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'''Harpoon reactions'''<ref>[http://www.iupac.org/goldbook/H02746.pdf IUPAC Goldbook definition of harpoon reaction]</ref> are a type of [[chemical reaction]] between two substances one of them prone to form a [[cation]], generally a [[metal]], and the other one prone to form an [[anion]], generally a [[halogen]].

Their main feature is that these reactions, unlike most reactions, have [[steric factor]]s 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 section (physics)|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.<ref>{{cite journal|title=Cooperative photoabsorption induced charge transfer reaction dynamics in rare gas solids. I. Photodynamics of localized xenon chloride exciplexes|last=Fajardo|first=Mario E.|coauthors=V. A. Apkarian|journal=The Journal of Chemical Physics|pages=5660–5681 |volume=85 |issue=10 |date=November 15, 1986|doi=10.1063/1.451579|bibcode = 1986JChPh..85.5660F }}</ref><ref>{{cite journal|title=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)|last=Fajardo|first=Mario E.|coauthors=V. A. Apkarian|journal=The Journal of Chemical Physics|pages=4102–4123  |volume=89 |issue=7 |date=October 1, 1988| doi=10.1063/1.454846|bibcode = 1988JChPh..89.4102F }}</ref>

The calculated [[rate constant]] can be improved by using a better estimation of the steric factor. A rough approximation is that the largest separation R<sub>x</sub> 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 ΔE<sub>0</sub>

:<math>\frac{-e^2}{R_x}+\Delta E_0 = 0</math><ref>[http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/3_Com.pdf 3<!-- Bot generated title -->]</ref>

With <math>\Delta E_0 = IP - EA</math>, where IP is the [[ionization potential]] of the metal and EA is the [[electron affinity]] of the halogen.

==Examples of harpoon reactions==
* Generically: Rg + X<sub>2</sub> + ''h''ν → RgX + X,<ref>{{cite journal | last = Okada | first = F. | coauthors = L. Wiedeman and V. A. Apkarian | date = February 23, 1989 | title = Photoinduced harpoon reactions as a probe of condensed-phase dynamics: iodine chloride in liquid and solid xenon | journal = Journal of Physical Chemistry | volume = 93 | issue = 4 | pages = 1267–1272 | doi = 10.1021/j100341a020 | url = http://pubs.acs.org/cgi-bin/abstract.cgi/jpchax/1989/93/i04/f-pdf/f_j100341a020.pdf?sessid=2492
 | format = PDF}}</ref> where Rg is a [[rare gas]] and X is a halogen
* Ba...FCH<sub>3</sub> + ''h''ν → BaF<sup>(*)</sup> + CH<sub>3</sub><ref>{{cite journal | last = Skowronek | first = S. | coauthors =J. B. Jiméne and A. González Ureña | date = 8 July 1999 | title = Resonances in the Ba...FCH<sub>3</sub> + ''h''ν → BaF + CH<sub>3</sub> reaction probability | journal = Journal of Chemical Physics | volume = 111 | issue = 4 | pages = 460–463 | doi =  10.1063/1.479326| url = http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JCPSA6000111000002000460000001&idtype=cvips&prog=normal
 | format = PDF |bibcode = 1999JChPh.111..460S }}</ref>
*K + CH<sub>3</sub>I → KI + CH<sub>3</sub><ref>{{cite journal | last = Wiskerke | first = A. E. | coauthors = S. Stolte, H. J. Loesch and R. D. Levine | year = 2000 | title = K + CH<sub>3</sub>I → KI + CH<sub>3</sub> revisited: the total reaction cross section and its energy and orientation dependence. A case study of an intermolecular electron transfer | journal = Physical Chemistry Chemical Physics | volume = 2 | issue = 4 | pages = 757–767 | doi = 10.1039/a907701d|bibcode = 2000PCCP....2..757W }}</ref>

==References==
{{Reflist}}

[[Category:Chemical kinetics]]
[[Category:Chemical reactions]]