Hydrated electron precursors

Radiolysis of aqueous nitrate solution has been investigated not only from practical viewpoints in nuclear technology, but also from scientific interests in order to understand the radiolysis of water because nitrate ion is an efficient scavenger for hydrated electron [114] and even for the precursor of the hydrated electron [115]. In practical process, the concentration range of nitrate covers from millimolars to 10 M and the radiation effect not only in diluted nitrate solutions but also in concentrated ones should be considered. 

Pastina B, LaVerne JA, Pimblott SM (1999) Dependence of molecular hydrogen formation in water on scavengers of the precursor to hydrated electron. J Phys Chem A 103 5841-5846 Paul T, Young MJ, Hill IE, Ingold KU (2000) Strand cleavage of supercoiled DNA by water-soluble peroxyl radicals. The overlooked importance of peroxyl radical charge. Biochemistry 39 4129-4135... 

Pimblott SM, LaVerne JA (1998) On the radiation chemical kinetics of the precursor to the hydrated electron. J Phys Chem A 102 2967-2975... 

Hamill had suggested that there was a precursor of the hydrated electron that could be scavenged and called this species the dry elec-tron. Work by the Hunt group with his stroboscopic pulse radiolysis... 

Most recently, Mizuno et al. presented a femtosecond version (250 fs time resolution, 160 cm spectral resolution) of the RR experiment to probe the O-H band of the electron as it hydrates following 2 X 4.66 eV photon excitation. Mizuno et al. conclude that the precursor of the hydrated electron that undergoes continuous blue shift on the time scale of 1-2 ps also yields a downshifted O-H stretch signal whose resonance enhancement follows the efficiency of Raman excitation as the absorption spectrum of the s-like state shifts to the blue (thus indirectly confirming its identity as a hot s-like state). The comparison of anti-Stokes and Stokes Raman intensities indicates that the local temperature rise is < 100 K at 250 fs. This estimate agrees with the estimates based on the evolution of the spectral envelope during the thermalization, using the dependence of the absorption maximum of thermalized electron on the bath temperature. 

The hydrated electron, symbolized by e,", is stated by Hart ° to be an ideal analytical reagent. It is highly specific in its reactions and intensely colored. It can be produced photochemically by pulsed radiolysis, electrochemically, and by reduction of water where it is the precursor to formation of hydrogen atoms. In water the hydrated electron has a half-life of about 8 x 10 s. 

Figure 8 Absorption spectra of the electron at different delays after photo-ionisation of liquid water at 21 °C (from [26]). The solvation process is very fast as the hydrated electron (A ax = 720 nm) is observed within / ps from precursors absorbing in the infrared domain (A > 1200 nm).

A specific free radical can be produced from a precursor molecule either in an initiation step or a propagation step in which a reagent radical reacts with the precursor. Initiation requires either removal or addition of an electron or homolysis. Chemically this can be done in a number of ways, by using one-electron oxidants or reductants or by inducing homolysis in some way examples of these types of reactions include autoxidation [84-86], photochemical oxidation and reduction [87-90], and oxidation and reduction by metal ions and their complexes [91-93], In propagation reactions, the reagent radical might be the hydroxyl radical, the hydrated electron, or any other suitably reactive species that will interact with the precursor molecule in the desired manner. We will consider initiation reactions first. 

One of the most common bimolecular reactions of radicals is their association with other nonradical molecules. We have used the term hemicolligation to describe this reaction type elsewhere.11 This mode of reaction is particularly important because many radical precursors can react in this way. For example, bromine atoms are often generated by oxidizing bromide ions, so the reaction Br + Br- = Br2 is an unavoidable component in such systems. Association of radicals with 02 is another common process that can be important when atmospheric oxygen is not completely excluded from the reaction mixture. When the radical is the hydrated electron, the association reaction is simply a reduction and is treated separately (Table 9.5). 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

Ten years ago, femtosecond IR spectroscopy of an excess electron in pure water showed the existence of an ultrashort-hved prehydrated state (61). This IR nonequihbrium electronic configuration is built up in less than 120 fs in H2O and represents a direct precursor of the hydrated electron ground state (equation 6). In the infrared (0.99 eV), the monoexponential relaxation of the signal toward an s-hke ground state of the hydrated electron (240 20 fs) has been analyzed in the framework of a two-state model (61, 65). With a similar model, an indirect estimate of the infrared electron relaxation in the red spectral region gives a deactivation rate of 2 X 10 s (62, 66). The very fast appearance of the infrared electron (efn) is comparable to any nuclear motion, solvent dipole orientation, or thermal motion of water molecules. The relaxation of... 

However, yields of nitrogen were obtained in both neutral and alkaline solutions, indicating that the hydrated electron is produced in this system and is the precursor of hydrogen, viz. 

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

Laboratory in Japan. These machines operate with electron pulses of several tens of ps and led to the observation of scavenging of the precursor to the hydrated electron. Using sub-ps laser techniques this species, in fact a variety of precursors, were later discovered upon photo-ionization (37,38). These early events are presently a matter of discussion in the literature and faster time resolution will be required in order to directly observe and identify the radiolytic precursors to the hydrated electron. 

Both of these paths can be studied by pulse radiolysis when selective reductants can be used to reduce the metal complex or the radical source. For example, one can use hydrated electrons to reduce RX (present in excess over M P) and C02 radicals to reduce M "P. On the other hand, if COj is the sole reducing radical in the system, since this does not react rapidly with RX, one can prepare M P and follow its subsequent reaction with RX. Of course, R- can be prepared also by reactions of various precursors other than RX. 

As shown in figure 3A, the kinetics at 1250 and 720 nm reflect the evolution of the infrared and solvated species respectively. The infrared band appears with a time constant of 110 +/- 30 fs and quickly relaxes following a first order kinetics with a time constant of 2 0 +/- 0 fs towards the fully solvated species. These data are the first evidence of at least one precursor of hydrated electron. 

One can investigate the reactivity of a precursor of the hydrated electron with biological molecules. So, one can measure a lifetime in the... 

One-electron oxidation of the adenine moiety of DNA and 2 -deoxyadenos-ine (dAdo) (45) gives rise to related purine radical cations 46 that may undergo either hydration to generate 8-hydroxy-7,8-dihydroadenyl radicals (47) or deprotonation to give rise to the 6-aminyl radicals 50. The formation of 8-oxo-7,8-dihydro-2 -deoxyadenosine (8-oxodAdo) (48) and 4,6-diamino-5-formamidopyrimidine (FapyAde) (49) is likely explained in terms of oxidation and reduction of 8-hydroxy-7,8-dihydroadenyl precursor radicals 47, respectively [90]. Another modified nucleoside that was found to be generated upon type I mediated one-electron oxidation of 45 by photoexcited riboflavin and menadione is 2 -deoxyinosine (51) [29]. The latter nucleoside is likely to arise from deamination of 6-aminyl radicals (50). Overall, the yield of formation of 8-oxodAdo 48 and FapyAde 49 upon one-electron oxidation of DNA is about 10-fold-lower than that of 8-oxodGuo 44 and FapyGua 43, similar to OH radical mediated reactions [91]. 

The reversibly formed re-complexes precursor to the hydration of olefins have the proton imbedded in the re-electron cloud of the double bond somewhere between the two carbon atoms. They are therefore... 

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