Electron pulse, species involved

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... 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

The species involved in the electron transfer to be studied are both generated by the electron pulse interacting with the appropriate aqueous solution of the precursors. The electron pulse creates the following primary species in the solvent ... 

The aqueous chemistry of NO has been extensively studied. Nitric oxide has a modest solubility of 1.9 mM in water under 1 atm of NO. Descriptions of its reactivity have tended to be highly variable. Recent work indicates that NO is not highly reactive, despite having an unpaired electron, but reacts with O2 to form reactive species. Traces of Oj are the probable source of different conclusions about the reactivity of NO. It should be noted that there have been some recent studies of the general properties of species involved in these sterns, such as the pulse radiolysis of NO, the pK, values of HNOj and 1 0, the reduction potential of NO. and the AGf of ONOjH. ... 

In an elegant experiment carried out at the Argonne Laboratory, the absorption spectra of both divalent americium and tetravalent americium were obtained [356]. The technique involved irradiation of americium (iii) solutions with single electron pulses and recording the spectra with a streak camera at post-irradiation times of 50 /zs for Am(ii) and 100 s for Am(iv). Am(ii) disappeared by reaction with water while the Am(iv) species disproportionated, that is, reacted with each other to yield Am(iii) and Am(v). 

Discovery of the hydrated electron and pulse-radiolytic measurement of specific rates (giving generally different values for different reactions) necessitated consideration of multiradical diffusion models, for which the pioneering efforts were made by Kuppermann (1967) and by Schwarz (1969). In Kuppermann s model, there are seven reactive species. The four primary radicals are eh, H, H30+, and OH. Two secondary species, OH- and H202, are products of primary reactions while these themselves undergo various secondary reactions. The seventh species, the O atom was included for material balance as suggested by Allen (1964). However, since its initial yield is taken to be only 4% of the ionization yield, its involvement is not evident in the calculation. 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

On the other hand, M or M can be selectively generated in radiation chemical reactions of any M in solution via pulse radiolysis (PR) and y-radiolysis (y-R) techniques [1,36-41], which are used in the present study. Pulse radiolysis has been widely used for the kinetic study involving M . Various processes, such as ionization, excitation, electron transfer, solvation, relaxation, decomposition, etc., occur initially in the radiation chemical reaction in solutions. Chemical species generated from the initial processes react with M as a solute molecule to generate effectively and selectively M, M , or M in the triplet... 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

Concerning the first field of application, the kinetics and equilibrium constants for several halide transfer reactions (equation 1) were measured in a pulsed electron high pressure mass spectrometer (HPMS)4 or in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR)5. From measurements of equilibrium constants performed at different temperatures, experimental values were obtained for the thermochemical quantities AG°, AH° and AS° for the reaction of equation 1. The heat of formation (AH°) of any carbocation of interest, R+, was then calculated from the AH0 of reaction and the AH° values of the other species (RC1, R Cl and R +) involved. 

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