Harpoon Reactions with Surfaces

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. 

In physical terms, the formation of LiH in the ground state from its constituent atoms occurs by means of a transfer of an electron from the Li atom to H when the internuclear distance decreases below a critical separation R. This same concept underlies the harpoon mechanism which is used to explain the very large cross-sections for reaction which are observed for such processes as K -i- Br2 - KBr + Br. As the reactants approach, the covalent K + Br2 potential surface is intersected by an ionic K Br surface. Accordingly, an electron transfers from K to Br2. Subsequent production of KBr and Br is immediate. This model is also in accord with the observation in beam scattering experiments that the distribution of KBr product is strongly forward-peaked . 

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