Electron harpooning

E.E.Nikitin, Charge-exchange indueed reactions, Teor.Eksp.Khim. 4, 593 (1968) E.E.Nikitin, Quantum effects in electron-harpoon reactions, Teor. Eksp. Khim. 4, 751 (1968)... 

NakatsujI H, Kuwano R, Merita H and Nakal H 1993 Dipped adcluster model and SAC-CI method applied to harpooning, chemical luminescence and electron emission in halogen chemisorption on alkali metal surface J. Mol. Catal. 82 211-28... 

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). 

Figure 2. The harpoon-like tooth of Conus, a. An unusual photograph of a radular tooth at the tip of the proboscis of Conus pur-purascens. Normally, the tooth would not be ejected from the proboscis until the prey had been harpooned. Photograph by Alex Ker-stitch. b. A scanning electron micrograph of the tip of the radular tooth of Conus purpurascens, showing its harpoon-like form.

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. 

The product we monitor is again the I atom using femtosecond-resolved mass spectrometry (the other product is the Bzl species). We also monitor the initial complex evolution. The initial femtosecond pulse prepares the system in the transition state of the harpoon region, that is, Bz+h. The iodine atom is liberated either by continuing on the harpoon PES and/or by electron transfer from iodine (I2-) to Bz+ and dissociation of neutral I2 to iodine atoms. We have studied the femtosecond dynamics of both channels (Fig. 17) by resolving their different kinetic energies and temporal behavior. The mechanism for the elementary steps of this century-old reaction is now clear. 

Figure 17. (a) Generic reaction path for charge transfer reactions with both channels of harpooning and electron transfer indicated. Molecular dynamics of the Bz/l2 bimolecular reaction is shown at the bottom, (b) Observed transient for the Bz/l2 reaction (I detection) and the associated changes in molecular structure. Note that we observe the two channels of the reaction, shown in (a), with different kinetic energies and rises of the I atom. 

The results, which are detailed elsewhere, reveal that the transition state of CT reactions can be studied directly at well-defined impact geometries. The dissociative CT reaction of benzenes with iodine occurs with an elementary harpoon/electron transfer mechanism. The time scales for the CT and for the product (I) formation define the degree of concertedness and, as reported elsewhere, are significant to the recent elegant studies in condensed media by Wiersma and colleagues and by Sension. So far we have studied the electron donors of benzene, toluene, xylene, mesitylene, and cyclohexane and we plan extension to other systems. We have also studied the effect of solvation in clusters and in solutions. 

The above CT systems represent the case for intermolecular electron transfer. There is some analogy to proton transfer in acid-base reactions [5]. We have also examined intramolecular electron transfer systems and studied the influence of IVR and geometric changes this work is detailed elsewhere [5]. Other reactions involving ultrafast electron transfer are those of harpooning in Xe + I2) Nal, and more recently Xe/Ch (see Ref. 1). 

Manifestations of nuclei tunneling in chemical reactions in gaseous, liquid, and solid phases are consecutively considered in Sects. 4.2-4.5. Also discussed in this chapter are (1) manifestations of nuclear tunneling in the vibrational spectra of ammonia-type molecules (Sect. 4.6), (2) electron tunneling in gas-phase chemical reactions of atom transfer (the so-called "harpoon reactions, Sect. 4.2), and (3) tunneling of hydrated electrons in the reactions of their recombination with some inorganic anions in aqueous solutions (Sect. 4.4). 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

For harpoon reactions of alkaline metal atoms with iodine molecule I2, the interaction radii, Re, calculated using the formula Re = (ajji) 12 from the experimentally measured cross-sections a, are compared in Table 3 with the distances, Ru, calculated with the help of eqn. (40) and the sums of the gas-kinetic radii i M + i l2 of the reagents. In these calculations, effective radii of alkaline metal atoms have been used as RM, while the radii of the molecule I2, calculated from the data on the viscosity of I2 vapour at T > co and at T = 273 K, have been used as i l2 (the values of RM + i ,2 given in brackets correspond to the latter) [71], It is seen that the values of Re exceed Rm + Rh, i.e. electron transfer occurs at large impact parameters. 

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 ... 

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. 

Pure rotational and vibrational Raman spectra of At2 Raman spectroscopic study of kinetics of Ar, formation in a supersonic expansion seeded with Nj Electronic absorption spectrum of HgAr Rotational Zeeman effect in ArHBr t HgCl2 collision complex formed in harpoon reaction of Hg with Clj investigated via excitation of the HgCL van der Waals complex... 

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, which describes harpoon reactions in the gas phase, has strong relationships with Figure 1, which pictures the Marcus mechanism of an electron-... 

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]. 

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