Recoil tritium

With fluoro-compounds, there has been work on the radiolysis of fluorinated methanes , hexafluoroethane , tetrafluoroethylene - , fluorobenzene , cyclic fluorocarbons , trifluoroiodomethane - - , and perfluorocar-bons . Most studies have involved the use of y-radiation, but Yi-Noo Tang and Rowland have discussed the recoil tritium-excitation of ethyl fluoride (and ethyl chloride), providing evidence for 1,2-HX elimination. 

Radiolysis studies on chloro-compounds have included those on chlorinated methanes - - - , 1,2-dichloroethane , chlorotriphenylmethane - , chloroolefins , benzyl chloride - , a-chlorotoluene , and chloroacetic acids . Recoil tritium reactions with dichloromethane and chloroethane have also been reported. 

The gas-phase reactions of recoil tritium atoms (generated in the He(n,p) H nuclear reaction) with gaseous arsine, AsHj, at high pressure has been studied in mixtures either with or without the scavenger ethylene. In the absence of scavenger about 67.3% of the tritium has been found as H H(HT) and about 31.8% as ASH2T. 

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. 

E. Dovble-Bond Addition Reactions Reactions of recoil tritium atoms in alkenes have been studied and have brought to light an additional reaction type. The energetic tritium atom can add to the double bond. Urch and Wolfgang (1959) have substantiated this hot addition by studying scavenger and moderator effects in alkenes. Decomposition of the hot radical once formed follows a pattern similar to that of the decomposition of thermally excited free radicals, namely... 

The reactions of organic molecules with recoiling tritium atoms are summarized in Table 6. 

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... 

A. Production of Energetic Carbon Atoms Many of the same difficulties involving work with recoiling tritium atoms also beset work with recoiling carbon atoms. Information... 

Discrepancies may arise in comparing theoretical and experimental yields if the hot product initially formed does not survive thermalization. Collisicmal dissociation of excited HT is believed to account for anomalies in the effects of differmt moderators on HT yields from the reaction of recoil tritium with alkanes. Despite these limitations, the formalian of the kin c theory is employed as a useful, if approximate, means of summarizing data in terms of the parameters a and Z. - ... 

The only experimental infarmation on the stereochonistry of substitution relates to methane with fiiree heavy substituents, when reaction with recoil tritium takes place with retention of configuration. However, C3h>u et al. have shown that in the reaction of T with CH3F at energies below 3.2 eV, replacement of F by T is more probable than T-for-H substitution, and have argued that this points to substitution with inversion when the atoms other than that being replaced are all H, whose mobility facilitates the process. 

A barrier height of 51 kJ/mol for the identity reaction H3P+H H + H2PH was estimated from the empirical Morse function of the H2P-H bond and geometrical parameters of the activated complex [14]. The reaction of recoil tritium atoms with PH3 yields PH2T and HT, see Section 1.3.1.5.1.5, p. 215. 

O2 scavenger effect on recoil tritium reaction with i-C4Hio... 

It is interesting to see the pressure dependence of hot tritium reaction in cyclobutane C4H8. Recoil tritium attacks this molecule to give tritiated cyclobutane (substitution reaction) in an excited state, which either stabilizes by collision with a third body or decomposes into two molecules of ethylene. This is shown in the following scheme ... 

Similar experiments nsing various organic substances give mean excitation energy in recoil tritium reaction. O Table 24.4 lists selected cases for such studies (Tominaga and Tachikawa 1981). Note that the recoil reaction of for fluorine gives the largest values around 10 eV. 

For the H substitution reaction of recoil tritium, binding energy also plays an important role. In the T + CH3X reaction, where X = SH, NH2, Cl, OH, and F, the yield of substituted CH2TX increases with binding energy of C-X. This is due to weakening of the C-H bond by introduction of X. 

In Table 24.5 (Wolfgang 1965 Seewald et al. 1968 Small and Rowland 1968 Chou and Rowland 1971 Tachikawa and Yanai 1972 Saeki and Tachikawa 1973 Numakura et al. 1973 Numakura and Tachikawa 1973) several examples of the isotope effect experimentally obtained are listed. The effect ranges between 1.25 and 1.41 for recoil tritium substitution reaction, but is much larger (3.1-7.2) for photochemically produced tritium (2.8 eV) in hydrocarbon mixture gases. 

Recoil tritium produced by Li(n,a)Treaction in enriched LiF reacted with H2-D2 mixture at 4.2 K giving HT and DT as hydrogen abstraction products. From the isotope effect of the rate constants it was deduced that more than 90% of recoil T atoms react with hydrogen in the hot... 

Substitution of thermal and recoil tritium atoms into cyclo-octene shows differences from that into cyclohexene. ... 

An N2H intermediate was proposed to form during the photodecomposition of N2H2 [14] (see p. 61) and dissociation of N2H2O isomers [12]. Formation of a vibrationally excited N2T intermediate in the reaction of recoil tritium with N2 was suggested [15]. 

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