harpoon reaction

{{Short description|Redox reaction in which an electron jumps between both reagents}}

A harpoon reaction is a type of chemical reaction, first proposed by Michael Polanyi in 1920,{{Cite journal |last=Polanyi |first=M. |date=1920-01-01 |title=Zum Ursprung der chemischen Energie |url=https://doi.org/10.1007/BF01356227 |journal=Zeitschrift für Physik |language=de |volume=3 |issue=1 |pages=31–35 |doi=10.1007/BF01356227 |s2cid=120940201 |issn=0044-3328}}{{Citation |last=Herschbach |first=D. R. |title=Advances in Chemical Physics |chapter=Reactive Scattering in Molecular Beams |date=2007-03-14 |chapter-url=https://onlinelibrary.wiley.com/doi/10.1002/9780470143568.ch9 |series=Advances in Chemical Physics |pages=319–393 |editor-last=Ross |editor-first=John |place=Hoboken, NJ, USA |publisher=John Wiley & Sons, Inc. |doi=10.1002/9780470143568.ch9 |isbn=978-0-470-14356-8 |access-date=2022-04-13 |archive-date=2022-04-13 |archive-url=https://web.archive.org/web/20220413063421/https://onlinelibrary.wiley.com/doi/10.1002/9780470143568.ch9 |url-status=live }} 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.{{GoldBookRef|file=H02746|title=harpoon mechanism}} 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.{{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.|author2=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 }}{{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.|author2=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 }}

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 $\backslash Delta\; E\_0$.

:$\backslash frac\{-q\_e^2\}\{R\_x\}+\backslash Delta\; E\_0\; =\; 0${{cite book|last1=Atkins|first1=Peter|title=Atkins' Physical Chemistry|date=2014|publisher=Oxford|isbn=9780199697403|pages=875}}

With $\backslash Delta\; E\_0\; =\; E\_i\; -\; E\_\{ea\}$, where $E\_\{i\}$ is the ionization potential of the metal and $E\_\{ea\}$ is the electron affinity of the halogen.