Alkaline solutions, pulse radiolysis

Aqueous alkaline luminol solutions can be excited to chemiluminescence by pulse radiolysis, the only additional requirement being oxygen 119h The suggested mechanism is that hydroxyl radicals attacking luminol monoanions, followed by reaction of the luminol radical anion thus formed with oxygen ... 

For those systems where Ri = R2 = H or Ri = H, R2 = CH3, i.e. where the number of alkyl groups at C, is <1, and R3 = H to NO3, the alkoxynitroxyl radicals formed according to Eq. 7 under steady-state-ESR or pulse radiolysis conditions do not give rise to nitrobenzene radical anions. This means that the rate constants for heterolysis of the nitroxyls are < 10 s . This is not only true in weakly acidic (pH 4) or neutral but also in strongly alkaline solution (pH 13-14). The latter observation means that the nitroxyls are not susceptible to base catalyzed heterolysis. From this the rate constant for OH catalyzed decomposition can be estimated to be < 10 M s [19]. This low number for... 

Ellison DH, Salmon GA, Wilkinson F (1972) Nanosecond pulse radiolysis of methanolic and aqueous solutions of readily oxidizable solutes. Proc R Soc Lond A 328 23-36 Erben-Russ M, Bors W, Saran M (1987) Reactions of linoleic acid peroxyl radicals with phenolic antioxidants a pulse radiolysis study. Int J Radiat Biol 52 393-412 Eriksen TE, Fransson G (1988) Radical-induced oxidation of glutathione in alkaline aqueous solution. Radiat Phys Chem 32 163-167... 

Faraggi M, Klapper MH (1993) Reduction potentials determination of some biochemically important free radicals. Pulse radiolysis and electrochemical methods. J Chim Phys 90 711-744 Faraggi M, Klapper MH (1994) One electron oxidation of guanine and 2 -deoxyguanosine by the azide radical in alkaline solutions. J Chim Phys 91 1062-1069 Faraggi M, Broitman F, Trent JB, Klapper MH (1996) One-electron oxidation reactions of some purine and pyrimidine bases in aqueous solutions. Electrochemical and pulse radiolysis studies. J Phys Chem 100 14751-14761... 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

In alkaline media Pb(II) is reduced to a hydrolyzed Pb(I) species, and in nonalkaline media at high Pb2+ concentrations, Pb+ reacts to form Pb23+ (300). By investigating the pulse radiolysis of Pb(II) in concentrated chloride solutions Sukhov and Ershov were able to identify PbCl and to set an upper limit to the association constant of 0.5 M l (302). 

Studies of proton transfers from carbon with f1/2 less than one microsecond are quite rare. The ionization of hydrogen cyanide in aqueous alkaline solution has been studied [20] using the ultrasonic stationary method which is applicable to reactions with tl/2 in the range 10 5 to 10 9 sec. Several reactions of benzyl carbanion having f 1/2 in the range 10-6—5 x 10-8 sec have been studied in tetrahydrofuran. The carbanion was generated by pulse radiolysis of solutions of dibenzyl mercury and its subsequent reaction with water and alcohols was followed spectrophotometrically [21]. 

Pulse radiolysis studies of solutions containing alkali or alkaline earth salts have shown that the solvated electron does not reduce these metal ions. The main effect of salts is to produce a blue shift of the solvated electron spectrum. 

Generation of peroxynitrite by pulse radiolysis involves very tricky adjustments of concentrations of additives and many other experimental conditions. Radiolysis of aerated aqueous solutions containing sodium formate and potassium nitrite in the pH range 3 to 10 produced peroxynitrite according to Eqs. (21) to (23), (40), (41) and (11). For this, initial concentrations of nitrite and formate have to be adjusted in such a way that the radiolytically produced concentration of the radicals of [0 ] -l- [HOj] > NO. However, reactions of peroxynitrite with antioxidants are not generally studied by pulse radiolysis technique. Chemical methods such as ozonolysis of alkaline sodium azide solutions are commonly used to produce peroxynitrite in large quantities and its reactions studied by mixing techniques. 

The radiation chemistry of 2-propanol is analogous to that of methanol, that is, the main reactive species are Cs and (CH3)2 COH. In alkaline solution, (CH3)2 COH deprotonates to (CH3)2CO . In the presence of N2O or acetone, es is converted to (CH3)2 C0H/(CH3)2C0 by the reactions in Eqs. 30 and 18, or the reaction of Eq. 20, respectively. The solvated electron in 2-propanol has been utilized to study electron-transfer reactions between aromatic radical anions (donor) and aromatic molecules (acceptor) [16]. The donor-acceptor pairs studied were pyrene-anthracene, pyrene-9,10-dimethylanthracene and w-terphenyl-/ -terphenyl. In the first two cases an equilibrium was established and the parameters forward and kback were measured this was the first example of the measurement of an equilibrium constant by use of pulse radiolysis. The rate constants for the electron-transfer reactions were examined in terms of the Marcus theory [17]. 

To measure the back reaction, it is necessary to generate H in alkaline solution, and this was achieved by Matheson and Rabani [71] who developed a pulse radiolysis cell capable of operating at a pressure of 100 bar. In this way they were able to produce H effectively during the pulse (0.4 ps) by the following reaction (Eq. 60) ... 

Nevertheless, the information obtained on the relative reactivities of CrIvaq and other Cr species232 is very important for understanding the general mechanisms of interconversions of various oxidation states of Cr, and the roles of 02 in these reactions 9 Generation of Crlv species by pulse radiolysis of Cr111 hydroxo-aqua complexes in alkaline aqueous solutions has also been described.237... 

Reaction of eaq , produced by pulse radiolysis, with bromophenols in alkaline solutions exhibited completely different pathways . When the hydroxyl group of the hydroxyphenyl radical is dissociated, the negative charge is partly delocalized from to the site of the radical on the aromatic ring and this site then undergoes very rapid protonation by water to form a phenoxyl radical (equation 8). 

Some years later the detection and production of transient hexavalcnt technetium in aqueous alkaline media by pulsed radiolysis and very fast scan cyclic voltammetry were reported [32]. More recently, the mechanism of the stepwise reduction of TCO4 to Tc(-i-6), Tc(-i-5) and Tc(h-4) in alkaline solution was reinvestigated in detail by pulsed and dc polarography [33]. 

It is known that the first idea of solvated electrons appeared to explain the properties of alkaline metal solutions in liquid ammonia. In this connection the pulse radiolysis of liquid ammonia is of special interest because it is possible to compare properties of intermediates formed by different methods. 

M. This shows that the decay of trapped intermediates takes place in a manner which is similar to the case of the pulse radiolysis of frozen alkaline and LiC104 solutions, observed earlier 7,8). It is significant that in this case, the absorption spectrum of the intermediates does not differ significantly from that in pure ammonia and NaCl solutions. 

A general review of pulse radiolysis studies on electron transfer in solution is presented together with some recent unpublished data. Electron transfer processes occurring in irradiated solutions of metal ions, inorganic anions, and various aliphatic and aromatic organic compounds are discussed with respect to general redox phenomena in radiation and free radical chemistry. Specific topics include the measurement of peroxy radical formation, the use of nitrous oxide in alkaline radiation chemistry, and cascade electron transfer processes. Some implications of the kinetics of electron transfer are discussed briefly. 

Pulse Radiolysis of Aqueous Fluorescein Dyes. The xanthene dyes of the fluorescein type were investigated using l-/xsec. pulses of 30 Mev. electrons (4, 5, 8). The transient spectra obtained with deaerated fluorescein solutions show three characteristic sets of bands. A prominent peak that shifts from 355 m/x in neutral solutions to 395 m/x in alkaline solutions corresponds with the semiquinone monoanion (pKa = 9.5) (13). This band is quenched by e q scavengers, such as oxygen or H202, and was attributed to reduction of the dye by eaq. A band at 415-420 m/x which does not change with pH was identified with the semioxidized radical monoanion, a phenoxyl derivative first observed in flash photolysis also (13). This band is quenched by formate and was attributed to the oxidative attack of OH. The remaining transient consists of a diffuse... 

Mayer and Kiaslukianis [44] carried out pulse radiolysis of p-benzoquinone, 1,4-naphthoquinone, naphthazarin, 9,10-anthraquinone and quinizaiin, in fi-eon 113 (CF2CI-CFCI2) solution at room temperature. Blue shifted UV-visible absorption spectra were assigned to the radical cations. High formation rate constants ( 10 ° dm mof s ) have been reported. The value of pK(l) is the most important parameter as pK(2) and pK(3) are normally measurable in very strongly alkaline solutions only [31]. 

Radiolytic oxidation of Cr "-porphyrins in alkaline solutions also led to the formation of oxo-Cr -porphyrins. Further irradiation led to production of oxo-Cr -porphyrins. Pulse radiolysis studies indicated that Cr P under all conditions is oxidized first to the 7r-radical cation, Cr P-. This radical cation is unstable in aqueous solutions and undergoes disproportionation, but in CH2CI2 it is stabilized by the HCl (produced by fixe radiolysis). The radical cation was further oxidized by irradiation and the product was suggested to be a dication, Cr p2, also stabilized by the HCl. Furthermore, it was found that addition of base to Cr P converts this species into Cr P, and the process can be reversed by the addition of acid. 

The kinetics of several reactions have been examined by pulse radiolysis methods. The HO" radical-induced oxidation of o-, m-, and p-cresol in acidic and alkaline solution has been studied by pulse radiolysis and found to be a two-step process. The first step was determined to be the diffusion-controlled addition of HO" to the aromatic ring and was independent of pH. Subsequent elimination of HO in alkaline solution occurs considerably faster than the elimination of H2O in the acidic medium. 

The decay of the ozonide radical ion in aqueous, alkaline solution has been characterized. A complication arises from carbonate impurities in the alkaline solution, which are converted to CO3 radical ions in the pulse radiolysis experiments. The ozonide radical ion OJ (formed by reaction between 0 and O2) reacts with the carbonate radical ion to give ozone and carbonate. " This may explain why carbonate has a stabilizing effect on ozone, as both O3 and CO3 radicals have been detected during the decomposition of ozone in alkaline solutions with carbonate present, and this reaction between the two radical species may occur. The mechanism of formation of hypobromite from ozone and bromide has been studied. ... 

The luminescence properties of the decatungstolanthanate anions, especially [Eu(W5Oi8)2]9 , have attracted considerable attention, both in the solid state and solution (Blasse et al., 1981a, 1981b Blasse and Zonnevijlle, 1982 Ballardini et al., 1983 Darwent et al., 1986 Blasse, 1988 Sugeta and Yamase, 1993 Ozeki and Ya-mase, 1993c Lis et al., 2003). Electroluminescence cells based on alkaline earth metal salts of the europium anion have been described (Yamase and Ueda, 1993) and the results of pulse- and continuous-radiolysis of solutions of the europium and neodymium anions have been reported (Mulazzani et al., 1985). 

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