Effect different electron scavengers

Table II. Effect of Different Electron Scavengers on H Atom Yields in 7-lrradiated Frozen Aqueous Solutions at 77° K.

Von Bunau and Kuhnert studied the y-radiolysis of cyclopropane both in direct radiolysis and in Kr- and Xe-sensitized radiolysis. The sensitized radiolysis was studied earlier by Smith and coworkers who showed that the addition of a rare gas increases the percent of cyclopropane consumed for constant irradiation time. The sensitization increases with increasing pressure of the rare gas for constant pressure of cyclopropane. The degree of rare gas sensitization was found, as expected, to be in the order of its energy absorption characteristics (electron density), i.e. Xe > Kr > Ar. Von Bunau and Kuhnert studied the effect of electron scavenger (SFe) and radical scavenger (NO) on the yield of the various products in direct radiolysis and in the sensitized radiolysis. They explained the different effects of the additives in the three systems by assuming the formation of two reactive species, an excited cyclopropane molecule and an excited cyclopropane ion, whose yields are different in the three systems as can be seen in Table 3. 

In the early 1980s, one of the authors of this chapter began to study argon matrix isolation of radical cations235 by applying the radiolytic techniques elaborated by Hamill and Shida. A central factor was the addition of an electron scavenger to the matrix which was expected to increase the yield of radical cations and the selectivity of the method. For practical reasons, X-rays replaced y-rays as a radiolytic source and argon was chosen as a matrix material because of its substantial cross section for interaction with keV photons (which presumably effect resonant core ionization of Ar). Due to the temporal separation of the process of matrix isolation of the neutral molecules and their ionization, it was possible to obtain difference spectra which show exclusively the bands of the radical cations. 

The term ionization may refer to different processes depending on the context. For radiation effects in the gas phase, it usually implies the removal of the least bound electron to infinity. Such a theoretical definition is not feasible in the condensed phase and it is necessary use a heuristic or operational procedure. Thus, in liquid hydrocarbons, one may use the electron scavenging reaction or a conductivity current to quantify the electrons liberated from molecules. It has only been possible to extrapolate the conductivity current at a low irradiation dose and at a relatively low external field to saturation in the cases of liquefied... 

In this short review it is perhaps sufficient to indicate that in organic materials the primary processes and products of alpha and gamma radiation are usually fairly similar. However, major differences occur in the presence of so-called protecting additives, which are far less effective in the case of alphas. This is to be expected from the high concentration of radicals built up in the dense alpha track, far higher than in gamma or electron spurs. From the data on the effect of radical scavengers on the final product, the diameter of the alpha track can be estimated. 

Investigation of replacement of the 5-methoxy group by substituents with different electronic and lipophilic properties and methylation of the indole nitrogen or its replacement by a sulfur atom was evidence for the shift of the 5-methoxy group to the 4-position of the indole nucleus led to the most active radical scavenger but much less effective as a cytoprotectant [135]. 5-alkoxy-2-(N-acylaminoethyl)indole (Fig. 15) appeared as the key feature to confer both antioxidant and cytoprotective activity to the structure. Antioxidant activity seems essential for cytoprotection, but it is not sufficient, and there is no statistically significant correlation between the two types of activity. 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

The effect of the electron scavenger, CS2, is explained as due to preventing the neutralization of the cations in the system and subsequent reaction of in the state with cyclopropane or with the radiolytic products of the substrate. In the presence of CS2 the major product is methyl iodide similarly to the major product of Br found by De Jong and coworkers . However the analog to the main product in the case of I2 or O2 as scavenger, namely, n-propyl bromide was not found at all by De Jong and co workers. They found the second and third major products to be allyl bromide and cyclopropyl bromide while Certout and Schleifer did not find the respective iodides at all. It is not clear if this is due to the difference between Br and I or due to the different amount of radiolysis induced by each of them. 

The effect of benzene in the Febetron irradiations is markedly different from that of the electron scavengers. The difference between the cyclohexene and bicyclohexyl yields is much smaller and implies that benzene decreases the yield of thermal hydrogen atoms. Thus, benzene is not acting as an electron scavenger as has been suggested (17). 

Hayon endeavored to prove that Ge is equal to 2.3 at low concentrations of electron scavenger, where keaq+s[S] < 107, and at higher concentrations follows G(e ) = 2.3 + f(ke +s[S]). The function (/(fc6 q+s[S] ) is given as an experimental curve (47), Hayon believes that the observed spread in Ge is caused only by the different values of fceaq+s[S] in the systems which were studied and proposes a theory (45). However, on checking his data, it can be shown that in these systems, an appreciable—if not the entire—effect of the decrease in Geaq as the concentration of S decreases, is caused by eaq competition with the irradiation products formed. 

By modelling the TR MFE fluorescence decay curves in low-permittivity solvents using new simulation techniques, it has been shown that the spin-lattice relaxation time can be significantly decreased by this cross-combination effect, depending on the number of radical pairs in the spur. It is hypothesised that this effect acts as an extra source of spin relaxation in hydrocarbons where the recombination fluorescence is slowed down by an electron scavenger, such as hexafluorobenzene. It has also been hypothesised that different spin-lattice relaxation times are to be expected for photolytic and radiolytic pairs. 

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