluous to review here the story of e aq in the radiation chemistry of aqueous solutions. High energy radiations cause ionizations and the free electrons so generated dissipate their excess energy and are eventually trapped in solvation shells. The discovery of hydrated electrons showed that electrons in water were chemical entities (as distinct from possessing purely physical characteristics) in having diffusion properties, size and sphere of influence, associated ion atmosphere, and reaction rate parameters all of which are comparable to normal chemical reagents. 

Fig. 3 Energy diagram of strongly doped n-Si/Si02 electrode in contact with aqueous electrolyte solution. The diagram contains the formal redox potentials of the primary radicals and combined redox and luminescence properties of Ru(bpy)3, and 2,6-bisPM,N-bis(carboxymethyl)amino-methyl]-4-benzoylphenol (ligand L). The diagram demonstrates clearly that on thermodynamical basis, both of these luminopbores could be excited even to their singlet excited states in the simultaneous presence of hydrated electrons and sulfate radicals via both ox-red and red-ox pathways. However, in practice the ox-red pathway seems to be dominant in both cases. E-type electron centers in silica fully correspond to F and F electron centers in alumina [36]...

Structure and electronic properties of hydrated mesityl oxide a sequential quantum mechanics/molecular mechanics approach... 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

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. 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

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