Preparation pulse radiolysis

Ulanski P, Kadlubowski S, Rosiak JM (2002) Synthesis of poly(acrylic acid) nanogels by preparative pulse radiolysis. Radiat Phys Chem 63(3-6) 533-537... 

Reactions such as these are of interest in themselves. Beyond that, one can use the pulse radiolysis experiment as a preparative technique for other species. Thus, the reactions of numerous aliphatic, carbon-centered radicals have been evaluated.22 If one employs a reasonably high concentration of solute, say 0.1-1 M CH3OH, the formation of CH2OH is complete within the electron pulse. Following that, reactions such as the following can be studied ... 

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,... 

Neta and Evans (214) studied the anion radicals of some bianthronylidenes. The anion radical was prepared in the Ar form by pulse radiolysis in 2-propanol, and its conversion to BT was followed by fast spectrophotometric technique. The rate was determined at 21°C for 129c, its 3,3 -dimethyl, 3,3 -dimethoxy, and 1,1 -dimethyl analogs, and free energy barriers of 10.6, 10.6, 11.5, and 13.1 kcal/mole, respectively, were found, which were considerably lower than that for the B - A conversion in the neutral molecules. 

In order to further probe the above route for electron transfer, there is a need to study derivatives modified at Tyr83. One such derivative in which the phenolic group of Tyr83 has been N02-modified has so far been prepared [139], On pulse radiolysis reduction with Cd to give the radical Tyr83NOj, fast electron transfer through to the Cu(II) is observed, k>10 s [140]. 

For comparison within the chalcogens, the oxidation potentials of the Ph-E-Me series 94, 95b, 96b, and 97b, where E = O, S, Se, Te, respectively, were evaluated by pulse radiolysis.Consistent with the analogous diaryl series, the values for this series indicate that the compounds are increasingly easy to oxidize, with telluride 97b most easily oxidized (0.74 V) and ether 94 least easily oxidized (1.62 V), with these values vs. NHE (Table 10). Eor a broader comparison, series of four or five para-substituted arylmethylsulfides, selenides, and tellurides, 95a-e, 96a-e, and 97a-d were prepared and their values determined in the same manner (Table 10). The same trends were observed, with the thioethers least... 

Pulse radiolysis experiments have shown that "OH radical adds preferentially at C5 of the uracil moiety, giving rise to the reducing 5-hydroxy-5,6-uracil-6-yl radical. Interestingly, the two cis diastereomers of 6-hydroperoxy-5-hydroxy-5,6-dihydrouridine, two of the expected final products of the latter radicals in aerated aqueous solutions, have been prepared by trifluoroacetic acid treatment of uridine (3, R = H, = ribose) in the presence of H202 (equation 14). The mechanism of the reaction that involves transient formation of an epoxide-type intermediate followed by nucleophilic attack by a perhy-droxyl group at C6 presents similarities with the substitution of thymine bromohydrin by... 

Nickel oligomers prepared in the presence of PA (Amax = 540 nm) (Section 20.4.2) may also act as catalysts for the reduction of Ni by hypophosphite ions. This requires, as shown by pulse radiolysis, a critical nuclearity, while free Ni cannot be reduced directly by H2PO2. Very low radiation dose conditions, just initiating the formation of a few supercritical nuclei, will lead to large particles of nickel [96]. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

Complexes of Ni(III) are synthesized by the oxidation of Ni(II) analogues in aqueous or nonaqueous media. Electrochemical oxidation (51, 105,106), chemical oxidation using (NH4)2S208, cone. HN03, NOBF4, or Co(H20)e+, and pulse radiolysis (107-110) are the most commonly used methods. Ni(III) complexes can be prepared from Ni(II) complexes when the Ni(II)/Ni(III) oxidation potentials are less than +1.1 V vs SCE. For instance, the Ni(III) species can be prepared from [Ni (cyclam)]2+ and l-3a, but cannot be obtained from the polycyclic complexes (9-14), which have Ni(II)/Ni(III) oxidation potentials in the range +1.25-+ 1.50 V vs SCE. 

The rapid-freezing EPR data obtained in pulse radiolysis experiments had been interpreted by half-site reactivity In the meantime preparations have, however, been obtained with a definitely higher specific activity (second-order rate constant... 

X 10 against 2.4x 10 M s" for the preparations used in the pulse radiolysis experiments) so that the half-site reactivity has been seriously questioned by the presence of inactive enzyme in the earlier experiments Large differences in specific activity (up to a factor of two) and in the ability to protect mouse bone marrow stem cells against X-irradiation have been reported for five SOD preparations... 

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... 

In general, cadmium(I) is a very unstable species but more examples of it are known than of zinc(I). The species Cd+ has been produced by pulse radiolysis of solutions of cadmium(II) complexes,1095-1096,10%a and the complexes [ZnL]+ and [CdL]+ (L = [14] ane-l,4,8,ll-N4) have been prepared by this means.1096 It is thought that the initial step in the pulse radiolysis is the decomposition of water to hydrated electrons and [OH], and that it is the former species which reduces the Cd11 to Cd1. 

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. 

Relative rate constants for reactions of OH have been measured by competitive methods in 7-irradiated solutions where product formation or reactant destruction have been monitored. These methods have generally been of low accuracy and can sometimes be misleading because of the possible complications in the processes between the initial reaction and the final products. Several competitors that allow the competition at the initial step to be followed became available for use with the pulse radiolysis technique in 1965 (Adams et al., 1965). Most of the rate constants for OH reported in the literature have been determined by this method. Numerous rates have also been determined by pulse radiolysis in an absolute way, i.e. by directly observing the kinetics of the formation of transient absorption or of the decay of the parent compound absorption. Direct observation of OH (or of H) by pulse radiolysis cannot help in obtaining reaction rates, because the absorption is in the far ultraviolet and one can observe only the tail of this absorption which at 200-250nm has a very low extinction coefficient ( 500 M -1 cm-1) (Pagsberg et al., 1969). A review of the experimental methods and summary of the rate constants of OH reactions has recently been published (Dorfman and Adams, 1973) and another compilation of rate constants is currently being prepared (Farhataziz and Ross, 1975). 

Transient quantities of 02 in aqueous solution can be generated by pulse radiolysis of 02 and by photolysis of H202 in aqueous media. In aprotic media stable solutions of OJ can be prepared by electrochemical reduction of molecular oxygen and by the base-induced decomposition of H202. Superoxide species can also be made from basic dioxygen-saturated solutions of aniline in dimethyl sulfoxide 34... 

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. 

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. 

Radiation chemistry also made it possible to prepare radicals and ions of interest and study their properties. With the advent of pulse radiolysis, it was possible to directly explore the reactivity of such intermediates. In fact, many reactions that were suggested to be of importance in solar energy conversion could be more cleanly studied using radiation chemistry. Similarly, questions about the mobility of actinide species in the biosphere often depended on the reactivity of different oxidation states of materials such as plutonium. Thus, it was possible to show that plutonium oxides were unlikely to move quickly through water in the earth, because the soluble oxides were very reactive and the equilibrirrm values were far to the side of the insoluble compounds. 

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