Recoil reactions

Boyd, G. E., J. W. Cobble and S. Wexler Recoil Reactions with High... 

All these values indicate that the Si-Si bond is a relatively weak bond, as is confirmed by tritium recoil reactions with Si2me6" ... 

Under suitable conditions, the recoiled reaction products can be collected on a collector plate during the Irradiation. 

Similarly, in the case of Sharman and McCallum s (74) studies of recoil reactions of carbon in anhydrous sodium carbonate it was found... 

This reaction schane is perhaps typical, both in its length and in its tentative nature, of those suggested for polyvalent atoms. It is being closely scrutinized, and parts of it are undergoing revision. However, it was the hope of making silylene SiHs via recoil reactions that lured this author into hot atom chemistry. 

Investigation of Systematics of Polyvalent Atom Recoil Reactions... 

Identification of Reactive Intermediates. Comparison of Intermediates IN Recoil Reactions with Chemically Generated Species. The reactive intermediates formed from recoiling polyvalent atoms must, for now, be identified by chemical means. Reaction substrates are chosen so that the structures of the products derived from them help us imder-stand the nature of the intermediates. For this purpose it is useful to know the reactivities of the molecules used as reaction substrates toward short-lived intermediates of known structure. The most direct way in which a reactive intermediate in a recoil reaction can be identified is to generate the same intermediate by chemical means and to compare their behavior. 

If a reactive intermediate in a polyvalent atom recoil reaction has been independently characterized, as has methylene or cyclopropyl-methylene, above, then acceptable evidence for its intermediacy in a recoil system is the observation of a product spectrum characteristic of the given intermediate. Enough difiFerent substrates must be used, however, to ascertain that all the major reactions of the suspected intermediate are obtained from the recoil system. 

In the case of species that have not been characterized in other kinds of experiments, conclusions about their presence or absence in recoil reactions may be considerably more tenuous. 

Thus the study of chemically generated silicon ions, stimulated by earlier recoil studies that raised the possibility of ionic intermediates, has provided mechanisms for their possible participation in recoil reactions. However, it is also possible that the apparent methyl group abstraction is actually the result of insertion followed by scrambling of methyl groups and hydrogens (32). 

Often the intermediates in recoil reactions behave somewhat differently from species of the same structure generated chemically. This is not surprising since tbe species formed in recoil reactions are often quite different in energy or electronic state from thermally generated species. 

Cases in Which Postulated Intermediates in Recoil Reactions Cannot Be Chemically Generated. Turning to other approaches for the mechanistic study of recoiling polyvalent atoms, among the most interesting and diflBcult cases arise when observed products and postulated intermediates in the recoil systems cannot be generated by chemical means. Then the resourcefulness of the hot atom chemist is pressed to the utmost. 

One stimulus for future effort is the ability to produce a number of different molecules via recoil reactions— unusual stable molecules as well as reactive intermediates. There is growing interest in labeled molecules both for fundamental mechanistic studies and for biomedical purposes. The finding that polyvalent recoiling atoms undergo quite specific and novel reactions ensures considerable interest in the many atoms whose... 

Meth3me. Although the reactions of substituted methyne such as EtOCOC have been extensively studied (31-33), the chemistry of unsubstituted meth)me, (CH), is essentially unknown. The first clues for its possible chemical interactions can be found in recoil reactions. For example, during the reactions of recoil atoms with ethylene in the presence of H2, the observation of C-labeled 1-pentene can be attributed to the successive interaction of with two molecules of C2H4 followed by H-abstraction (34). 

Another more direct approach has recently been used to define the excitation function for a reaction product from a recoil reaction (21). Instead of using an RRKM method for a single competitive unimolecular process, the sequential decomposition of excited CF4 was studied. The fragments formed were scavenged with CI2 to give direct yield data for each path as indicated below ... 

Recoil reactions present a highly specialized approach to certain kinetic problems that are of both fundamental and practical interest. The broad distribution of energies available provides access to both high energy and to thermal kinetic parameters. The ultratrace nature of the... 

Modeling of the recoil process provides a detailed rationale for the success of this important new approach to thermal kinetics. Initial work was designed to explore both the conditions under which best results might be obtained and the limitations anticipated as applications are broadened (42). Calculated results based on earlier methods for determining recoil reaction yields (25) and parameterized forms for cross-section functions (15,16) were used for this analysis. The cross sections used, while not precisely vahd for real chemical systems, were approximated representations for hydrogen atom abstraction and addition reac-... 

Kinetic descriptions of high energy recoil reactions have also been developed. While the results obtained are not directly relevant to conventional thermal kinetics, they do provide considerable insight into high-energy collision dynamics. The recoil process itself consists of an irreversible dispersion of energy via elastic and inelastic collisions, and... 

Early interest in stereochemistry of recoil reactions focused on developing a model for hot processes. The conventional view was that such reactions were direct in nature in the gas phase, occurring on a time scale that promoted retention of configuration. In condensed phases the picture was complicated by the expectation that the local environment caged fragments generated in the recoil process, and thus the stereochemical outcome of the reaction was either random or dependent on specific features of the nearest neighbors. 

Libby s proposals were essentially non-inechanistic. The formation of ethyl bromide by the recombination of an ethyl radical and a Br-atom was, in the strictest sense of the term, a no-mechanism reaction. Perhaps the difficulty with the early models was that they attempted to explain everything. The approach most prevalent today is to consider each recoil species separately by investigating the nature of the labeled products and the effects of various physical and chemical parameters on the yields of these products. As we shall see later, rationalization of product distribution in terms of mechanism has had considerable success. Application of known mechanisms of displacement reactions, insertion reactions, free radical reactions, etc. to recoil reactions has been quite useful. 

The desire to eliminate the nitrogen source and the necessity for reducing the radiation dose led Suryanarayana and Wolf (1958) to use the C(n,2n) C reaction in investigating the carbon recoil reaction in benzene. Previous work (Wolf et al., 1956) using benzene in admixture... 

An investigation of recoil reactions in gaseous ammonia was undertaken to see if any of the compounds of carbon could be formed without the necessity of a hquid or solid cage vide supra). Again, contrary to the predictions of the physical theories, a unique high yield of one product was observed (Yang and Wolf, 1960) ... 

The large amounts of tars usually accompanying recoil reactions in... 

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