Absorption spectrum hydrated electron

A surprising observation was made in the first experiments on the flash photolysis of CdS and CdS/ZnS co-colloids Immediately after the flash from, a frequency doubled ruby laser (X = 347.2 nm photon energy, = 3.57 eV) the absorption spectrum of the hydrated electron was recorded. This species disappeared within 5 to 10 microseconds. More recent studies showed that the quantum yield increased... 

The core of the iron storage protein ferritin consists of a hydrated ferric oxide-phosphate complex. Various models have been proposed which feature Fe111 06 oct., Fe111 O4 tet. or Fe111 O4 tet. Fe111 06 oct. complexing the first listed is preferred by Gray (99) on the basis of the electronic absorption spectrum. The protein very closely related to ferritin which occurs in the mold Phycomyces blakesleeanus contains... 

FIGURE 6.2 Absorption spectrum of the hydrated electron. The spectrum is structureless, broad (half-width 0.84 eV), intense (oscillator strength 0.75), and has a single peak at 1.725 eV. (See text for details.)... 

The absorption spectrum of ehis intense, has a broad peak at about 720 nm (halfwidth -1 eV), and is structureless (see Sect. 6.1 and Figure 6.2). It covers at least 220 to 1000 nm and possibly extends on either side. There is some evidence that the absorption rises somewhat in the UV, which has been interpreted as the water absorption perturbed by the hydrated electron (Nielsen et al, 1969,1976). 

The intensity of absorption gives the product G , where G is the observed yield and is the molar extinction coefficient. The absolute value of was determined by Fielden and Hart (1967) using an H2-saturated alkaline solution and an alkaline permanganate-formate solution, where all radicals are converted into Mn042. They thus obtained = 1.09 x 104 M- cm1 at 578 nm, which is almost identical with that obtained by Rabani et al. (1965), who converted the hydrated electron into the nitroform anion in a neutral solution of tetrani-tromethane. From the shape of the absorption spectrum and the absolute value of at 578 nm, one can then find the absolute extinction coefficient at all wavelengths. In particular, at the peak of absorption, (720)/ (578) = 1.7 gives at 720 nm as 1.85 X 104 M 1cm 1. 

The first subnanosecond experiments on the eh yield were performed at Toronto (Hunt et al., 1973 Wolff et al., 1973). These were followed by the subnanosecond work of Jonah et al. (1976) and the subpicosecond works of Migus et al. (1987) and of Lu et al. (1989). Summarizing, we may note the following (1) the initial (-100 ps) yield of the hydrated electron is 4.6 0.2, which, together with the yield of 0.8 for dry neutralization, gives the total ionization yield in liquid water as 5.4 (2) there is -17% decay of the eh yield at 3 ns, of which about half occurs at 700 ps and (3) there is a relatively fast decay of the yield between 1 and 10 ns. Of these, items (1) and (3) are consistent with the Schwarz form of the diffusion model, but item (2) is not. In the time scale of 0.1-10 ns, the experimental yield is consistently greater than the calculated value. The subpicosecond experiments corroborated this finding and determined the evolution of the absorption spectrum of the trapped electron as well. 

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

Other transient radicals such as (SCN)2 [78], carbonate radical (COj ) [79], Ag and Ag " [80], and benzophenone ketyl and anion radicals [81] have been observed from room temperature to 400°C in supercritical water. The (SCN)2 radical formation in aqueous solution has been widely taken as a standard and useful dosimeter in pulse radiolysis study [82,83], The lifetime of the (SCN)2 radical is longer than 10 psec at room temperature and becomes shorter with increasing temperature. This dosimeter is not useful anymore at elevated temperatures. The absorption spectrum of the (SCN)2 radical again shows a red shift with increasing temperature, but the degree of the shift is not significant as compared with the case of the hydrated electron. It is known that the (SCN) radical is equilibrated with SCN , and precise dynamic equilibration as a function of temperature has been analyzed to reproduce the observation [78],... 

Two-photon absorption chemistry of 2AP, specifically photoionization processes, can be induced by intense nanosecond 308-nm XeCl excimer laser pulses [10]. Typical transient absorption spectra of 2AP in deoxygenated neutral aqueous solutions are shown in Fig. 1. The stronger (385 nm) and weaker (510 nm) absorption bands were assigned to 2AP radicals derived from the ionization of 2AP (bleaching near 310 nm) [10], whereas a structureless absorption band from -500 to 750 nm corresponds to the well-known spectrum of the hydrated electron (eh ) [41]. 

The transient absorption spectra of duplexes with [2AP]A4GGAs are depicted in Fig. 5. At a delay time of 100 ns, the transient absorption spectrum is attributed to the superposition of the spectra of the 2AP(-H) and G /G (-H) radical products and the hydrated electrons. The structureless tail of the eh absorption in the 350-600 nm region decays completely within At<500 ns. The formation of G VG(-H) radicals monitored by the rise of the 310-nm absorption band and associated with the decay of the 2AP V 2AP(-H) transient absorption bands at 365 and 510 nm (Fig. 5) occurs in at least three well-separated time domains (Fig. 6). The prompt (<100 ns) rise of the transient absorption at 312 nm due to guanine oxidation by 2AP was not resolved in our experiments. However, the ampHtude, A((=ioo), related to the prompt formation of the G /G(-H) radicals (Fig. 6a) can be estimated using the extinction coefficients of the radical species at 312 and 330 nm (isosbestic point) [11]. The kinetics of the G VG(-H) formation in the yits and ms time intervals were time-resolved and characterized by two well-defined components shown in Fig. 6a (0.5 /zs) and Fig. 6b (60 /zs). 

Fig. 98. Lower parr, difference spectra of Q-CdS after the reaction with hydrated electrons (full line) and after the reaction with OH radicals (dashed line, lOx magnified). Upper part absorption spectrum of the investigated sample [576]...

The blue color of these "type 1" copper proteins is much more intense than are the well known colors of the hydrated ion Cu(H20)42+ or of the more strongly absorbing Cu(NH3)42+. The blue color of these simple complexes arises from a transition of an electron from one d orbital to another within the copper atom. The absorption is somewhat more intense in copper peptide chelates of the type shown in Eq. 6-85. However, the -600 nm absorption bands of the blue proteins are an order of magnitude more intense, as is illustrated by the absorption spectrum of azurin (Fig. 23-8). The intense blue is thought to arise as a result of transfer of electronic charge from the cysteine thiolate to the Cu2+ ion.520 521... 

Figure 1.5 Absorption spectrum of the hydrated electron the ordinate is the molar decadic extinction coefficient (m 1 cmr1)...

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. 

Absorption Spectrum of e aq. The absorption spectrum of the hydrated electron is shown in Figure 1. The evidence that this spectrum is that of eaq is at least four-fold. First, the spectrum is suppressed by known electron scavengers, such as H30+, 02, N20 (4, 18). Second, it resembles in form the absorption bands of the solvated electron in liquid ammonia and methylamine (4, 18). Third, the rate constants calculated from the decay of this absorption in the presence of scavengers... 

Finally, the development of pulse radiolysis enabled a direct observation of e aq, and a direct distinction between e aq and H could easily be made. Matheson (37) (with spectroscopic data obtained by Keene) suggested that e ag has optical absorption in the visible. Hart and Boag (26) used spectrographic plates and studied this absorption. The effect of solutes, which were known as electron scavengers led to the conclusion that the absorption was due to e aq. It was confirmed later, that the absorption belonged to unit negatively charged species by means of a salt effect (20), as well as by conductivity measurements (49). Many more papers on the absorption spectrum and rate constants of the hydrated electron have since appeared (16). 

Keene JP (1964) The absorption spectrum and some reaction constants of the hydrated electron. Radiat Res 22 1-13... 

Subsequently, the absorption spectrum of the transient hydrated electron was observed in pulse radiolysed deaerated water43 44. Its spectrum is shown in Fig. 5. This absorption may also be observed with continuous radiolysis at high dose rates. Thus it has been observed46 with hydrogen-saturated solutions of 10 3 M NaOH when irradiated in a 15,000 curie 60Co source. 

In the presence of soft-core micelles at high water content (Wo = 60), the absorption spectrum consists of a broad band with a maximum around 710 nm. To explain the blue-shift and the broadening of the hydrated electron absorption spectra, the effect of sodium perchlorate in aqueous solution was studied. Without any additive, the absorption spectrum of the hydrated electrons was centered at 720 nm, and on adding 5M sodium perchlorate to the water solution, a blue-shift of the absorption spectrum was observed with a maximum centered at 650 nm. The blue-shift observed in the reverse micelles at low Wo is due to the high concentration of sodium ions (and ion-pairs) in the micellar core, at Wo = 5 and [Na] = 10 M of the sodium sulfonate (Pileni et al., 1982). 

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. 

Cavanagh MC, Martini IB, Schwartz BJ. (2004) Revisiting the pump-probe polarized transient hole-burning of the hydrated electron Is its absorption spectrum inhomogeneously broadened Chem Phys Letts 396 359-366. 

Romero C, Jonah CD. (1989) Molecular dynamics simulation of the optical absorption spectrum of the hydrated electron. J Chem Phys 90 1877-1887. 

The first publication on the effect of non-reactive metal cations on the absorption spectrum of the hydrated electron was published in 1965. A systematic study of the salt concentration effect on the hydrated electron absorption spectrum in very concentrated (up to 15 M) aqueous solutions of LiCl was done by IV Kreitus in 1985, then, resumed by P Krebs and co-workers in 1999. With the development of ultrafast laser pump-probe setup, a few publications noted... 

Hart EJ, Boag JW. (1962) Absorption spectrum of the hydrated electron in water and in aqueous solutions. J Am Chem Soc 84 4090 095. 

Kreitus IV. (1985) Effect of solution microstructure on the hydrated electron absorption spectrum. /Pftyr Chem 89 1987-1990. 

---