Intermediates recoil reaction

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. 

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... 

Reactions of the recoil C1] with several olefins have been studied, including ethylene, propylene, cyclopentene, and cfs-butene-2, as well as with several paraffins. The type of products observed indicated the existence of several general modes of interaction, such as CH bond insertion, interactions with CC double bonds, formation of methylene-C11. The most important single product in all systems is acetylene, presumably formed by CH insertion and subsequent decomposition of the intermediate. Direct interaction with double bonds is shown by the fact that, for example, in the case of propylene, yields of stable carbon atom addition products were significantly higher than in the case of propane. The same was true for ethylene and ethane. 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

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. 

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). 

Another example involves the soxurce of acetylene as a major product from the reaction of recoiling carbon atoms and cyclopropane (18). MacKay and Wolfgang attributed the formation of acetylene from carbon-atom reactions with alkanes to an insertion into a C—bond followed by fragmentation of the intermediate carbene (18). The hi yield of labeled acetylene from cyclopropane was in accord with this mechanism, since the intermediate cyclopropylmethylene was believed to cleave into two stable molecules ethylene in addition to acetylene. 

Recently these workers have also foimd evidence for formation of chloromethyne, CCl, in reactions of alkyl chlorides with recoiling carbon atoms (6). Chloromethyne, like the fluoromethyne earlier implicated as a reactive intermediate by Wolf (37) and Wolfgang (38), is a species whose reactivity has not been studied in conventional kinetic experiments. 

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 ... 

It is likely that the enormous sensitivity of laser spectroscopy (109) will in the near future make possible the detection of reactive intermediates as well as the reactants and products of atomic reactions. It has been estimated that 10 molecules/cm can be detected by laser absorption spectroscopy under realistic laboratory conditions for a moderately strong electronic transition in the visible region of the spectrum (IIO). If this sensitivity can be attained, then even the direct detection of recoil species appears to be possible. 

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