Recoil atoms, thermalized, reactions

The thermalization of energetic recoil atoms in excess moderator is a useful tool to measure kinetic parameters for abstraction, substitution, and addition reactions. For thermal experiments, the bulk (>90%) of the sample must consist of a compound that is (1) inert for hot and thermal reactions with the recoil atom and (2) able to supply the radioactive atom. For example, Ne, CF4, C2F6, SF6 + Ar, CF2CI2, and CF3C1 meet these requirements for radioactive recoil F and Cl atoms. 

After the recoil atoms have come to rest, subsequent reactions are stopped. Further reactions including recombination of reactive atoms or molecular fragments are possible after diffusion of the reactive species or after dissolution of the solid. Diffusion can be enhanced by increasing the temperature (thermal annealing) or by iiTadiation with y rays or electrons (radiation annealing). Dissolution may lead to recombination, reaction with other species or reaction with the solvent. Due to these processes secondary retention may increase or decrease. As an example, the retention of in the form of sulfate after neutron irradiation of ammonium sulfate and dissolution is plotted in Fig. 9.13 as a function of the time of thermal annealing at... 

Although we have not distinguished between ionic and molecular crystals in the above discussion, for the reason that the latter also show typical thermal annealing effects, it is possible that their annealing reactions may be discovered to arise from quite different mechanisms. If we take the ratio Ta/Tm as a measure of the tendency of a crystal to anneal recoil atoms, where Ta is the absolute temperature at which an-... 

The stereochemistry of certain thermal reactions involving atom addition processes and resultant radical intermediates can also be studied conveniently by recoil techniques. In this type of experiment the recoil atom used to initiate the process must be thermalized before reaction, by diluting the system with a large concentration of nonreactive bath molecules. Two recently reported results on geometric isomerization serve as examples of the type of processes that can be studied (55,56). Both involve recoil chlorine atom reactions with unsaturated substrates. 

The enhanced velocities possessed by hot atoms dramatically shorten the time scales associated with nonthermal collisions. Approximate recoil F mean thermalization times, , corresponding to simulated MNR conditions have been calculated (4) using the local equilibrium steady-state hot atom kinetic theory (21,22,41). The values can be compared with the mean reactive lifetimes for thermal Reaction 5, , obtained from absolute and MNR sample composition data. At readily accessible moderator concentrations, the were shown to be tenfold or more larger than the , even for very reactive R species. A calculated [/] ratio of 160 was obtained for C2H6 present at 5.0 X lO" mol fraction in C2F6, showing that hot atom moderation is effectively completed prior to the onset of Reaction 5 in this system. 

In general the reaction between two species will have a variety of alternative channels leading to different products, each characterized by a threshold and energy-dependent cross-section. Several products corresponding to these alternative pathways are observed in experiments with nuclear recoil atoms, whereas thermal... 

When comparing recoil atom reaction yields of (n,y) and (n,2n) reactions, there is a distinct difference in the thermal reaction yield, whereas little if any difference can be observed in the hot reaction yield. Note that an (n,2n) reaction provides recoil energy 10 -10 times larger than that an (n,y) reaction. Probably this result indicates a difference of thermal radical concentration in the cages formed along the track of the recoil atom. As hot reactions occur at the end of the recoil track after slowing down and is not affected by recoil energy so much. This is shown in Fig, 24,14 for the Br recoils produced by (n,y) and (n,2n) reactions (Cole et al. 1966). 

If the sulphur in a thiol cannot be rendered radioactive itself, it might be exchanged for thermally activated radioactive S atoms. For instance, the sulphur atoms of mercaptobenzothiazole exchange with S recoil atoms generated in situ by the nuclear reactions, Cl(n,p) S (where CgHgCls is used as the Cl source) or S(n,y) S where elemental sulphur... 

Iyer, R. S., Ph. D. Thesis, University of California, Irvine, 1974. This thesis, entitled "Hot and Thermal Reactions of Atomic f in the Gas Phase , contains the most complete description of F recoil chemistry as practiced by Rowland and coworkers. 

Later sections describe the modeling procedure and results obtained for nuclear recoil atoms reacting with pure (70) and inert gas moderated (71) H2. These reaction systems have been chosen for study because of the availability of quasl-classical trajectory reaction cross sections (51) together with results from thermal (21.72) and nonthermal ( 7 nuclestr recoil experiments. 

The hot-atom chemistry of phosphorus in sulphur and phosphorus compounds, reactor-irradiated in benzene solutions, indicates that a wide range of products is produced. - In one case, greater oxidation of the recoil P is observed in dilute solutions, whereas in the other case thermal reactions of P with carbon atoms dominate for low concentrations but hot reactions seem more important in the higher concentration range. A detailed reaction sequence is given for the phenyl-phosphorus compounds. ... 

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