Harpoon reaction explained

A harpoon reaction is a type of chemical reaction, first proposed by Michael Polanyi in 1920,^[1] whose mechanism (also called the harpooning mechanism) involves two neutral reactants undergoing an electron transfer over a relatively long distance to form ions that then attract each other closer together. For example, a metal atom and a halogen might react to form a cation and anion, respectively, leading to a combined metal halide.

The main feature of these redox reactions is that, unlike most reactions, they have steric factors greater than unity; that is, they take place faster than predicted by collision theory. This is explained by the fact that the colliding particles have greater cross sections than the pure geometrical ones calculated from their radii, because when the particles are close enough, an electron "jumps" (therefore the name) from one of the particles to the other one, forming an anion and a cation which subsequently attract each other. Harpoon reactions usually take place in the gas phase, but they are also possible in condensed media.^{[2] [3]}

The predicted rate constant can be improved by using a better estimation of the steric factor. A rough approximation is that the largest separation R_x at which charge transfer can take place on energetic grounds, can be estimated from the solution of the following equation that determines the largest distance at which the Coulombic attraction between the two oppositely charged ions is sufficient to provide the energy

\Delta E _{0} .(- q ^{2} _{e})/(R _{x}) + \Delta E _{0} = 0 ^[4]

With

\Delta E _{0} = E _{i} - E _{ea} , where E _{i} is the ionization potential of the metal and E _{ea} is the electron affinity of the halogen.

Examples of harpoon reactions

• Generically: Rg + X₂ + hν → RgX + X,^[5] where Rg is a rare gas and X is a halogen
• Ba...FCH₃ + hν → BaF^(*) + CH₃^[6]
• K + CH₃I → KI + CH₃^[7]

Notes and References

1. Polanyi . M. . 1920-01-01 . Zum Ursprung der chemischen Energie . Zeitschrift für Physik . de . 3 . 1 . 31–35 . 10.1007/BF01356227 . 120940201 . 0044-3328.
2. Cooperative photoabsorption induced charge transfer reaction dynamics in rare gas solids. I. Photodynamics of localized xenon chloride exciplexes. Fajardo. Mario E.. V. A. Apkarian. The Journal of Chemical Physics. 5660–5681 . 85 . 10 . November 15, 1986. 10.1063/1.451579. 1986JChPh..85.5660F .
3. Charge transfer photodynamics in halogen doped xenon matrices. II. Photoinduced harpooning and the delocalized charge transfer states of solid xenon halides (F, Cl, Br, I). Fajardo. Mario E.. V. A. Apkarian. The Journal of Chemical Physics. 4102–4123 . 89 . 7 . October 1, 1988. 10.1063/1.454846. 1988JChPh..89.4102F .
4. Book: Atkins. Peter. Atkins' Physical Chemistry. 2014. Oxford. 9780199697403. 875.
5. Okada . F. . L. Wiedeman . V. A. Apkarian . February 23, 1989 . Photoinduced harpoon reactions as a probe of condensed-phase dynamics: iodine chloride in liquid and solid xenon . Journal of Physical Chemistry . 93 . 4 . 1267–1272 . 10.1021/j100341a020.
6. Skowronek . S. . J. B. Jiméne . A. González Ureña . 8 July 1999 . Resonances in the Ba...FCH₃ + hν → BaF + CH₃ reaction probability . Journal of Chemical Physics . 111 . 4 . 460–463 . 10.1063/1.479326 . 1999JChPh.111..460S .
7. Wiskerke . A. E. . S. Stolte . H. J. Loesch . R. D. Levine . 2000 . K + CH₃I → KI + CH₃ revisited: the total reaction cross section and its energy and orientation dependence. A case study of an intermolecular electron transfer . Physical Chemistry Chemical Physics . 2 . 4 . 757–767 . 10.1039/a907701d. 2000PCCP....2..757W .