Harpoon reaction

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. 

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... 

Figure 2, which describes harpoon reactions in the gas phase, has strong relationships with Figure 1, which pictures the Marcus mechanism of an electron-... 

These reactions were pioneered by Grice and co-workers who first addressed the reactivity of alkali metal dimers with halogen molecules [108-111]. These reactions are four-center harpoon reactions. They are interesting because of the possibility of two sequential electron jumps [112. ... 

This effect is best viewed in single harpoon reactions such as those of alkali metal atoms with halogen-containing molecules discussed in Section 2.3.1. A series of studies conducted in a crossed-beam experiment by Lee s group at Berkeley have demonstrated how the electronic excitation of sodium affects the dynamics of these reactions. 

Within the framework of this chapter, a section on stereodynamics must focus on the heart of harpoon reactions the electron transfer itself. Hence we review studies which directly inform on the extent to which the electron transfer is affected by the relative polarization of the donor (HOMO) and acceptor (LUMO) orbitals. 

The van der Waals initiated reaction explores ideally excited-state harpoon reactions. The latter occur at intermolecular distances in the range 3-TO A, a region which is Franck-Condon accessible from ground-state van der Waals complexes, whose equilibrium distance is typically R = 3-4 A. 

Within the framework of the harpoon reactions described at the beginning of this chapter, ground-state reactions occur between an electron donor and an electron acceptor, because an electron is totally transferred from the donor to the acceptor. Of course, situations must exist where an electron is transferred but the reaction does not occur owing to energetics. Thus the charge-transfer complex can be stabilized before reaction. Other situations can be encountered where the electron transfer is only partial. These situations can be named prereactive to highlight the fact that the reaction is blocked somewhere along the reaction path. 

Prereactive behaviors were identified very early and an impressive series of examples was listed in a book by Klabunde in 1980 [266]. Electron spin resonance (ESR) studies reveal that in low-temperature matrices electron-transfer reactions are blocked as a rule and most, if not all, charge-transfer complexes involved in standard gas-phase harpoon reactions have been stabilized and observed in matrices. The ESR spectra of these systems revealed nearly complete electron transfer. Similar conclusions have also been drawn from infrared spectroscopy. For example, outside the field of alkali metal atoms, evidence of an AHNO complex has been obtained by this technique [267]. It should not be thought that every metal atom is able to make charge transfer with every molecule. For example, no indication exists of a charge transfer between Cu and NO in an argon matrix [268]. 

Let us consider an illuminating example concerning an alkali metal atom the harpoon reaction... 

I am, however, only one of many who have seen the extraordinary possibilities offered by harpooning reactions. For example, Dudley Herschbach began his life as a dynamicist by studying that type of reaction. One should also add that my father himself was part of a continuous progression. What drew him to sodium reactions was that Fritz Haber had been studying an unexplained chemiluminescence from them. This was in Berlin and my father was in Haber s Institute as a young researcher. The history, as is usual in science, constitutes an unbroken chain. 

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)... 

A.A.Zembekov and E.E.Nikitin, On dynamies of harpoon reactions, Doklady Akad. Nauk SSSR 205,1392 (1972)... 

A.A.Zembekov, E.E.Nikitin, U.Havemann, and L.Zuelicke, Dynamics of harpoon reactions as a prototype of chemiionization in Khimiya Plazmy 6, ed. B. M. Smirnov, Moscow, Atomizdat, 1979, p.3... 

This latter mechanism can be ruled out in a number of ways through judicious choice of experimental conditions, as most notably demonstrated by studies of the photo-induced harpoon reactions of ICl in liquid and solid xenon (Okada et al. 1989). Here it has been shown that Xel is produced even at wavelengths well beyond the threshold for atomic charge transfer between... 

R. Brooks [42-44] the process of electron transfer for K to oriented t-butyl bromide is found strongly dependent on the orientation. Systems involving metal atoms are traditional favorites of molecular beam studies, particularly of stereodynamics. In recent experiments [45], with brute force oriented ICl, experimental determination was made of the cone of acceptance for reactivity (steric effect) in a "harpooning" reaction, Sr + ICl leading to electroiucally excited products detected via their chemiluminescence... 

A schematic diagram of the potential curves involved in the harpoon reaction is depicted in Figure 22.2. 

Reactions of this type are, for example, initiated in van der Waals clusters, M---XR (M = alkali metal X = F, Cl, Br R = H,CH3,Ph), by absorption of visible light. This approach has allowed the study of the classic alkali-metal atom harpooning reactions. The metal atom M acts as the chromophore and, thereafter, charge transfer to the halide molecule XR results in dissociation of the complex, i.e. 

This van der Waals approach has also been extended to study alkali-metal atom harpooning reactions with complexes of Na with CH3CI, CH3F and PhF (J.C. Polanyi s group). In these investigations, the photodepletion of the complexes through... 

Photoinduced charge-transfer reaction. In this scheme the absorbed photon induces the harpoon reaction, e.g. 

Farmanara P, Stert V, Radloff W, Skowronek S, Gonzalez-Urena A. 1999. Ultra-fast dynamics and energetics of the intra-cluster harpooning reaction in Ba FCH3 . Chem. Phys. Lett. 304(3-4) 127-133. 

Skowronek S, Pereira R, Gonzalez-Urena A. 1997. Spectroscopy and dynamics of excited harpooning reactions the photodepletion action spectrum of the Ba - FCHs complex . J. Phys. Chem. A 101(41) 7468-7475. 

Caging Reactions Harpoon Reactions Exclmera Exciplexes Polyivier Reactions Semiconductor Reactions... 

Fajardo ME, Withnall R, Feld J et al (1988) Condensed phase laser induced harpoon reactions. Laser Chem 9 1-3... 

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