Harpooning model

The possibility of a barrier which inhibits a reaction in spite of the attractive ion-dipole potential suggests that one should make even crude attempts to guess the properties of the potential hypersurface for ion reactions. Even a simple model for the long range behavior of the potential between neutrals (the harpoon model ) appears promising as a means to understand alkali beam reactions (11). The possibility of resonance interaction either to aid or hinder reactions of ions with neutrals has been suggested (8). The effect of possible resonance interaction on cross-sections of ion-molecule reactions has been calculated (25). The resonance interaction would be relatively unimportant for Reaction 2 because the ionization potential for O (13.61 e.v.) is so different from that for N2 (15.56 e.v.). A case in which this resonance interaction should be strong and attractive is Reaction 3 ... 

An important class of the gas-phase electron-transfer reactions is well described, at least qualitatively, by the model named pictorially the harpoon model , which was proposed originally by Michael Polanyi [8] to account for exceptionally large cross-sections in the oxidation reaction of alkali metal atoms by halogen molecules. It is striking to observe that such a simple model is still a useful tool to rationalize these reactions. It is illustrated in Figure 2. 

Figure 2. Schematic energy diagram of the harpoon model for describing the Na + CI2 — NaCl + Cl reaction.

Of course, the very simple harpoon model at the beginning has been considerably refined and more sophisticated approaches are currently used also. This chapter aims to review such reactions. This has obliged us to omit interesting reactions which are not directly relevant to the harpoon model, such as the electron-transfer reactions of buckminsterfullerene, Cso [9], and charge-transfer reactions, which have also produced a considerable literature [10]. Reactions of metal ions are also not treated here. They appear elsewhere in this Part (see Chapter IV.3.5). Moreover, although not centered exclusively on electron-transfer reactions, extensive reviews are available on the ion chemistry of transition metals, either bare [11] or ligated [12]. 

The harpoon model has been used on a large variety of systems, far beyond the alkali metal-halogen systems for which it was proposed originally. It is therefore necessary to describe the model on a more general basis by considering the reaction... 

After these two examples, it should not be thought that reactions described by the harpoon model are necessarily direct. This was the case with reactions 4 and 5 because they are very exoergic and the molecular negative ion is unstable. In contrast, in the almost thermoneutral reaction of Cs with NO2, NOj is a stable ion, and the charge-transfer complex Cs - NOj corresponds to a deep well along the reaction coordinate. Hence the reaction proceeds from a persistent complex, with a forward-backward symmetry in the angular distribution of the reaction product CsO [72]. 

Beyond the Harpoon Model the HOMO/LUMO Matching to Describe Transition Metal Reactivity... 

The model of electron transfer in gas-phase metal-molecule reactions can be extended to more complex systems such as the collisions of metastable rare gas atoms with molecules to produce negative molecular ions [306], In surface chemistry the harpoon model describes the forces between the reagents after the electron transfer has been applied to reactions of molecules with metal surfaces [120]. Another domain, involving the reaction of metal ions with complex systems could be interpreted in the framework of electron transfers in the porphyrin site of the heme within hemoglobin, addition of oxygen to the Fe " " results in an electron transfer from the metal to the oxygen. The dynamics of this attachement and of the photo-induced detachment could be viewed in that perspective. 

The harpoon model uses the ionization potential (IP) and the electron affinity (EA) as the measures of electron donating and accepting. In terms of the energy of the A-electron molecule, the IP and EA are defined by... 

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