Excitation decomposition

B. Excitation-Decomposition in Svbstitution Hoflf and Rowland (1957), in studying the reactions of tritium recoils with methanol, ethanol, and acetone, suggested the same reactions in order to account for most of their observed products in the liquid phase. In addition, they postulated an excitation-decomposition reaction as given in eqs. (10) and (11) in order to explain the formation of labeled acetaldehyde from ethanol. 

The excitation-decomposition reaction can also occur in alkanes. Lee et al. (1960b) showed that the yield of labeled propylene from cyclopropane increased with decreasing cyclopropane pressure. 

Recent work on cyolobutane (Lee and Rowland, 1963) supports and extends these arguments. The excitation-decomposition reaction has also received support from recent work on the efiFect of various additives and of phase on the radiochemical yields of benzene, cyclohexane and a variety of cyolohexenes (Avdonina, 1962 Pozdeev et al., 1962a, b). As we shall see later, the implications of this residual energy in an excited intermediate has given rise to a controversy on the details of the mechanisms of the reactions involved in product formation. 

Heavy Atom or Group Substitution RX + Tt - RT + X-Excitation-Decomposition... 

Urch and Wolfgang (1961a) have considered excitation-decomposition as an explanation for the observed labeled-radical formation but prefer a mechanism involving direct displacement of two hydrogen atoms, or an alkyl group and a hydrogen atom by the recoiling tritium atom, namely... 

The methylene produced is almost certainly excited and it probably also has excess kinetic energy. The insertion-excitation-decomposition reaction (46) again seems to be a reasonable mode of formation, but a two-hydrogen transfer (possibly a four-center reaction as in eq. 47), an... 

Hot methyl radicals may be formed by the insertion-excitation-decomposition reaction. It is, however, difficult to say whether or not we see any reactions characteristic of specifically hot methyl radicals. Some evidence for the presence of methyl radicals has been obtained by... 

In this case both gas evolution occurs and additional sites are generated. Clearly, many different reactions of this type could be proposed. To reiterate, it will be assumed that once a singly excited decomposition site has acquired a second excitation it may enter into a decomposition reaction or it may produce an additional decomposition site without destroying itself. 

EXCITED DECOMPOSITION SITES PRODUCED BY EXPENDABLE GENERATORS... 

The same type of reasoning that led to the assumption that excited decomposition sites may generate additional decomposition sites points to the possibility that excited decomposition sites may be generated from precursors that are used up in the site-creation process. For example, consider a crystal with a substitutional OH impurity. It is conceivable that the OH ion will interact with a BrOj ion to form BrO by a photolytic process, e.g.,... 

It is reasonable to assume that the rate at which precursors are converted into excited decomposition sites is proportional to the product of the light intensity and the concentration of unconverted precursors. Thus at time t... 

In the derivation given below it will be assumed that the excited decomposition sites generated from precursors are the same as those generated during decomposition. Actually this assumption is not necessary. It is conceivable that two or more independent processes are operating. In fact two or more different types of precursors may be involved and each type may or may not be related to the process responsible for the steady-state behavior. 

The data obtained for unirradiated NaBr03 requires that P be slightly negative. With irradiated NaBr03 the constant P was considerably more negative. Thus this data supports the concept of excited decomposition-site generation. 

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