Benzene pulse radiolysis

The branching of the adduct decomposition was studied by measuring the pKa of the mixtures. Thus for pulse radiolysis of N20-saturated methyl phenyl sulfoxide the results yield a p/Ca obs value of 1.50 while the values for methane sulfinic and benzene sulfinic acids are 2.28 and 1.29, respectively. The fraction of each branch can be calculated from the equation,... 

FIGURE 14.10 Transient absorption spectra observed following pulse radiolysis of 1 x 10-4 M ASTA with 1 x 10-5 M LYC in argon flushed benzene. 

The 0.23 pmolJ-1 of reaction 15 increases the yield of radical 5 from 0.29 in pulse radiolysis to 0.52 pmolJ-1 in y-radiolysis. Summing up all the yields of radical 5 gives 2 x (0.085 + 0.056 + 0.067 + 0.021) + 0.02 = 0.48. In this calculation we assume that benzene is formed solely from 5 + 5 disproportionation and none from reaction 16. In order to obtain closer agreement between the pulse and continuous radiolysis yields, less than 0.23 pmolJ-1 should be formed in reaction 15. 

Using basic pH leads to higher plateau rate constants, indicating that the ratedetermining step is reaction 18. Reaction 17 must be at least as fast as the rate of 02 addition in the highest 02 concentration used, kn 8 x 105 s 1, which is the limit of the instrument measurement. The G of benzene in pulse radiolysis was found to be equal to that of the nitroform anion (1.6 x 10-7 molJ-1) as can be expected from reactions 17-19. Since the yield of the cyclohexadienylperoxyl radical is 2.9 x 10-7 mol. 1 1 it means that only a fraction (ca 60%) of the cyclohexadienylperoxyl radicals eliminates HO2. The H02 elimination occurs by H-transfer of the allylic hydrogen to the oxygen... 

This conclusion is supported by results of detailed study on the decay of hydroxyhexa-dienylperoxyl radicals, formed by addition of OH to benzene, followed by addition of dioxygen molecule. It was found that in the high dose rate of pulse radiolysis, hydro-quinone is the major product whereas catechol was not observed, indicating that only the 1,3-isomer loses HO2" and hence does not lead to dihydroxybenzene. The observation that the yield of 02 is 60% of the yield of the cyclohexadienyl radicals indicates that when dioxygen molecules react with the cyclohexadienyl radical, the radical is 60% trapped in the mesomeric form of 5b, whereas the results from the final products of dimerization in /-radiolysis show that 60% react in the form 5a. 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

The solute benzene radical cation was formed on pulse radiolysis of an acidic aqueous solution of benzene. The transient optical absorption bands (A-max = 310, 350-500 nm) were assigned to the solute benzene radical cation which is formed on acid-catalysed dehydration of the OH adduct. The radical cation is able to undergo an electron-transfer reaction with Br and was found to be a strong electron oxidant. Pulse radiolysis has been used to study the complex reaction that follows electron addition to hydroxybenzophenones (HOBPs). The various radical species involved have been characterized spectrally and their p/fa values evaluated. The differences... 

Aromatic radical-cations are generated by pulse-radiolysis of benzene derivatives in aqueous solution. Radiolysis generates solvated electrons, protons and hydroxyl radicals. The electrons are converted by reaction with peroxydisulpbate ion to form sulphate radical-anion, which is an oxidising species, and sulphate. In another proceedure, electrons and protons react with dissolved nitrous oxide to form hydroxyl radicals and water, Hydroxyl radicals are then made to react with either thallium(i) or silver(i) to generate thallium(ii) or silver(ll) which are powerfully... 

Table 6.2. Long wavelength uv absorption bands of benzene radical-cations in water, determined by pulse-radiolysis techniques.

A third important reaction of aromatic radical-cations is carbon-carbon bond formation with a further aromatic substrate. This reaction is limited to the oxidation in acetonitrile of substrates with electrondonating substituents. Radical-cations from benzene, naphthalene and anthracene form a-complexes but do not form a a-bonded reaction intermediate. Tlie dimerization reaction has been investigated both by pulse-radiolysis [22] in water and by electrochemical methods [27] in acetoni-... 

Direct evidence for photoassociation is limited to those systems that exhibit either excimer (or exciplex) luminescence or excimer absorption which has recently been observed in benzene following pulse radiolysis.141... 

Sens, (pulse radiolysis of benzene Determination of energy transfer 369... 

The self-recombination reactions of HOj, CF3CFHO, and CF3O) have been studied using pulse radiolysis/time-resolved UV absorption spectroscopy.215 The addition of the cumylperoxy radical to a range of alkyl-substituted biphenyls has been studied and the rate constants compared with reactions with related monosubstituted benzenes.216... 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

Dorfman, C.M., Taumb, I.A., and Buhler, R.E., Pulse radiolysis studies. I. Transient spectra and reaction rate constant in irradiated aqueous solution of benzene, /. Chem. Phys., 36, 3051-3061, 1962. 

Gordon S, Schmidt KH, Hart EJ (1977) A pulse radiolysis study of aqueous benzene solutions. J Phys Chem 81 104-109... 

Pan X-M, von Sonntag C (1990) OH-Radical-induced oxidation of benzene in the presence of oxygen R <=> R02 equilibria in aqueous solution. A pulse radiolysis study. Z Naturforsch 45b 1337-1340... 

Pan X-M, Schuchmann MN, von Sonntag C (1993) Oxidation of benzene by the OH radical. A product and pulse radiolysis study in oxygenated aqueous solution. J Chem Soc Perkin Trans 2 289-297... 

Fluorescence detected magnetic resonance effects observed during the pulse radiolysis of anthracene-d10 in the presence of tetramethylethene portray an additional facet of the cyclobutane radical cation system [345, 346]. The spectra [eight ( ) equivalent methyl groups, ad = 8.2 G approximately one half of the monomer splitting, am = 17.1 G] are compatible with a dimer cation. In analogy to the benzene dimer radical cation [347, 348] they were interpreted as evidence for a sandwich , one molecule above the other [346],... 

Fig. 24. ODMR spectrum obtained upon pulse radiolysis of hexamethyl-(Dewar)-benzene in solution [368]...

Mantaka A, Marketos DG, Stein G. Continuous gamma and pulse radiolysis of aqueous benzene solutions some reactions of the hydroxycyclohexadienyl radical. J Phys Chem 1971 75 3885-3889. 

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