Endothermic dissociative charge transfer

A number of endothermic dissociative charge transfers have also been studied by several investigators. Such studies have proved to be very important since they provide a powerful means for determining bond energies, in addition to their shedding light on the problem of energy transfer in chemical reactions. 

In order to see the threshold behaviour of such a reaction and its marked contrast with the exothermic dissociative charge transfer, the cross-section curves for the two reactions 

Maier found that the equation fits the experimental data for almost all the reactions studied. Although some reactions with C2H2 and C2H4, such as reaction (118), may have more than three separating products (e.g. CH2, CH, H, and Xe), he found that eqn. (122) is still applicable to some of these reactions if suitable values of n, which are sometimes greater than 2, are chosen. On the other hand, the equation never fits the data for reaction (113) in which only two products are separating, although for the latter case a,similar discussion leads to the same equation as (122) with n = 1/2. 

Vance and Bailey [94] studied an endothermic dissociative charge transfer with a non-rare-gas reactant 

While eqn. (122) has been shown to fit the experimental data of many reactions, it has no sound theoretical basis, and at present there is no way to predict the values of n theoretically. In fact, in many cases, different sets of n and Q, or n and B give as good fit as that reported, and might give even better fits. Further investigation of the phase space approach is desirable. 

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Fig. 22. Comparison of values and energy dependence of cross-sections for endothermic dissociative charge transfer with those for exothermic dissociative charge transfer. (From ref. 180.)...

As to another important aspect of the study of endothermic dissociative charge transfer, i.e. its application to the determination of bond dissociation energies of Vcirious molecules, we do not have enough space to describe it, and the reader is referred to the original papers [176, 181] for detailed discussion on individual molecules. 

Mass spectrometric studies of the ionic species which arrive at the cathode of both glow and corona discharges yield useful information regarding ion-molecule reactions which occur within these systems. Glow discharges have been used to study endothermic reactions, and their usefulness and limitations have been demonstrated by studies of the dissociative charge transfer reactions Ar+ + N2 N+ + N + Ar N2+ + N2 N+ + N + N2 N2+ + 02 0+ + O + N2. Exo-... 

Ar" " and Nj have similar recombination energies and for some reactions have similar reactivity even though one is atomic and the other diatomic. The similarities and differences in the reactions of these two ions with O2 was described above. The reactions of these ions with CO2 and SO2 have also been studied in the HTFA. " The reactions with CO2 proceed exclusively by charge transfer and the SO2 reaction is mainly charge transfer except at high temperature/energy where SO" " is produced by dissociative charge transfer, a process which is endothermic at room temperature. 

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]. 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

Interestingly, from protonated systems, in order to explain the observed nontotal regios-electivity observed for labeled hexene elimination (nonspecific H/D exchanges) from 2,2,4,4-D45CC-hexyl /n-dimethylamino ethers, the contribution of ion-dipole complex formation prior to dissociation is considered without (or with) charge retention at the initial protonation site (i.e., Me2N-site, Scheme 17.3). This ion isomerization requires as the first step an endothermic proton transfer from the protonated tertiary amine to the aromatic system (Scheme 17.3). This complex is able to undergo reversible proton (or deuteron)... 

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