Harpoon reaction
A harpoon reaction or harpoon mechanism is a type of chemical reaction that involves two neutral reactants undergoing an electron transfer over a relatively long distance to form ions that then attract each other closer together.[1] The initial electron-transfer is a thermally- or photochemically-induced redox reaction. The subsequent ionic combination begins as a result of Coulombic forces. For example, a metal atom and a halogen might react by this type of reaction mechanism to form a cation and anion, respectively, which then form an associated salt of the metal halide. It was first proposed by Michael Polanyi in 1920.[2][3]
The main feature of these redox reactions is that, unlike most reactions, they have steric factors greater than unity; that is, they take place reaction rate 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" (analogous to firing a harpoon weapon) from one of the particles to the other one, forming an anion and a cation. Harpoon reactions usually take place in the gas phase, but they are also possible in condensed media.[4][5]
The accuracy of predicted rate constants can be improved by using a better estimation of the steric factor. A rough approximation is that the largest separation Rx 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 .
With , where is the ionization potential of the metal and is the electron affinity of the halogen.
Examples
- Generically: Rg + X2 + hν → RgX + X,[7] where Rg is a rare gas and X is a halogen
- Ba...FCH3 + hν → BaF(*) + CH3[8]
- K + CH3I → KI + CH3[9]
References
- ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "harpoon mechanism". doi:10.1351/goldbook.H02746
- ^ Polanyi, M. (1 January 1920). "Zum Ursprung der chemischen Energie". Zeitschrift für Physik (in German). 3 (1): 31–35. Bibcode:1920ZPhy....3...31P. doi:10.1007/BF01356227. ISSN 0044-3328. S2CID 120940201.
- ^ Herschbach, D. R. (14 March 2007), "Reactive Scattering in Molecular Beams", in Ross, John (ed.), Advances in Chemical Physics, vol. 10, Hoboken, NJ, USA: John Wiley & Sons, Inc., pp. 319–393, doi:10.1002/9780470143568.ch9, ISBN 978-0-470-14356-8, archived from the original on 13 April 2022, retrieved 13 April 2022
- ^ Fajardo, Mario E.; V. A. Apkarian (15 November 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.
- ^ Fajardo, Mario E.; V. A. Apkarian (1 October 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.
- ^ Atkins, Peter (2014). Atkins' Physical Chemistry. Oxford. p. 875. ISBN 9780199697403.
- ^ Okada, F.; L. Wiedeman; V. A. Apkarian (23 February 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. doi:10.1021/j100341a020.
- ^ 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. Bibcode:1999JChPh.111..460S. doi:10.1063/1.479326.
- ^ 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.