Scavenger studies

This sequence of formation of radical cation which is followed by a C—S bond scission into alkyl radical and alkyl sulfonyl cation was previously suggested by the same authors for the radiolysis of poly(olefin sulfone)s in the solid state and was confirmed by scavenger studies . Seavengers are ineffeetive in erystalline solids such as dialkyl sulfones and hence eould not be used in this study. 

Tritiated water in solution has been used in radiolysis scavenger studies are rare, but a few are known (Appleby and Gagnon, 1971 Lemaire and Ferradini, 1972). Electrons from the /3-decay of tritium have a broad spectrum between 0 and 18 KeV, with a peak at 5.5 KeV. Over this distribution, the energy is partitioned between spurs, blobs, and short tracks as 0.2 5 0.08 0.67, which... 

Initially, TNM solutions tended to give a somewhat larger yield, but now it is believed that the yields obtained by absorption spectroscopy and by chemical scavenging studies give identical values under ideal conditions. 

Although the time dependence of G(e q) and G( OH) can be obtained by direct observation using pulse radiolysis, as well as by adding scavengers, the evolution of the yields of H", H2, and H2O2 can only be derived from scavenger studies. 

The time dependence of the yield of hydrated electrons during the chemical stage of water radiolysis (ca. 10 to 10 sec) can be obtained from scavenging studies and by direct observation using pulse radiolysis. In the former case, the scavenger dependence G(S) is converted into the time dependence using the inverse Laplace transform of F(S) to F(t), which is given by Eqs. (19) and (20) ... 

By comparison with G e, relatively few independent measurements of G( OH) have been made. In contrast to, only the relative change in G( OH) with time has been reliably measured by pulse radiolysis [51]. In practice, absolute values of G( OH) have been obtained from scavenger studies or by material balance (reaction (7)). Fig. 7 shows data for aerated solutions of formate ion [52] and hexacyanoferrate(II) [53] taken from Fig. 1 of Ref. 54. The data for formic acid, which were included by LaVerne and Pimblott [54], have been omitted here because they were obtained at low pH where the primary yields are different (see Section 3.4). The solid line shows the best fit obtained using Eqs. (16) and (17) and the broken line is the best fit when the term u[5]/2 is omitted from Eq. (17). The respective sets of parameters are a = 1.64 and 1.69 nsec, g( OH) = 2.53 and 2.50 molecules (100 eV) and G°( OH) = 4.48 and 4.86 molecules (100 eV) These values differ significantly from those obtained by LaVerne and Pimblott [54], which were a = 0.258 nsec, g(" OH) = 2.66 molecules (100 eV) and G°( OH) = 5.50 molecules (100 eV) The reason for the difference is that LaVerne and Pimblott [54] chose G°( OH) = 5.50 molecules (100 eV) ... 

The second-order rate coefficient for the above reaction was found to be (6.2 0.6) x 109 l.mole-1.sec-1 at 23 °C. Scavenger studies indicated that G(CH3COCH3)T = 1.1. The anthracene negative ion was also observed. 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

Perhaps the best qualitative and quantitative scavenger studies in these ionic polymerizations have been made with ammonia and several amines. Busier, Martin, and Williams (8) have shown that, at —78°C. the initial polymer yield from irradiated cyclopentadiene increases linearly with the ratio of [C5H6]/[NH3] over the range 1-10. This would be consistent with a mechanism in which all growing chains are terminated by reaction with ammonia, according to ... 

A recent study of the radiation-induced polymerization of nitro-ethylene (38) indicates that the predominant propagating species in this monomer is an anion. This is based on scavenger studies utilizing HBr, substantiated by a study of the electron-trapping capabilities of this monomer in irradiated 2-methyltetrahydrofuran glass (38). 

Assuming a single-ion (carbonium ion) propagation step and based on scavenger studies with ammonia and amines and on electrical conductance measurements, rate constants for the propagation reaction (ion-molecule reaction) can be estimated. These estimates are free of the correction that one has to apply (or ignore) for chemically generated ion pair ionic polymerization, but are subject to other limitations imposed by different assumptions in the treatment of the data. 

When Pt2(pop)44- is irradiated at 450 nm in the presence of DNA, strand scission occurs, and scavenger studies show that no diffusible intermediates are involved (22). Thus, scission must involve abstraction of hydrogen atoms from the sugar functionalities by the metal complex itself. Because Pt2(pop)44- is an anion, binding of the complex to DNA does not occur. This presents the possibility that Pt2(pop)44- may be a particularly sensitive probe of solvent accessibility and hence nucleic acid structure. 

Scavenging Studies. The solvent excited states produced by radiolysis of aromatic liquids could be produced directly, or formed, via charge neutralisation of solvent ions. The low oscillator strengths of the first and second excited states of benzene, toluene and p-xylene preclude direct excitation into these states. However, the third excited state could be excited with a yield as high as unity. 

Table 4 Acid Scavenger Study for Photolysis of ImM Nvoc-GGG-NBzb...

The photochemistry of benzoin ethers (a-alkoxy-a-phenyl-acetophenones) has been examined in considerable detail, recently, by quenching, sensitization (, CIDNP (, and radical scavenging studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This abscission is not retarded by conventional triplet... 

In 1971 a new variation on the transmission method was introduced by Sanche and Schulz (13). The technique incorporated the trochoidal monochromator of Stamatovlc and Schulz (14) and a modulation scheme to obtain the derivative of the electron current transmitted through a gas cell. This combination provides a relatively simple and very sensitive means of locating resonances as they appear in the total scattering cross section. In particular the energy resolution (20-50 meV), which is substantially better than that found in most trapped electron and SFg scavenger studies, is sufficient to observe the vibrational structure possessed by anions long lived enough to... 

The production of H2 in the radiolysis of water has been extensively re-examined in recent years [8], Previous studies had assumed that the main mechanism for H2 production was due to radical reactions of the hydrated electron and H atoms. Selected scavenger studies have shown that the precursor to the hydrated electron is also the precursor to H2. The majority of H2 production in the track of heavy ions is due to dissociative combination reactions between the precursor to the hydrated electron and the molecular water cation. Dissociative electron attachment reactions may also play some role in y-ray and fast electron radiolysis. The radiation chemical yield, G-value, of H2 is 0.45 molecule/100 eV at about 1 microsecond in the radiolysis of water with y-rays. This value may be different in the radiolysis of adsorbed water because of its dissociation at the surface, steric effects, or transport of energy through the interface. 

The product yield from positive-ion scavenging studies has been known for some time to be approximately proportional to the square root of the solute concentration, and recently it has been found to be well described by an empirical expression identical in form to Equation II over the concentration range 10"4 to 10"2Af (product yields 0.13 to 0.30) (5 4)—i.e.,... 

Weiss (18), Kevan, Davis, and Libby (3, 9) proposed ion-molecule reactions to explain dimer formation. Radical scavenging studies have lead Falconer and Salovey (4, 5, 12) to the conclusion that radical reactions produce at least 40% of the dimer. Other authors (17, 19) give much more importance to radical processes. 

A test of the relative appropriateness of the curves in Figure 2 is the value of Gfi that each predicts. It appears that G(e soiv)o 4.0 in ethyl alcohol (16), so the predicted values of Gfi are 1.5 for Curve 1, 1.4 for Curve 2, 0.9 for 3 and 0.5 for 4. Many workers have experimentally estimated G/f = 1.0 in ethyl alcohol, and the results of a recent, more complete, scavenger study are consistent with a value of Gfi = 1.5 (16). It may therefore be concluded that the lifetime spectrum lies above Curve 3, and may be in the vicinity of Curve 2. 

M. The amount formed was nonlinear in nitrate concentration but agreed well with Equation B. In itself, this strongly indicated that it was not produced by an OH radical reaction (Reaction 2), and this was confirmed by scavenging studies. However, these studies did show that it has a precursor—i.e., it was not itself a primary species of the direct effect, formed by a reaction such as... 

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