Harpoon reaction

Harpoon reactions^[1] are a type of chemical reaction between two substances: one (generally a metal) prone to form a cation, and the other (generally a halogen) prone to form an anion.

Their main feature is that these reactions, unlike most reactions, 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 ΔE₀

$\frac{-e^2}{R_x}+\Delta E_0 = 0$^[4]

With $\Delta E_0 = IP - EA$, where IP is the ionization potential of the metal and 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]

References

1. IUPAC Goldbook definition of harpoon reaction
2. Fajardo, Mario E.; V. A. Apkarian (November 15, 1986). "Cooperative photoabsorption induced charge transfer reaction dynamics in rare gas solids. I. Photodynamics of localized xenon chloride exciplexes". The Journal of Chemical Physics 85 (10): 5660–5681. Bibcode:1986JChPh..85.5660F. doi:10.1063/1.451579.
3. Fajardo, Mario E.; V. A. Apkarian (October 1, 1988). "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)". The Journal of Chemical Physics 89 (7): 4102–4123. Bibcode:1988JChPh..89.4102F. doi:10.1063/1.454846.
4. 3
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" (PDF). Journal of Physical Chemistry 93 (4): 1267–1272. doi:10.1021/j100341a020.
6. Skowronek, S.; J. B. Jiméne and A. González Ureña (8 July 1999). "Resonances in the Ba...FCH₃ + hν → BaF + CH₃ reaction probability" (PDF). Journal of Chemical Physics 111 (4): 460–463. Bibcode:1999JChPh.111..460S. doi:10.1063/1.479326. Cite uses deprecated parameter |coauthors= (help)
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. Bibcode:2000PCCP....2..757W. doi:10.1039/a907701d.