Polyvalent atom recoil reactions

Investigation of Systematics of Polyvalent Atom Recoil Reactions... 

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. 

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). 

Anyone who imdertakes to design a polyvalent atom recoil experiment must keep clearly in mind that the observed reaction products will rqp-... 

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,... 

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. 

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. 

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). 

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. 

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. 

Carbon Monoxtoe Formation from Carbon Atoms. In certain cases quite detailed pictures of primary reactions of recoiling polyvalent atoms have been developed. "End-on attack of carbon atoms on oxygen molecules rather than attack normal to the oxygen-oxygen bond was deduced from the formation of carbon monoxide instead of carbon dioxide, even in condensed phases capable of stabilizing excited CO2 (51). 

Diatomic molecules offer important advantages that have not been fully utilized for probing the primary reactions of recoiling polyvalent atoms. The primary products of the interaction of diatomic molecules with polyvalent atoms wall, of course, be diatomic and triatomic reactive radicals whose gross structures and electronic states can, in favorable cases, be deduced from their subsequent reactions, as discussed earlier. 

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... 

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). 

Since the necessity for examining reaction sequences initiated by polyvalent recoiling atoms via their end products has been a severe experimental constraint, it may be worthwhile to repeat the prediction that high sensitivity laser absorption spectroscopy will in the near future permit direct detection of reactive intermediates in both chemical and recoil studies of atomic reactions. If this proves to be the case, then an exciting new era will have dawned in which recoil experiments may be able to answer many questions about the dynamics as well as the sys-tematics of polyvalent atomic reactions. 

Polyvalent recoiling atoms incorporate themselves into chemically stable product molecules via reaction sequences that generally contain multiple steps. The elucidation of the primary reactions of the polyvalent recoiling atoms requires the identification and study of the reactive intermediates that participate in these reaction sequences. Unusual product molecules as well as a wide variety of reactive radicals and ions are formed. There is a mutual reinforcement between recoil studies and chemical experiments that is illustrated by several detailed mechanistic studies of the chemistry of carbon and silicon atoms. 

The study of polyvalent recoiling atoms presents the opportunity of creating and studying a wide range of reactive radicals and ions at ambient temperatures in the presence of virtually any desired reaction partner. There are a number of examples of species such as methyne, CH (5) chloromethyne, CCl (6) sUylsilyene, SiHsSiH (7) and phase reaction products were obtained for the first time from recoil studies. 

Primary Reactimis of Polyvalent Recotling Atoms. We have dwelled thus far on the identification of reactive intermediates in the reactions of polyvalent recoiling atoms. Only in the cases of carbon, silicon, germanium, nitrogen, and phosphorus have complete reaction... 

A Postulated Single-step Reaction. An unusual primary reaction of a polyvalent recoiling atom, simultaneous abstraction of three hydrogen atoms by phosphorus atoms, has been suggested by Tang (68). This is a reaction without a reactive intermediate ... 

This brings us to some chemical methods for the generation of free atoms, and these have not been fully exploited in providing information useful for the interpretation of the reactions of polyvalent recoiling atoms. 

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