Saadia Amiel

Moderation Processes in Hot Atom Chemistry

The various moderation processes involved in hot atom chemistry are discussed. Two main types are considered: (i) moderation processes of hot atoms before entering into a chemical combination, (ii) moderation processes of hot molecules formed with various degrees of translational and internal energies. For the first case, it is proved rigorously that under certain conditions the ratio αm1 / αm2 related to two moderators is equal to (1 − β1) / (1 − β2,) where αmi, i = 1, 2, is the average energy loss per collision and βi = [(Mi − m) / (Mi + m)]2, with m and Mi the masses of the projectile and two different moderators. For the second case, the medium dependence of the survival probability, or alternatively of the decomposition probability, is derived. As an application of this treatment the reaction CH4+80Br→CH380Br+H, studied experimentally in different noble gases, is discussed.

On the Energy Losses of Hot Atoms Colliding with Different Molecules

Experimental α values for hot halogen atoms colliding with methyl halides disagree with the calculated values, assuming elastic collisions. This is attributed to the asymmetry of the target molecules which either render the interaction inelastic or are governed by a different scattering potential.

A CRITERION FOR SELECTIVITY IN RECOIL LABELING

and [CO(NH3)5C1]C12. Samples irradiated with pulses showed unexpectedly less thermal annealing, bu t low specific activities severely limit the scope and accuracy of studies on this point. Theories which propose t h a t the recoiling a tom is torn from all, or most, of its a t tached atoms are supported by the occurrence of very low retentions (e.g. near 1 percent for Co in Vitamin B12 [29], and [Co(en)3]Cl3 irradiated for 0.5 hours, Table 7). The widespread thermal and radiation annealing which rebuilds complex species exactly like the parent compound is harder to comprehend on this theory, although surrounding groups might favor the f ragments recombining to the parent species. In connection with hot-atom annealing in organic solids, it should be noted t ha t NMR [30] studies have shown tha t molecular reorientation takes place in stages, involving rotat ion about one or more axes, and finally translation or self-diffusion of whole molecules through the lattice. All this occurs well below the melting point, and thus diffusive recombination may be an important factor in determining product yields from recoil effects. Y O S H I H A E A and H A H B O T T L E [1] have outlined with great detail an electron donor-acceptor mechanism which accounts for retention values and for thermal and radiation annealing reactions in neutron irradiated [Co(NH3)e]Br3. The Co(II) found on analysis of dissolved complex arises from reaction with water of the Co(NH3)e++ species which is formed during irradiation. Annealing by heat involves the Co(NH3)6+++ being reformed. Strong support for this theory comes from the observation t h a t analyses of most Co(III) complexes yield as major products only the parent ion

Reactions of Hot (n, γ)‐Produced 80Br with CH3Br. Comparison with the Reactions between Hot 38Cl and CH3Cl, and Calculation of the Excitation Functions

The reactions of hot (n, γ)‐produced 17.6 m 80Br with methyl bromide were studied in the presence of He and Kr as moderators. The survival probability of the reaction products was found to be the same in both moderators, suggesting that no observable decomposition took place upon moderation (within a 15% error). The reactivity integral for halogen displacement in the reaction was found to be equal to that in the system 38Cl + CH3Cl, which seems reasonable since the activation energies and steric factors are almost equal. The ratio of hydrogen displacement to halogen displacement was smaller in the case of 80Br than in that of 38Cl, as can be predicted on the basis of the translational inertial factor. From the reactivity integrals and the activation energies the excitation functions for halogen displacement were calculated by the energy‐dependent hard potential model of Baer and Amiel.

Survival Probability of Products of Hot Reactions of (n, γ)‐Produced 38Cl with CH3Cl in the Gas Phase

The hot reactions of (n, γ)‐produced 38Cl with CH3Cl have been studied in the presence of three noblegas moderators: He, Ar, and Kr, using Br2 as a scavenger. The main purpose of this work was to study the extent of decomposition of the newly formed products upon collision with Ar and Kr. Applying the method of Baer and Amiel, it was established that the average survival probability of the various hot products in Ar and Kr is 100%, within the experimental error. Kinetic parameters for the three hot products, CH338Cl, CH2Cl38Cl, and CH238ClBr were determined as well.