Absorption of solvated electrons

In a pulse-radiolysis optical-absorption method, the value of kje, where 8 is a molar absorption coefficient, is measured by the time-resolved measurement of the optical absorption of solvated electrons, and then the kr value is determined by the observed value of kr/s and the value of s known separately. 

Fig. 4. Spectra of optical absorption of solvated electrons in liquid ammonia at —65-70 °C ...

Kira and coworkers25 found that in deaerated DMSO solution of frans-stilbene both the solute cation and anion are produced and the anions are eliminated by aeration. Since they found26 that the absorption spectra of the anthracene cation and anion are quite similar, they suggested25 that the absorption spectrum observed by Hayon for anthracene solution in DMSO is a superposition of the spectra of the solute cation and anion. This observation casts a serious question on the yield of solvated electrons found by Hayon23. 

FIGURE 5. Absorption spectra of solvated electrons in DMSO/H20 mixtures 0,0.20,0.28,0.43,0.72,0.93 and l.Omole fraction DMSO. To fit into the Figure, the data for pure water have been multiplied by a factor of 0.65 relative to the others. These spectra represent the short-lived solvated electron band only, the longer-lived 600nm band and UV bands having been substracted from the observed absorbances. Reproduced by permission of the authors from Reference 30. 

Early pulse radiolysis studies of alkanes at room temperature showed that the solvated electron absorption begins around 1 pm and increases with increasing wavelength to 1.6 pm for -hexane, cyclohexane, and 2-methylbutane [77]. More complete spectra for three liquid alkanes are shown in Fig. 4. The spectrum for methylcyclohexane at 295 K extends to 4 pm and shows a peak at 3.25 pm [78]. At the maximum, the extinction coefScient is 2.8 x 10 cm The spectrum for 3-methyloctane at 127 K, shown in Fig. 4, peaks around 2 pm. The peak for methylcyclohexane is also at 2 pm at lower temperature. Recently, the absorption spectra of solvated electrons in 2-methylpentane, 3-methylpentane, cA-decalin, and methylcyclohexane glasses have been measured accurately at 77 K [80]. For these alkanes, the maxima occur at 1.8 pm, where the extinction coefScient is 2.7 x 10 cm. ... 

Figure 4 Absorption spectra of solvated electrons in alkanes vs. wavelength. Solid line is for methylcyclohexane at 295 K [78]. Dashed line is for 3-methyloctane at 127 K [79]. Dash-dot line is for 3-methylpentane at 77 K [80]. Spectra have been normalized to unity at the peaks.

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

The kinetic behavior of solvated electrons has been followed directly using flash radiolysis (44, 45, 58) or flash photolysis technique (62, 94, 107). The former method is more universally applicable owing to the high absorption coefficient of e soiv in a spectral region where most reactants contribute little to the overall optical density. Stopped-flow spectrophotometry has also been applied in the specific case of the eaq + H20 reaction (43), but it is not applicable to reactions where the e soiv half-life is below 0.1 msec. 

From the preceding it is apparent that the nature of the first excited state has been clarified for CTTS transitions of simple anions, and that both for these and for intramolecular processes following on primary excitation the events are known with some certainty up to the stage of asymmetrization. Also, after the pair of solvated electron and parent species have been established by at least one diffusive translation (so that at least one solvent molecule separates them) all steps following have been elucidated. There remains the question of the detailed nature of the processes in the time interval between 10 u and 10 n to 10-9 sec. after primary absorption. 

A photoprocess rather common with inorganic compounds is the formation of solvated electrons, e [ in organic solvents and eat in aqueous solutions.43,44 The photoprocess is most commonly observed with anions whose absorption spectrum exhibits a characteristic charge transfer to solvent, CTTS, band in the ultraviolet. It is the typical photoprocess of the halide anions shown in Equations 6.89 and 6.90 where X = Cl, Br, and I-. 

The CTTS band can also be found in the absorption spectrum of some polyatomic anions together with transitions to the excited states described above43 44 In the case of SCN, an intense absorption band with 2max = 225 nm (s = 3.5 x 103 M 1 cm-1) has been assigned to a charge transfer to solvent transition. The wavelength-dependent photochemistry of SCN induces, however, the formation of solvated electrons according to Equation 6.89 and the detachment of S (Equation 6.91) in a parallel process. 

The optical absorption spectrum resembles that of solvated electrons in liquid water and consists of a diffuse featureless band with a broad maximum in the range of 5500-7000 A and possibly an increase in absorption towards the near ultraviolet. Since the good resolution of the e.s.r. spectrum shows that the ground states of all the trapped electrons are very similar, the broad optical absorption band must be caused by large variations in the excited states of different traps. 

The temperature dependence of the absorption spectrum of the solvated electron has been recorded not only in water but also in alcohols (Fig. 3). Measurements are performed using nanosecond pulse radiolysis with a specific cell for high temperature and high pressure in a temperature range up to around 600 K depending on the solvent. Indeed, by increasing the temperature, the decay of solvated electrons becomes faster for example, this decay is much faster in alcohols than in water, so, the data obtained with nanosecond set-up are limited at lower temperatures for alcohols compared with water. 

Several publications have also dealt with the changes that occur in the optical absorption spectra of solvated electrons in liquid amines. 

Fig. 6. Optical absorption spectra of solvated electrons at 10°C in pure water and in aqueous solutions containing (a) 2 mol kg of monovalent (Na ), divalent (Mg ) and trivalent (Tb ) metal chloride salts, (b) magnesium perchlorate, Mg(C10.j)2, with increasing concentration (1, 2 and 3 mol kg ). [Reprinted from Ref. 68, Copyright 2004, with permission from American Chemical Society.]...

Fig. 11. Time-resolved absorption spectra in neat methanol after two-photon absorption at 273 nm and energy level scheme used to account for the measmed dynamics during the generation of solvated electrons. [Reprinted ftom Ref. 94, Copyright 2003, with permission from Elsevier.]...

Jou FY, Freeman GR. (1979) Temperamre and isotope effects on the shape of the optical absorption spectrum of solvated electron in water. J Phys Chem 83 2383-2387. 

Anbar M, Hart EJ. (1965) The effect of solvent and solutes on the absorption spectrum of solvated electrons. JPhys Chem 69 1244-1247. 

Han Z, Katsumura Y, Lin M, He H, Muroya Y, Kudo H. (2005) Temperature and pressure dependence of the absorption spectra and decay kinetics of solvated electrons in ethanol from 22 to 250°C studied by pulse radiolysis. Chem Rhys L 404 267-271. 

Figure 1 shows tJie corresponding optical absorption spectra of these radical cations as taken in the pulse radiolysis of the pure solvents. To avoid the influence of solvated electrons, in the case of alkanes, an electron scavenger such as tetrachloromethane is usually added [see Eq. (5b)]. Alkyl chlorides are internal electron scavengers and do not need further additives. In most of the cases, for the study of electron transfer reactions of type 2 or 3, the solvent derived radicals do not disturb because of their much lower reactivity compared with those of the ions. 

The possibility that subexcitation electrons may be responsible for the formation of hydrogen atoms was mentioned previously. Attempts to confirm reports of solvated electrons have not been successful. No visible color is observable when metallic potassium, distilled onto the walls of a silica absorption-cell, reacts with water at 0°. 

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