Harpooning mechanism

In the harpoon mechanism for the reaction between potassium and iodine to form potassium iodide, as a K atom approaches an I, molecule (a), an electron passes from the K atom to the I, molecule (b). The charge difference now tethers the two ions together (c and d) until an I ion separates and leaves with the Kf ion (e). 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

In a sticky collision, the reactant molecules orbit around each other for one revolution or more. As a result, the product molecules emerge in random directions because no memory of the approach direction is retained. However, a rotation takes time—about 1 ps. If the reaction is over before that, the product molecules will emerge in a specific direction that depends on the direction of the collision. In the collision of K and I2, for example, most of the products are thrown off in the forward direction. This observation is consistent with the harpoon mechanism that had been proposed for this reaction. In this mechanism, an electron flips across from the K atom to the I2 molecule when they are quite far apart, and the resulting K+ ion draws in the negatively charged I2 ion. We V ... 

This experiment shows that the harpoon mechanism can be observed through the spectroscopic observation of the charge transfer state Hg+Cl2. Moreover, it shows the interest of starting from a fixed geometry to understand the spectroscopy of the reactive collision complex. 

During the last ten years, reactive collisions Rg + X2 or Rg + Xf leading to the RgX excimer through the harpoon mechanism have been widely studied (Ishiwata et al. 1984 Johnson et al. 1986a,b, 1987 Le Calve et al. 1985 O Grady and Donovan 1985 Tamagake et al. 1979 Wilkinson et al. 1986 Yu et al. 1983). These systems and mainly the Xe + Cl2 or Br2 have been the first ones where the collision complex was excited in the gas phase by one photon (Dubbov et al. 1981 Grieneisen et al. 1983 Wilcomb and Burnham 1981) or two photons (Inoue et al. 1984 Ku et al. 1983 Setser and Ku 1985). The same two-photon excitation technique has been used for the excitation of the Xe-X2 van der Waals complexes (X2 = Cl2, Br2,12). These results can be then compared with the collisional ones. 

Harpoon mechanism Reaction sequence (thermal or photoinduced) between neutral molecular or atomic entities in which long-range electron transfer is followed by a considerable reduction of the distance between donor and acceptor sites as a result of the electrostatic attraction in the ion pair created. 

Alkaline earth metal atoms have fairly low ionization potentials, as have alkali metal atoms (e.g., 5.21 and 5.14 eV for barium and sodium, respectively [89]). Hence the reactions of alkaline earth metal atoms with oxidizing molecules are also expected to be initiated by an electron transfer and should follow the harpoon mechanism. However, alkali metal atoms are monovalent species, whereas alkaline earth metal atoms have two valence electrons. Hence peculiarities are to be expected in the alkaline earth metal reaction dynamics, especially when doubly charged products such as BaO are to be formed [90]. The second valence electron also opens up the possibility of chemiluminescent reactions, which are largely absent in alkali metal atom reactions [91, 92]. The second electron causes the existence of low-lying excited states in the product. 

Reactions that produce RgX excimers via a harpoon mechanism have been widely studied [203-217]. Among them, those involving Xe and CI2 have been examined most extensively, both in the gas phase [203-209, 212, 215-217] and using weakly bonded complexes [48, 49, 214]. 

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 . 

The electron-jump or harpooning mechanism [233], which has been discussed in Section 2.4 (see refs. 53—55), involves an electron exchange reaction... 

Much of this chapter concerns ET reaetions in solution. However, gas phase ET proeesses are well known too. See figure C3.2.1. The harpoon mechanism by which halogens oxidize alkali metals is fundamentally an eleetron transfer reaction [2]. One might guess, from this simple reaction, some of the struetural parameters that eontrol ET rates relative electron affinities of reaetants, reaetant separation distanee, bond length changes upon oxidation/reduction, vibrational frequeneies, ete. 

D24.1 The harpoon mechanism accounts for the laigesteric factor of reactions of the kind K+Bf2 — KBr+Br... 

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