Recoil chemistry

Study of the recoil chemistry of organometallic compounds for its own sake, began really in 1955 with the publication of a study (56) by Mad-dock and Sutin on neutron activation of triphenylarsene. Since this time, most of the published work has been focussed on those radioactive atoms which did not permanently escape their ligands. Thus, in one way or another, they end up in molecular form. It is with these that this review is largely concerned. 

The early work of Sutin and Dodson (85) on neutron-irradiated ferrocene exemplifies the results and problems of recoil chemistry. After dissolving their samples in hexane and extracting with aqueous solutions they isolated, after further purification, radioactive FeCp2 and a species which emerged as ionic iron(III). Adsorbed on the walls of the glass vessels remained another species soluble in acetone which accounted for up to 50-60% of the radioactive iron. This species has not yet been identified. The FeCp2 activity accounted for some 10-12% of the Fe, which increased on standing several weeks at room temperature or 2-3 days at 110° C, as is shown in Table III. 

Cl (12), I (13), N (14), Si (9), other polyvalent atoms (15), and of muonium (16). Reviews have also been published on the reactions of recoil atoms with arenes (17), (halo)ethylenes (18), and (halo)-methanes (19). The capture of ir in hydrogenated species is sometimes considered as a part of recoil chemistry (20), and so also are reactions of species formed after decay of multiply labeled (T, 14C) molecules (21-23), for example,... 

Because energies of the order of 1 eV to 1 MeV are transmitted to the atoms by nuclear reactions, corresponding to temperature equivalents of the order of 10" to 10 ° K, these atoms are called hot atoms and their chemistry is called hot atom chemistry , or recoil chemistry if the recoil effects are considered. 

D. S. Urch, Nuclear Recoil Chemistry in Gases and Liquids, in Radiochemistry, Vol. 2, Specialist Periodical Reports, The Chemical Society, London, 1975 P. Glentworth, A. Nath, Recoil Chemistry of Solids, in Radiochemistry, Vol. 2, Specialist Periodical Reports, The Chemical Society, London, 1975 T. A. Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1975... 

Recoil Chemistry and Mechanistic Studies with Polyvalent Atoms... 

While such a reaction is in principle possible, it is expected to occur with very low eflBciency. In the recoil chemistry of polyvalent atoms, chemically stable reaction products are likely to be formed in multistep reaction sequences rather than in single reactive collisions. This implies the formation of reactive intermediates, which is the feature that most clearly distinguishes the study of polyvalent atoms from the investigation of monovalent atoms. 

Formation of Reactive Intermediates The Distinctive Feature of Polyvalent Atom Recoil Chemistry. The possibility that reactions of recoiling silicon and germanium atoms would produce novel reactive intermediates originally attracted the author of this chapter to hot atom chemistry. Several such intermediates are shown in a reaction scheme for recoiling silicon atoms that has been under investigation in our laboratory for several years (3,4). 

The most important mechanistic clue for any chemical reaction is the structure of the reaction products, and a great strength of the recoil technique is that there is no more convenient or effective way to determine what products are formed from a polyvalent atom and a chosen reaction substrate than by examining its recoil chemistry. The harshness of the conditions required to liberate polyvalent atoms chemically severely limits the study of their reactions by other methods (vide infra). 

These examples point to the symbiotic relationship between polyvalent atom recoil chemistry and the study of chemically generated reactive intermediates. Hot atom experiments identify products that suggest the intermediacy and delineate the reactivity of exotic species such as cycloproplyidene, methyne, and silylene. Thus, hot atom experiments have stimulated the search for other ways of generating these species, and in turn these chemical experiments can help interpret the hot atom experiments. 

Early in the study of silicon atom recoil chemistry, ethylene was used as a scavenger to diflFerentiate between silyl radicals, SiHs, and silylene, SiH2, as the intermediate responsible for the formation of silane and disilane in phosphine-silane systems (21). 

CASPAR Recoil Chemistry arid Mechanistic Studies... 

Many kinds of metals have been studied by evaporation-cocondensation (120y 124-126) but thus far, little recoil chemistry has been carried out. The mechanistic interest and commercial importance of olefin metathesis reactions (127) suggests... 

Conclusion—A Positive View of Polyvalent Atom Recoil Chemistry... 

In conclusion, the author would like to reiterate his belief that several factors argue for more rather than less intensive activity in the field of polyvalent atom recoil chemistry. These factors include the sharpening of our perceptions about the problems that face us and about what can be achieved, as well as the availability of new tools to facilitate our work. 

A natural area of interest for chemists studying high-energy polyvalent atoms is cosmochemistry. With growing indications that high-energy reactions of carbon and silicon atoms are important processes in the formation of interstellar grains, it can be predicted with confidence that the recoil chemistry of polyvalent atoms will claim the attention of cosmochemists and laboratory astrophysicists (129-133). 

The recoil chemistry of tritium is perhaps one of the best understood in terms of a clear idea of the product spectrum under various conditions. Realistic models have been tested and correlations with theoretical approaches have been made. Thermal-neutron absorption reactions of lithium-6 and helium-3 have been used to produce hot tritium atoms. Some physical parameters are listed in Table 2. 

Harbottle, G. (1962). In Radioisotopes in the Field of Recoil Chemistry , International Atomic Energy Agency, Vienna, p. 375. 

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