Solvated electron optical absorption

The optical absorption of the solvated electron, in the continuum and semicontinuum models, is interpreted as a Is—-2p transition. Because of the Franck-Condon principle, the orientational polarization in the 2p state is given... 

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. 

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. 10.16 Potential seen by a solvated electron according to the model of Jortner (1959). The wave function r of the electron is also shown. The optical absorption is due to the ls-2p transition. The radius R of the cavity is approximately 3.2 A. From Cohen and...

Gillis et al. [394g] observed the decay of the solvated electron s optical absorption in the infrared (800—2100 nm) in liquid propane at 88 and... 

K. They noted a decay over timescales 95 and < 35 ns, respectively, which was attributed to geminate ion-pair recombination (see Fig. 33). The decay of the optical absorption is independent of the dose of radiation received and continues for about lps. Rather than displaying a dependence on time as eqn. (153), i.e. at f 3/2, the experimental results are more nearly represented by either at f 1 decay to an optical density about one tenth of the maximum or by a decay as t 1/2 to zero absorption. These effects may be the recombination of ions within a spur (or cluster of ion-pairs), which is more nearly like a homogeneous reaction. The range of electrons in propane at 100 K is 10 nm [334] and the extrapolated diffusion coefficient is 10 11 m2 s 1 [320]. The timescale of recombination is 10 ps. The locally greater concentration of ions within a spur probably leads to a faster rate of reaction and is consistent with the time-scale of the reaction observed. Baxendale et al. [395] observed the decay of the infrared optical absorption of the solvated electron in methylcyclo-hexane at 160 K. They noted that the faster decay occurring over < 50 ns was independent of dose and depended on time as t 1/2, i.e. the reaction rate decays as t 3/2, see eqn. (153). It was attributed to recombination of... 

Hummel and Luthjens [398] formed electron—cation pairs in cyclohexane by pulse radiolysis. With biphenyl added to the solvent, biphenyl cations and anions were formed rapidly on radiolysis as deduced from the optical spectra of the solutions. The optical absorption of these species decreased approximately as t 1/2 during the 500 ns or so after an 11ns pulse of electrons. The much lower mobility of the molecular biphenyl anion (or cation) than the solvated electron, es, (solvent or cation) increases the timescale over which ion recombination occurs. Reaction of the solvated electron with biphenyl (present in a large excess over the ions) produces a biphenyl anion near to the site of the solvated electron localisation. The biphenyl anion can recombine with the solvent cation or a biphenyl cation. From the relative rates of ion-pair reactions (electron-cation, electron—biphenyl cation, cation—biphenyl anion etc.), Hummel and Luthjens deduced that the cation (or hole) in cyclohexane was more mobile than the solvated electron (cf. Sect. 2.2 [352, 353]). 

The 77-SCF MO Cl method has also been used446 to interpret spectral transitions of a series of possible intermediates in the reaction of uracil and cytosine with the solvated electrons eaq, produced by radiolysis of water. Experimentally this reaction has been investigated by Hayon,447 who used the technique of flash radiolysis. Hayon measured the optical-absorption spectra of the transient species in the UV range to obtain information on the site of attack of eaq on the pyrimidine base. At pH 5.0 the solvated electrons react with the pyrimidine molecules mainly at the C-2 and C-4 carbonyls, and the intermediates are rapidly protonatcd to give the corresponding ketyl radicals. For uracil Hayon found two absorption maxima (at 305 and < 280 nm) at pH 5.1 and one peak at 310 nm at pH 11.7. In this last case, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. Several species, 44, 45, 46, have been suggested by... 

The solvated electron has been studied in a number of organic liquids, among which are the aliphatic alcohols (27, 28, 3, 2d, 2, 27), some ethers (25, 5), and certain amines (9, 22, 2). Of these systems, it is only in the alcohols, to which this paper is principally but not exclusively directed, that both the chemical reactivity and the optical absorption spectrum of the solvated electron have been investigated in detail. The method used in these studies is that of pulse radiolysis (22, 22), developed some five years ago. The way was shown for such investigations of the solvated electron by the observation of the absorption spectrum of the hydrated electron (6, 28, 19) and by the subsequent kinetic studies (2d, 22, 20) which are being discussed in other papers in this symposium. 

Table I. Optical Absorption Data and Radiation Chemical Yields for the Solvated Electron in the Aliphatic Alcohols at 23° C.

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. 

Rate studies of the reaction between cesium and water in ethylenediamine, using the stopped-flow technique, have been extended to all alkali metals. The earlier rate constant (k — 20 NT1 sec.-1) and, in some cases, a slower second-order process (k — 7 Af"1 sec.-1) have been observed. This is consistent with optical absorption data and agrees with recent results obtained in aqueous pulsed-radiolysis systems. Preliminary studies of the reaction rate of the solvated electron in ethylenediamine with other electron acceptors have been made. The rate constant for the reaction with ethylene-diammonium ions is about 105 NCl sec.-1 Reactions with methanol and with ethanol show rates similar to those with water. In addition, however, the presence of a strongly absorbing intermediate is indicated, which warrants more detailed examination. 

Earlier work (6) using this method yielded a second-order rate constant of 24.7 1.5 M""1 sec."1 for the reaction of dilute solutions of cesium with water in ethylenediamine. On the basis of optical absorption spectra (7) and other evidence (8, II), it was assumed that this reaction was that of the solvated electron as well as loosely bound electrostatic aggregates of electrons and cations with water. This permitted correlation with the results of aqueous radiation chemistry. 

Further, the results can suggest to the experimentalist the optical region for absorption by still other possible species of the solvated electron. 

The ferrocyanide ion, stable at neutral pH, shows well understood optical transitions in the visible and near u.v. It carries four negative charges, which should facilitate solvated electron formation. A survey of its known photochemistry indicated that at neutral pH we may hope that solvated electron formation will be the only photochemical primary process. Indeed, after we commenced the work, Matheson, Mulac, and Rabani (31) found on flash photolysis the absorption spectrum of e aq in aqueous solutions of ferrocyanide. 

Fig. 2. Wave functions and energy levels for the solvated electron in (a) methylamine (MeA) and (b) hexamethylphosphoramide (HMPA). The potential V(r) and wavefunction are based upon the model of Jortner (101) and computed using values of the optical and static dielectric constants of the two solvents. The optical absorption responsible for the characteristic blue color is marked by h v and represents transitions between the Is and 2p states. The radius of the cavity is 3 A in MeA, and —4.5 A in HMPA. 

All attempts (74,89) to find a sensible, quantitative relation between the wavelength of maximum absorption ( max) and typical macroscopic properties of the solvent (i.e., dielectric constant) have so far failed (146). However, the size of the solvent cavity in which the electron is trapped also plays a decisive role (101) in determining the transition energy [Eqs. (2), (3)], and the solvent dependence of A.max might well indicate a variation in cavity size from solvent to solvent. In this spirit, Dorfman and Jou (48) have evaluated cavity radii on the basis of the simple Jortner model for the solvated electron. The values are shown in Fig. 3, which shows a plot of the optical transition energy max versus... 

While the study of solvatochromic dyes is well established as a means of probing solvent polarity, these are not the only solutes that can be used in this fashion. A more exotic solvatochromic probe is an excess electron in solution. Optical absorption studies of the thermalized (solvated) electron generated in the pulse radiolysis of a series of ILs show a strong dependence on cation character, with a relatively low frequency for tetraalkylammonium systems and a higher frequency for cyclic (pyrrolidinium-based) cations [48, 207]. The solvated electron spectrum is often interpreted in a particle-in-a-box framework, which would imply that the cyclic cations (which possess smaller ionic volumes) simply coordinate more closely with the electron and so create a smaller domain in which the electron must localize. The breadth of the absorptions and their maximum fall within the range of values expected for moderately polar organic solvents. 

Figure 16-10. Left optical absorption spectra for the solvated electron for different densities in SCW at 645K. Solid line, 1.0g/cm3 dotted line, 0.5g/cm3 dashed line, 0.3g/cm3 dot-dashed line, 0.1 g/cm3. Empty circles are for ambient conditions. Right same for SCA at T =1.1 (reduced density are as marked). Filled circle are for the triple point of ammonia. The insert shows the density dependence of the absorption maxima in the low-density interval investigated. SCW diamonds, our work squares and triangles are from experiments. SCA diamonds our work circles experimental data. Left and right figures are from Ref. [27] and [28], respectively...

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. 

Since 1962, when it was first characterized by pulse radiolysis transient absorption measurements in water, the solvated electron has been widely studied in numerous solvents. The solvated electron, denoted by e, is a thermodynamically stable radical, but like most free radicals, it has a short lifetime due to its high chemical reactivity. The solvated electron is a unique chemical moiety whose properties may be compared in many solvents and are not dependent on the method creating the solvated electron. The solvated electron is an important reactive species as it is the simplest electron donor, its reactions correspond to electron transfer reactions and its reactivity may be used to probe electron transfer properties of acceptors. During the last 40 years, due to its optical absorption properties, the... 

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