Silicon recoiling atoms

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. 

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... 

Unusual compounds can be formed more readily in multivalent recoil atom systems than in monovalent recoil atom systems. In the latter case, unusual compounds are rarely formed during the primary abstraction and substitution processes. The only likely >vay for their formation to occur is through the reactions of reactive intermediates formed via secondary decomposition processes. Typical examples are the formation of CTF and fluorocyclopropanes described previously. On the other hand, multivalent recoil atoms and the reactive intermediates resulting from their abstraction or insertion reactions can frequently react to give unusual products that wrould not be easily formed by other means. Therefore, it is not surprising that various unusual compoimds are detected in recoil carbon and recofi silicon systems. 

In collisions where =M2 at 0 = 180° tlie incident particle is at rest after the collision, with all the energy transferred to the target atom. For 2.0 MeV helium ions colliding with silicon the recoil energy 2 is 0.88 MeV and from palladium is 0.28 MeV. 

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). 

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). 

However, we have found several products that are consistent with some participation by ion-molecule reactions in product formation. The reactions of recoiling silicon atoms in mixtures of phosphine and tri-methylsilane produce some provocative minor products (26). 

As an example, we have recently been comparing the reactivity of the intermediates derived from recoiling silicon atoms with the reactivity of thermally generated silylene SiH2 toward silane and methylsilane. This pair of substrates was chosen because Paquin and Ring have studied the temperature dependence of the relative rate of insertion of thermally generated silylene into the Si—bonds of these molecules (33). 

We were faced with a dilemma six years ago when the major product from reactions of recoiling silicon atoms with butadiene turned out to be one for which no authentic sample could be synthesized (4,39). 

We have recently found evidence for the formation of five-membered rings in the interaction of recoiling silicon atoms with butadiene in a long-sought spirononadiene product (4), which can be compared with the authentic compoimd (42). 

Figure 2, Product yields from reactions of recoiling silicon atoms in phosphine-butadiene mixtures as function of mole fraction butadiene at constant total pressure (1000 Torr) (4)...

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). 

Formation of Other Unusual Silicon Containing Compounds from Recoil Si Atom Rteactions... 

Phosphorus. Silicon. The reaction of PH3 with recoil P atoms (from the irradiation of PH3 with thermal neutrons) yields considerable quantities of PH3 via H abstraction details are given in Section 1.3.1.5.1.5, p. 215. The measured relative efficiency of the H abstraction in irradiated PH3 and PF3 was used to consider the likely reaction mechanism in [65]. A reaction of PH3 (from adding Ca3P2 to the solution) with recoil was also considered in order to explain the formation of 30 10% of when KCl crystals [66] or KCl-CaCl2 mixed crystals [67, 68] were irradiated with protons (635 to 660 MeV) and dissolved in water. The H abstraction between and PH3 might have occurred at the surface of the dissolving crystals however, a formation of PH by secondary reactions was also considered to be conceivable [66 to 68]. 

The ToF recoil spectrometer showing the two timing detectors separated by a flight tube is shown in Fig. 3.16. Recoiled target atoms enter the tube from the sample chamber, travel down the tube and finally into a silicon SBD (particle detector) at its end. 

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