Reactions of Hot Tritium Atoms

The reaction of hot tritium atoms with substituted cyclopropanes... 

The charge state of the tritium while it is undergoing reactive collisions has been thoroughly discussed by El-Sayed et al. (1958). While the reactions of hot tritium are considered to be those of the neutral atom, the reader s attention is directed to the comments of Rowland et al. (1961). The tritium atom is also assumed to be in its electronic ground state at the time of reaction. 

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... 

Physico-chemical studies of hot tritium atom reactions. 842... 

In the first mechanism most of the reaction energy transforms into rotation-vibration energy of KI, while in the second mechanism the energy distribution between the products is more random. A third example of organic reaction studied dynamically is that of a hot tritium atom on a methane molecule (studied by Polanyiand collaborators on the one hand and by Bunker and collaborators "... 

This reaction was studied thermally and found to have relatively low activation energy. In the case of thermal deuterium atoms, they did not show any exchange of D for H in the cyclopropane molecules ", indicating that reaction 51 has high activation energy. The kinetic energy of the hot tritium atom was found to overcome this activation energy, as 16 % of the hot tritium produced in the presence of c-CaH is incorporated in T-cyclopropane (equation 56) ... 

The recoil chemistry of tritium is perhaps one of the best understood in terms of a clear idea of the product spectrum under various conditions. Realistic models have been tested and correlations with theoretical approaches have been made. Thermal-neutron absorption reactions of lithium-6 and helium-3 have been used to produce hot tritium atoms. Some physical parameters are listed in Table 2. 

The radiolytically produced atoms, radicals, ions and molecular fragments form the third and unwanted class of reactants in the system. They serve only to confuse and obscure the reactions of the hot tritium atom with the substrate. 

Results with inert moderators also support the view that some of the abstraction and all of the substitution reactions are hot . Helium is an effective non-reactive moderator for tritium since its mass is close to that of tritium (cf. Section II, E). As the concentration of helium in an organic system being subjected to tritium recoils is increased, the probability of a reactive encounter between a hot tritium atom and the substrate is decreased. Product distributions should shift in the direction of what might be expected from thermal tritium atoms. In the presence of helium the yield of HT (Eq. (8)) goes up and the yield of RT (eqs. (9) and (13)) goes down (cf. Estrup and Wolfgang, 1960b) in accordance with what one would predict. 

A convenient way of thinking about the physical aspects of the probability of these hot reactions is stated by Rowland et al. (1961). A simple plot of reaction probability versus kinetic energy of the tritium atom is given in Fig. 3. The shape of the curve is unknown, as are the exact values of the energies where the probabilities are essentially zero. The action of the moderator in decreasing the number of collisions necessary to thermalize the atom and thereby decreasing the number of reactive encounters is clearly expressed. 

The product yields given by the authors indicate that hot tritium atoms insert preferentially into terminal methyl groups of propane and the time between insertion and decomposition of 37 is insufficient for full equilibration of tritium within the excited ion 37. This tritium effect , that is, the preferential presence of a tritium atom in one of the two terminal groups, should stimulate tritiated methane formation by rupture of the methylene carbon-CH4 bond due to electronic rearrangements and not due only to the mass effect of tritium. Similarly, the preferential (65/80) 100% = 81.25% localization of tritium in H2 in the reaction shown in equation 70a also indicates the non-equilibrium nature of this decomposition process. 

The billiard ball atom-atom collisions favouring the easier replacement of D do not explain the observed experimental deuterium isotope effects in substitution reactions with hot tritium. The linear structures, T— H—R , of the transition states have also been rejected. An attempt has been made to rationalize the experimental findings by... 

In terms of nonequilibrium interaction conditions the lack of the intramolecular deuterium isotope effect in the hot tritium atoms reaction with partial deuteriated methane, implies that the greater chance of encounter between fast moving... 

Threshold energy for hydrogen substitution reaction is more difficult to measure. Chou and Rowland (1969) used photolysis of TBr to obtain hot tritium atoms for the substitution reaction. They compared the yield of substituted CD3T in methane to the DT abstraction yield whose threshold energy is known to be smaller. The ratio substitution/abstraction was extrapolated to zero at which the threshold value of energy for substitution was determined to be 1.5 eV. 

Hot atom reactions have also been used to label organic compounds with T. Irradiation of helium-3 with neutrons according to the nuclear reaction produces very energetic tritium atoms that can displace ordinary hydrogen in organic compounds. This procedure is not very selective, and the labeling pattern must be determined to enable the tritiated product to be used effectively as a tracer (34). 

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