The main feature of theseredox reactions is that, unlike most reactions, they havesteric factors greater than unity; that is, they take placefaster than predicted bycollision theory. This is explained by the fact that the colliding particles have greatercross 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 thegas phase, but they are also possible in condensed media.[4][5]
The predictedrate constant 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.
^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.
^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.
^Atkins, Peter (2014).Atkins' Physical Chemistry. Oxford. p. 875.ISBN9780199697403.
^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.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.
