Radiolysis in aqueous solution

In a related manner, gamma radiolysis of tetrazolium salts also yields formazans.256,257 Pulse radiolysis in aqueous solutions leads to tetrazole... 

The results of the pulse radiolysis in aqueous solution of substituted phosphonium ions have been investigated38 in addition to those of the electrode reduction of benzyl triphenyl-phosphonium ion39 and quaternary phosphonium salts with hydrated electrons40-42. The radiolysis of methyltriphenylphosphonium, dimethyldiphosphonium and trimethyl-phenylphosphonium ions in neutral aqueous solutions leads to phosphoranyl radicals... 

It is then oxidized to afford phenol through cation 95. The pH of the reaction medium has a strong effect on conversion and product distribution since 94 may participate in an acid-catalyzed competitive dehydration to yield a radical cation (96). Compound 96 may be reduced to the starting material. If there is no other oxidant present, dimerization may also occur. This is the case when benzene undergoes radiolysis in aqueous solution.739,741... 

Complexes with macrocyclic ligands have been prepared using chemical oxidation, (NH4)2S208, HNO33037,3126-3128 and NOC104,3049,3129 electrochemical methods3052,3130 or pulse radiolysis in aqueous solutions.3054,3131,3132... 

X-ray ionization of o-vinylbenzaldehyde (136) in argon matrices leads to the quinoketene (137) via die radical cation, detected by IR spectroscopy.298 The product identity was confirmed by the independent preparation of (137) and (137+-) by the photo-stimulated ring-opening of 2-methylbenzocyclobutenone (138) (Scheme 21). The reactions of benzaldehyde, acetophenone, and benzophenone with OH, 0 and SC>4 have been studied by pulse radiolysis in aqueous solution.299 The addition of OH to the carbonyl moiety of benzaldehyde predominates over addition to the aromatic ring, whereas ring addition is predominant in the case of acetophenone. Disproportionation of the exocyclic OH adduct is proposed to explain the formation of benzoic acid, which is a major product in the reaction of benzaldehyde and OH or SO4T Rate constants for each reaction have been calculated. 

Quantities of pyridinyl radicals are best prepared by chemical reduction, defined concentrations of radicals by electrochemical reduction, by photolytic procedures, or by pulse radiolysis in aqueous solution. 

The basicity of pyridinyl radicals is of interest since some pyridinyl radicals react with one another in an acid catalyzed reaction (sect. 4.4). The absorption spectra of radicals generated from the pyridinium ion by pulse radiolysis in aqueous solution at different pH values allow the evaluation of the basicity of the radicals 97,98) I jjg Qf protonated form of l-methyl-3-carbamidopyridinyI radical, (CONHj) ), is 1.43 the protonated radical has alsorption maxima at 3(X) nm and 440 nm, at somewhat longer wavelengths than those for the unprotonated radical at 280 nm and 420 nm. The nicotinamide adenine dinucleotide radical (NAD ) has a pK, of 0 or less, with a shift in absorption maximum due to protonation being observed only in 1.5 M HCIO4. 

One-electron oxidation mechanism of methionine methyl ester (MME), N-acetylmethionine (NAM), N-acetylmethionine amide (NAMA), and N-acetylmethionine methyl ester (NAMME) employing OH radicals were studied by means of pulse radiolysis in aqueous solutions. 

A somewhat different behavior has been instead observed for the radical zwitte-rions of indol-3-ylacetic and 3-indol-3-ylpropionic acids generated by pulse radiolysis in aqueous solution (through Br2 oxidation), the former undergoing decarboxylation in acidic solution with / = 1.6 x 10" s , while no decarboxylation has been observed with the latter (Scheme 73) [234],... 

Neta and Behar measured the rate constants for intramolecular electron transfer for an extended series of aryl (ArX) and benzyl halide (ArCH2X) radical anions bearing an electron withdrawing ring substituent (NO2, CN, COCH3), generated by pulse radiolysis in aqueous solution (Table 12), conditions under which spectroscopic evidence for formation of the intermediate radical anions was obtained [281-284]. 

It can be seen that in contrast to its radiolysis in aqueous solution (see Table 1), in the solid state there is a larger contribution from fragmentation products which we attribute to the participation of oxyl radicals under these conditions. As expected, hydrogen is a major product and the material balance is poor despite the fact that a large number of products have been quantified (Table 3). Again, the missing complement should mainly consist of dimers which would escape analysis. 

The anion radicals of the metal complexes were observed by pulse radiolysis in aqueous solutions saturated with N2O. Excitation of the CT band for the zinc complex in the presence of methylviolgen (MV ) results in the oxidation of the complex... 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

An interesting example of the N(9)-C(8) prototropic tautomerism has been reported for the caffeine radical by pulse radiolysis studies in aqueous solution the transformation of the heteroatom-protonated electron adduct 25 into the carbon-protonated tautomer 26 occurred spontaneously in neutral media [95JCS(F)615]. 

Complexes of cations in unstable oxidation states in aqueous solutions as studied by pulse radiolysis. D. Meyerstein, Acc. Chem, Res., 1978,11,43-48 (95). 

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

The standard reduction potentials determined in aqueous solution give hierarchies slightly different from the antioxidant hierarchy established in DMF. For the potential determined by pulse radiolysis the ordering according to tendency of regeneration is (Jovanovic et al, 1994) ... 

Sumiyoshi and coworkers studied the radiolysis of aqueous solution of methyl methylthiomethyl sulfoxide [ H3S(0) H2S H3 MTMSO] at various pH by pulse radiolysis. They found that the reaction of e with MTMSO (in the presence of 1M t-butyl alcohol to scavenge OH" radicals) leads to formation of a transient with a broad absorption band of = 375 nm. The absorbance at 375 nm as a function of pH are of S shape, indicating an equilibrium due to reaction with H. Similar to the finding in dimethyl sulfoxide they suggested the scheme. 

The pulse radiolysis technique gives a direct way for measuring the hydrated electron yield. To get the stationary yield, one can simply follow the electron absorption signal as a function of time and, from the known value of the extinction coefficient (Table 6.2), evaluate g(eh). Alternatively, the electron can be converted into a stable anion with a known extinction coefficient. An example of such an ion is the nitroform anion produced by reaction of eh with tetrani-tromethane (TNM) in aqueous solution ... 

FIGURE 7.2 Relative molecular yields of H2 and H202 in the radiolysis of aqueous solutions (Schwarz, 1955). Full curve is theoretical for solute concentrations (X axis) in different experiments multiplied by suitable normalization factors. Notice that even after normalization experimental points remain considerably higher than theoretical at high concentrations (x > 0.1). From Schwarz (1955), with permission of Am. Chem. Soc. ... 

Azaquinoid structures have been detected by ESR along with several radical species formed by the in situ radiolysis of 3- and 5-hydroxy-2-pyridones in aqueous solutions (76T261 79JPC2407 90T2891). 

Michael and Hart10 found that the reaction of OH radicals (formed by pulse radiolysis of aqueous solutions saturated with N20) with 1,3- and 1,4-cyclohexadienes leads to formation of an intermediate absorbing at 310 nm. In the case of 1,4-cyclohexadiene, another band at A, < 240 nm was also found. In this system there are both H atoms and OH radicals, however the yield of the OH radicals is 10 times higher than that of the H- atoms. Michael and Hart10 assumed that the band at 310 nm is due to CeWi ... 

Tausch-Treml, R., Henglein, A., and Lilie, J., Reactivity of silver atoms in aqueous solution II a pulse radiolysis study, Her. Bunsen-Ges. Phys. Chem., 82,1335, 1978. 

The results of y-radiolysis of aqueous solutions of thymine in the presence of MNP exemplify the unusual, but by no means unprecedented, occurrence of accumulation of spin adduct after reaction has ceased . Thus, when irradiation is discontinued, and the sample is examined spectroscopically, the signal attributed to the MNP adduct of [38] slowly increases in intensity (Joshi et al., 1978). The explanation advanced in this instance was that the spin adduct itself scavenges radicals very efficiently during radiolysis, giving a diamagnetic product which is thermally unstable, and which slowly cleaves,... 

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