Pulse radiolysis results

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

There is still considerable controversy concerning the interpretation of pulse-radiolysis results,256-258 and none of these interpretations are in line with the principal conclusions drawn from the e.s.r.-spec-tral experiments.253 In Ref. 257, it was concluded that the principal radical 129 is already protonated at a rather high pH (pK = 4.25 that is, 0.15 unit above that of the parent compound), and that the neutral radical has a further pK value of 1.1. In the second, extended pulse-ra-diolysis study,258 the intermediacy of various OH-adduct radicals was discussed. This view might be supported by the tentative assignment253 of radical 130, observed by e.s.r. spectroscopy. However, the suggestion258 that the principal radical 129 should show a pK of 3 is not compatible with the interpretation of the e.s.r.-spectral results.253... 

The mechanism presented here is somewhat at variance with that proposed by the authors (Yamamoto et al. 1995) who suggested that the /BuOI I-derived radical adds to the primarily formed electron-adduct radical. Since this has been shown above to have only a very short lifetime, it will not be capable of undergoing bimolecular recombination reactions. An isomerization of C(8)-H -adduct [reaction (183)] followed by an addition of the tert-butanol-derived radical and water elimination [reactions (184) and (185)] is not in conflict with the above pulse radiolysis results [note that the tautomerization reaction (183) cannot be excluded on the basis of the pulse radiolysis data]. 

Polyhalide radical anions have recently been reviewed I4- and I6-have been observed in terf-butanol solution, but they are unknown in aqueous solution (127). The equilibrium constant for formation of I2-[reaction (31)] is the link between the reduction potentials of the iodine atom, the diiodine radical anion, and diiodine. Numerous measurements of this equilibrium constant have been made over the years. There are even two reports of the enthalpy of the reaction, obtained from the temperature dependence of the equilibrium constant (35). Published values for the formation constant of I2- are listed in Table IV (32, 36, 128, 129, 149, 314, 318). As noted in Fornier de Violet s review (127) and in Elliot and Sopchyshyn s paper (109), there is a systematic discrepancy between the flash photolysis results and the pulse radiolysis results. Fornier de Violet suggested that the pulse radiolysis results might be in error because of unrecognized adduct formation... 

Pulse radiolysis in chlorinated solvents leads to the formation of radical cations of almost all organic compounds including aryls. In most non-halogenated solvents, pulse radiolysis results in the formation of solvated electrons and radical anions of the solute. Therefore, depending on the selected solvent, the same donor-acceptor system could be used for the electron and hole transfer studies. 

Pulse radiolysis results were also obtained (5) suggesting that the role of methanol is to serve as a rapid proton donor to the anion radical converting the latter to a neutral propagating free radical. The resulting methoxide anion could neutralize the cation radical converting it to an additional neutral propagating free radical. Methanol has, indeed, been observed to scavenge both the cationic and anionic species ( ) and to increase the rate of free radical polymerization (11). 

Nature of the Reduced Species. Although the electrochemistry of Ni(bpy)3 species is not well estabhshed, we believe that the pulse radiolysis results give a clear indication that Ni(bpy)3 can be reduced in a single-electron step and that bpy is rapidly lost from the monoreduced species. 

Both the steady state (20) and the pulse radiolysis results can be accounted for on the assumption that the primary OH radical addition product H0C6H4N03-- undergoes the intramolecular rearrangement Reaction 6. It produces the OH benzene ring addition product... 

As indicated already, the optical transition energy are an extremely sensitive probe for the electronic and steric properties of the three-electron-bonded species and their respective relative contributions. However, the effect of substituents on the optical transitions becomes of much lesser importance in intramolecular radical cations derived from open-chain dithianes (type 7-9). Changing the terminal substituents in R-S-(CH2)3-S-R from methyl to isopropyl results in a just 15 nm change (440 vs. 455 nm), i.e., structure clearly appears to be the dominating parameter. This is fully corroborated by the pulse radiolysis results on 2-substituted-l,3-dithiacyclopentanes.l23 As mentioned already, the radical cation (11), derived from 1,3-dithiacyclopentane (12), is very unstable if formed at all ( niax > 650 nm). The analogous radical cation generated upon oxidation of l,3-dithia-2,2-dimethylcyclopentane (13), on the other hand, exhibits a pronounced and blue-shifted absorption at 610 nm as well as a considerable kinetic and thermodynamic stability. 

Since this time, further pulse radiolysis results have been reported in the ionic Hquids methyltributy-lammonium fcis(trifluoromethylsulfonyl)imide ([mtba] [TfjN]), N-butylpyridinium tetrafluoroborate ([bupy] [BF4]), and N-butyl-4-methylpyridinium hexafluorophosphate ([bupicllPF ]). The former Uq-uid was chosen because radiolysis produces solvated electrons that do not react with the parent ions and can thus be scavenged by reactive solutes (Equation 5.4). 

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,... 

The development of pulse radiolysis techniques have led to the determination of a number of the important kinetic processes of the species produced by the irradiation of H2O. The results that have been obtained for a number of the most important reactions are presented in Table II. These results demonstrate that the net effect of radiation is H2O decomposition in the absence of any reactive substrate. 

The pulse radiolysis of ODN conjugated with Py and Ptz showed a transient absorption spectrum with a peak at 470 or 520 nm, assigned to Py+ or Ptz + respectively (Fig. 9). The formation rates of Py+ and Ptz+ obtained for Py-1 and Ptz-1 were almost identical to that of the decay of TIOH+ and no secondary formation was observed. These results indicate that a hole migrates from... 

These results demonstrate that the hole transfer rate decreases with the increase of the distance between Py and Ptz (Scheme 7, path b). When Ptz and Py were separately conjugated to different ODNs, both Py + and Ptz + were observed at 100 /zs after the electron pulse during pulse radiolysis of the mixture of 0.2 mM each of Py- and Ptz-conjugated ODNs in other words no interstrand hole transfer occurred. Therefore, the observed results are accounted for by intramolecular processes. 

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. 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Much of the achieved advances result from the development and availability of instrumentation to study slow and fast reactions at pressures up to 300 MPa, including stopped-flow, T-jump, P-jump, NMR, ESR, flash-photolysis, and pulse-radiolysis instrumentation (1, 2, 4, 6, 7). Readers are advised to consult the quoted references for more detailed information, since these present a detailed account of the present instrumentation and commercial availability of such equipment. 

FDMR has also been used to detect the transient radical cations formed from secondary amines by pulse radiolysis. As mentioned earlier this technique has been used to study a variety of systems such as the radical cation of triethylamine. The radical cations of diethylamine, n-propyl amine and f-butylamine, have also been studied25. The results have shown that the FDMR signal is enhanced with increasing alkyl substitution of the amine as in the pyrrolidines (18) and the piperidines (19)25. 

Indoles can be also be converted into their radical cations by the use of C1C>2 as the oxidant produced by pulse radiolysis. From the reactivity of the resultant cation it was possible to establish the one-electron reduction potential of the indole in question. Typical results from this are illustrated in Table 234. As can be seen, the one-electron reduction potential is influenced by alkyl substitution. 

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