Nuclear Recoil Method

The majority of the information on silicon atom reactions is derived from studies involving the formation of Si yia nuclear transformation.Silicon-31 atoms have been generated by the following two nuclear processes (i) and (ii) Si( ,y) Si. Of the two, the first one is the most fre- 

For the Si( ,y) Si reaction, silicon atoms with 75-750-eV recoil energy are formed. Owing to the low recoil energy, both the bond rupture in the transmuted precursor molecule and the charge neutralization of Si ions may be incomplete. However, this process has the advantage that a phosphorus-containing precursor is not required to be present in the reaction mixture. 

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Unusual Compounds Synthesized via Nuclear Recoil Methods... 

Various unusual intermediates and compounds have been formed via the nuclear recoil methods. The unusual intermediates thus formed include free atoms with very high translational energies such as Si, and Ge ... 

The term unusual compounds is defined here as species that either cannot be formed or have not been formed using conventional synthetic methods. It also applies to species that were first synthesized via the nuclear recoil methods, even if they were subsequently prepared by other means. 

Atoms. Strictly speaking, all the atoms formed via the nuclear recoil methods are unusual species because they all possess kinetic energies far in excess of those that could be formed by conventional chemical means (5-12). On the qualitative side, such hot atoms will open various reaction channels that are not possible for their lower energy counterparts. On the quantitative side, the cross sections of various reactions for the hot atoms are normally very different from those for the thermal atoms. An additional unique feature of the atoms formed via nuclear recoil techniques is that they invariably include radioactive isotopes and, therefore, they are capable of functioning as tracers for their subsequent interactions. 

Carbenes and Silylenes. Carbenes such as CTF and CTCl were formed in recoil tritium systems (35-39), while carbenes such as CH F, CH C1, and CF F were formed in recoil halogen atom studies (40-44). Among them the truly unusual ones are the monofluorocarbenes, CTF and CH F, because their chemical interactions were first studied by these nuclear recoil methods (36 37y40y41,45). The details of such studies will be described in a later section. 

The formation of carbenes via the nuclear recoil method is illustrated below for CF F (40). 

Silylenes such as SiH2, SiF2, and SiHF were formed in recoil Si systems (13-21). Monomeric SiF2 is uniquely formed in these systems and react in the monomeric form (15,18,20) in contrast to the situation in which they are prepared by the high-temperature disproportion method (46,47). In the latter case, dimerization of SiF2 always predominates. The formation of SiHF via the nuclear recoil method is also observed. The details of these studies will be described later. 

There are two types of unusual compoimds that can be formed via nuclear recoil methods. The standard type includes the truly novel... 

From this point on, a collection of several cases in which unusual compoimds were synthesized via the nuclear recoil methods will be presented. They are arranged according to the valences of the recoil atoms involved. 

Monofluorocarbene, a reaction intermediate whose chemistry was first studied by using the nuclear recoil methods as a means of formation. 

Table II. Calculated Results for Tbermal Rate Constant Ratios Determined by the Moderated Nuclear Recoil Method in Model Systems...

In well-designed experiments the mean hot atom lifetime is much shorter than the mean thermal reactive lifetime. The MNR technique thus oflFers good utility for precise equilibrium kinetics studies wdth the radioactive atoms and unstable radicals produced using nuclear recoil methods. Small residual nonthermal reaction yields are invariably observed in recoil experiments, but these have no direct bearing on the validity of the MNR equilibrium hypothesis. 

Besides the nuclear recoil methods, the formation of silylenes from the reactions of elemental silicon with various halides as described in Section III. 6 can also be viewed as abstraction reactions. Thus equation (12) involves F-abstraction to give SiF2 " equations (13) and (14) involve Cl-abstraction to give SiCl2 and equation (15) involves 0-abstraction to give SiO." ... 

Weyenberg, but most of the recent studies were carried out mainly by Tang and coworkers and by Caspar and coworkers using hot Si atoms formed by the nuclear recoil method, and by Ring and coworkers with the recirculating flow pyrolysis method. 

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