Endothermic atom transfer

The HeH ion is simple enough to permit accurate quantum mechanical calculation of its dissociation energy and it is well established that reaction (100) is endothermic by 1.1 eV and reaction (102) is exothermic by 8.3 eV. Although the accuracy of the available values of the dissociation energy for the more complex NeH ion is poorer, reactions (101) and (103) are estimated to be 0.6 eV endothermic and 6.0 eV exothermic, respectively, based on the ionization threshold measurements [160]. 

Ionization efficiency data for the He + H2 system obtained by von Koch and Friedman [161] are presented in Fig. 17. The intensities are 

how dominant are these reactions A change of electron energy that reduces He (Ne ) by a factor of 2 decreased the HeH /H2 (NeH /H2) ratio by only about 4% (a few %). From this von Koch and Friedman [161] estimate that at most 8% (4%) of HeH (NeH ) is formed by reaction (102) ((103)), although they point out that these percentages may not be correct because the comparison was made with the efficiency curves for all the H ions produced. If, as will be shown below, only those H2 which have sufficient internal energy react with He (Ne), the proper comparison should be made with the curves for H2 which have such energy. This would yield much smaller percentages. 

Reaction (102) has also been searched for in a tandem mass spectrometer [95] and a flowing afterglow experiment [164]. In both studies it was not observed, in agreement with the result of single source mass spectrometer experiments. These studies set the upper limits of the cross-section for reaction (102) at 6 x 10 cm (for He energies from 1 to 10 eV) and at 10 cm (for thermal energy reactions), respectively. 

The observation that reactions (102) and (103) contribute to the HeH and NeH formation, respectively, to a negligibly small extent is consistent with Polanyi s conjecture [165] that the energy in exothermic reactions is deposited mainly as internal energy in the products. Namely, if most of the heat of reactions (102) and (103) (8.3 and 6.0 eV, respectively) goes to the internal degree of freedom of HeH and NeH, these products would dissociate immediately into H ions and rare gas atoms. In this connection, it is interesting to note that the much less exothermic reactions 

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Reaction sequences involving halogen transfer, followed by non-radical interception of the alkyl iodide or bromide formed can allow for the trapping of products arising from less exothermic or even endothermic atom transfer additions and are exemplified in Scheme 11. Yoon [30] and Curran [31] have demonstrated that the a-halo ethers formed upon addition to vinyl ethers can be trapped with alcohols, leading to formation of acetals. Substitution reactions on the heteroaromatics pyrrole and indole have been carried out through a sequence of steps involving I- or Br-... 

Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table II) causes an increased rate, with kH/k = 0.73 (37). The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope... 

It is more difficult to conduct the addition reactions of nucleophilic radicals to electron poor alkenes because the resulting atom transfer steps are often endothermic and are too slow to propagate chains, even with iodides. An exception is illustrated in Scheme 57 resonance-stabilized vinyl radicals (especially if they are secondary or tertiary) are reactive enough to abstract iodine from alkyl iodides.178... 

Such isomerization reactions are, of course, endothermic but the reaction energy in the case of sulfur-rich molecules is lower than ca. bOkJmol". Unfortunately, the activation energy is known only for disulfanes. However, in the case of ethylene episulfide a similar sulfur atom transfer has been shown to be the lowest energy pathway for the decomposition to elemental sulfur and an alkene with an activation enthalpy of llOkJmof. Compounds of type X-S(=S)-X are stable at 20 °C if X = F or OR. 

By moving from the transition state structure towards the product, we were able to localize the proposed radical intermediate in the IMOMM calculation (Figure 5). The energy of the intermediate is 4.9 kcal/mol down from the transition state, making the hydrogen atom transfer step endothermic by 8.7 kcal/mol. 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

The total energy needed for ion formation is even greater than this because metallic lithium and diatomic fluorine must first be converted to separate gaseous atoms, which also requires energy. Despite this, the standard heat of formation (A//f) of solid LiE is —617 kJ/mol that is, 617 kJ is released when 1 mol of LiF(5) forms from its elements. The case of LiF is typical of many reactions between active metals and nonmetals despite the endothermic electron transfer, ionic solids form readily, often vigorously. Figure 9.6 shows another example, the formation of NaBr. 

Efficient conversion of translational kinetic energy into internal energy has also been discovered for endothermic processes other than atom transfer, such as charge transfer [176—179], dissociative charge transfer [94,176, 180,181] and collision-induced dissociation [98, 169]. 

Most of these reactions are endothermic suggesting that, for many RO, the activation energy for cleavage (Ed) will be substantially greater than 5 kcal mole-1, and often greater than Ea, the activation energy for H-atom transfer to RO. Thus more stable RO will usually react by H-atom transfer [reaction (157)] rather than j3-cleavage [reaction (158)]. 

W.B. Maier II, Atom transfer in endothermic ion-molecule reactions, J. Chem. Phys. 46, 4991-4992 (1967). 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

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