Solvated electron absorption spectrum

The absorption spectrum of the solvated electron depends not only on the nature of the solvent but also on parameters that modify the structure and properties of the solvent, such as pressure and temperature. The optical absorption band shifts to higher energies (shorter wavelengths) with increasing pressure up to 2000 bar the shift is larger in primary alcohols than in water and it correlates with the increase in liquid density rather than with the rise in dielectric constant. A rise in the temperature induces a red shift of the solvated electron absorption spectrum. Thus, the absorption maximum in water is located around 692 nm at 274 K and 810 nm at 380... 

Asaad AN, Chandrasekhar N, Hashed AW, Krebs P. (1999) Is there any effect of solution microstructure on the solvated electron absorption spectrum in LiCl/HjO solutions JPhys Chem A 103 6339-6343. 

FIGURE 6.4 A typical trapped electron absorption spectrum in ethanol at 4 K and the corresponding solvated electron spectrum at 77 K. The irradiation is at 4 K in both cases. Reproduced from Hase et a1. (1972a), with permission from Am. Inst. Phys.O... 

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. ... 

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. 

It is worth noticing that, in contrast to what have been reported for the electron solvation dynamics in water and in alcohols at room temperature, we do not observe a hypsochromic translation of the electron absorption spectrum. 

Since the suggestion of the sequential QM/MM hybrid method, Canuto, Coutinho and co-authors have applied this method with success in the study of several systems and properties shift of the electronic absorption spectrum of benzene [42], pyrimidine [51] and (3-carotene [47] in several solvents shift of the ortho-betaine in water [52] shift of the electronic absorption and emission spectrum of formaldehyde in water [53] and acetone in water [54] hydrogen interaction energy of pyridine [46] and guanine-cytosine in water [55] differential solvation of phenol and phenoxy radical in different solvents [56,57] hydrated electron [58] dipole polarizability of F in water [59] tautomeric equilibrium of 2-mercaptopyridine in water [60] NMR chemical shifts in liquid water [61] electron affinity and ionization potential of liquid water [62] and liquid ammonia [35] dipole polarizability of atomic liquids [63] etc. 

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 n-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... 

In addition, only mild changes in the calculated absorption spectrum are seen. This suggests that, in this case, adapting the solute to the solvated situation obtained with ab initio dynamics essentially corrects the limitations of the classical force field. However, a more complex situation may arise. In the case of free base phthalocyanine the average and the distribution results obtained from a BOMD for the bond distances, bond angles and torsion angles were used to reparametrize the GROMOS53a6 [139] force field. Preliminary results for the electronic absorption spectrum [140, 141] well reproduced the data from the BOMD for free base phthalocyanine [142]. 

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. 

Although it is very hard to observe the absorption spectrum of eh when metal is dissolved in water because of its high reactivity, some attempts were made in water and ice (Jortner and Stein, 1955 Benett et al., 1964, 1967). Furthermore ESR (electron spin resonance) studies revealed that the trapped or solvated electron in ice interacts with six equivalent protons, thus ruling out H20-. 

Farhataziz et al. (1974a, b) studied the effect of pressure on eam and found that as the pressure is increased from 9 bar to 6.7 Kbar at 23° (1) the primary yield of e decreases from 3.2 to 2.0 (2) hv increases from 0.67 to 0.91 eV (3) the half-width of the absorption spectrum on the high-energy side increases by 35% and (4) the extinction coefficient decreases by 19%, which is similar to eh. The pressure effects are consistent with the large volume of ean (98 ml/M), whereas the reduction in the observed primary yield at 0.1 ps is attributable to the reaction eam + NH4+. Some of the properties of eam have been discussed by several authors in Solvated Electron (Hart, 1965). 

Solvated electrons are known to be formed in amines, amides, dimethyl sulfoxide, and many other liquids that will not be discussed here. Note that, except for the yield and time scale of observation, the production of es itself is not related to polarity. Thus, the es absorption spectrum has indeed been observed in nonpolar liquids both at low temperatures and room temperature (Taub and... 

Much worse than the oscillator strength is the line shape. The calculated absorption spectra has no similarity with what is experimentally seen. The calculated half-width is always smaller, typically by a factor of 2 the exact reasons for this are only speculated. It is common knowledge that a photodetachment process is capable of giving a very broad absorption spectrum, but a satisfactory method has not been developed to adopt this with the bound-bound transition of the semicontinuum models. Higher excited states (3p, 4p, etc.) have been proposed for the solvated electron, but they have never been identified in the absorption spectrum. 

The width of a band in the absorption spectrum of a chromophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Inhomogeneous broadening results from the fluctuations of the structure of the solvation shell... 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

Anion solvation has been studied by observing the shift in the absorption spectrum of the benzophenone anion in various solvents and as a function of temperature. The benzo-phenone anion was formed from the reaction of the benzophenone molecule and a precursor to the solvated electron. Approximately 0.25 M benzophenone is put into the solution so that all the presolvated electrons will react with the benzophenone and virtually none will form the solvated electron. This process occurs much more quickly than the solvation processes that are observed [14,20]. 

Upon dissolving the metal, a broad optical absorption line appears, peaked at 0.85 eV, and with a tail extending into the visible, which gives the characteristic blue colour (Fig. 10.13). The absorption does not depend on the nature of the solute, showing that the solvated electron dissociated from the cations is responsible for the absorption. The absorption spectrum is almost independent of concentration up to 10 1 MPM. 

Fig. 10.13 Absorption spectrum of the solvated electron as it appears in dilute Na-NH3 solutions (Burow and Lagowski 1965). The high-energy tail is the source of the characteristic blue colour of dilute solutions. From Cohen and Thompson (1968).

Finally, a striking property of metal-ammonia solutions is the large expansion of the liquid due to the solvated electrons. The apparent volume of the solvated electron remains roughly constant up to the metallic range, then shows a slight increase. It is about 100 cm3 mol. It is this effect that has led to the hypothesis that the electron forms a cavity for itself a cavity of radius 3.2 A accounts quantitatively for the excess volume. A model in which the electron moves in a cavity, and the surrounding liquid is polarized or solvated as it is round a cation, was first put forward by Jortner (1959), who showed that it was able to account for the absorption spectrum. Jortner s model, as modified by Mott (1967), Cohen and Thompson (1968) and Catterall and Mott (1969), will now be described. 

Blandamer et al (1964) pointed out that the absorption spectrum of iodine ions (I ) in NH3 has its maximum at Jtv=4.0eV the difference from that for an electron in a cavity, 4.0-0.8= 3.2eV, corresponds well to the electron affinity of iodine. In water the maxima for both I and a solvated electron are shifted by 0.8 eV to higher frequencies we deduce that the energy of the bottom of the conduction band in water is about 0.8 eV. 

The structure of a solvated electron depends on the solvent, and it is often difficult to describe accurately. The existence of the solvated electron as a distinct chemical species is however ascertained by its absorption spectrum this is a broad, structureless spectrum which covers the far VIS and NIR regions (Figure 1.5). 

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