Cage effect

{{short description|Behavior of molecules in solvent as encapsulated particles}}

File:CageEffect.tif

In chemistry, the cage effect{{Cite journal |last=Chemistry (IUPAC) |first=The International Union of Pure and Applied |title=IUPAC - cage effect (C00771) |url=https://goldbook.iupac.org/terms/view/C00771 |access-date=2022-03-28 |website=goldbook.iupac.org |doi=10.1351/goldbook.c00771|doi-access=free }} (also known as geminate recombination{{Cite web |last=Chemistry (IUPAC) |first=The International Union of Pure and Applied |title=IUPAC - geminate recombination (G02603) |url=https://goldbook.iupac.org/terms/view/G02603 |access-date=2022-03-28 |website=goldbook.iupac.org}}) describes how the properties of a molecule are affected by its surroundings. First introduced by James Franck and Eugene Rabinowitch{{Cite journal|last=Rabinowitch, Franck|year=1934|title=Some remarks about free radicals and the photochemisty of solutions|journal=Transactions of the Faraday Society|volume=30|pages=120–130|doi=10.1039/tf9343000120}}{{Cite journal|last=Rabinowitch|first=E|year=1936|title=The collison{{sic|nolink=y|expected=error is in original}} mechanism and the primary photochemical process in solutions|journal=Transactions of the Faraday Society|volume=32|pages=1381–1387|doi=10.1039/tf9363201381}} in 1934, the cage effect suggests that instead of acting as an individual particle, molecules in solvent are more accurately described as an encapsulated particle. The encapsulated molecules or radicals are called cage pairs or geminate pairs.{{Cite journal|last=Denisov|first=E.T.|year=1984|title=Cage effects in a polymer matrix|journal=Macromolecular Chemistry and Physics|volume=8|pages=63–78|doi=10.1002/macp.1984.020081984106}}{{Cite book|title=Introduction to Polymer Science and Chemistry: A problem solving approach|last=Chanda|first=Manas|publisher=CRC Press|year=2013|location=New York|pages=291, 301–303}} In order to interact with other molecules, the caged particle must diffuse from its solvent cage. The typical lifetime of a solvent cage is 10{{sup|-11}} seconds.{{cite journal | last1 = Herk | first1 = L. | last2 = Feld | first2 = M. | last3 = Szwarc | first3 = M. | year = 1961 | title = Studies of "Cage" Reactions | journal = J. Am. Chem. Soc. | volume = 83 | issue = 14| pages = 2998–3005 | doi=10.1021/ja01475a005}} Many manifestations of the cage effect exist.{{Cite web |title=Radical cage effects |url=https://macmillan.princeton.edu/wp-content/uploads/Radical-Cage-Effects-final-no-layering.pdf}}

In free radical polymerization, radicals formed from the decomposition of an initiator molecule are surrounded by a cage consisting of solvent and/or monomer molecules. Within the cage, the free radicals undergo many collisions leading to their recombination or mutual deactivation.{{Cite journal|last=Braden|first=Dale, A.|year=2001|title=Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity?|journal=Coordination Chemistry Reviews|volume=211|pages=279–294|doi=10.1016/s0010-8545(00)00287-3}} This can be described by the following reaction:

:$$

R\!-\!R

\;\;\underset{k_c}{\overset{k_1}{\rightleftharpoons}}\;\;

\underset{\text{cage pair}}{(R^{\,\bullet},^{\bullet}\!R)}

\;\;\underset{k_D}{\overset{k_d}{\rightleftharpoons}}\;\;

\underset{\text{free radicals}}{2R^{\,\bullet}}

\;\rightarrow\;

\text{Products}

After recombination, free radicals can either react with monomer molecules within the cage walls or diffuse out of the cage. In polymers, the probability of a free radical pair to escape recombination in the cage is 0.1 – 0.01 and 0.3-0.8 in liquids. In unimolecular chemistry, geminate recombination has first been studied in the solution phase using iodine molecules{{Citation |last1=Schwartz |first1=Benjamin J. |title=The Molecular Basis of Solvent Caging |date=1994 |url=https://doi.org/10.1007/978-94-011-0916-1_8 |work=Ultrafast Dynamics of Chemical Systems |pages=235–248 |editor-last=Simon |editor-first=John D. |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-011-0916-1_8 |isbn=978-94-011-0916-1 |access-date=2022-03-28 |last2=King |first2=Jason C. |last3=Harris |first3=Charles B.}} and heme proteins.{{Cite journal |last1=Chernoff |first1=D A |last2=Hochstrasser |first2=R M |last3=Steele |first3=A W |date=1980-10-01 |title=Geminate recombination of O2 and hemoglobin. |journal=Proceedings of the National Academy of Sciences |language=en |volume=77 |issue=10 |pages=5606–5610 |doi=10.1073/pnas.77.10.5606 |pmid=6932659 |issn=0027-8424|doi-access=free |pmc=350115 }}{{Cite journal |last1=Rohlfs |first1=R J |last2=Olson |first2=J S |last3=Gibson |first3=Q H |date=1988-02-05 |title=A comparison of the geminate recombination kinetics of several monomeric heme proteins. |journal=Journal of Biological Chemistry |volume=263 |issue=4 |pages=1803–1813 |doi=10.1016/s0021-9258(19)77948-4 |pmid=3338995 |issn=0021-9258|doi-access=free }} In the solid state, geminate recombination has been demonstrated with small molecules trapped in noble gas solid matrices{{Cite journal |last1=Apkarian |first1=V. A. |last2=Schwentner |first2=N. |date=1999-06-09 |title=Molecular Photodynamics in Rare Gas Solids |url=https://doi.org/10.1021/cr9404609 |journal=Chemical Reviews |volume=99 |issue=6 |pages=1481–1514 |doi=10.1021/cr9404609 |pmid=11849000 |issn=0009-2665}} and in triiodide crystalline compounds.{{Cite journal |last1=Cerullo |first1=Giulio |last2=Garavelli |first2=Marco |date=2017-05-27 |title=Caught in the act |url=https://www.nature.com/articles/nchem.2780 |journal=Nature Chemistry |language=en |volume=9 |issue=6 |pages=506–507 |doi=10.1038/nchem.2780 |pmid=28537591 |issn=1755-4349}}{{Cite journal |last1=Poulin |first1=Peter R. |last2=Nelson |first2=Keith A. |date=2006-09-22 |title=Irreversible Organic Crystalline Chemistry Monitored in Real Time |url=https://www.science.org/doi/abs/10.1126/science.1127826 |journal=Science |volume=313 |issue=5794 |pages=1756–1760 |language=EN |doi=10.1126/science.1127826|pmid=16946037 |s2cid=35002522 |doi-access=free }}{{Cite journal |last1=Xian |first1=Rui |last2=Corthey |first2=Gastón |last3=Rogers |first3=David M. |last4=Morrison |first4=Carole A. |last5=Prokhorenko |first5=Valentyn I. |last6=Hayes |first6=Stuart A. |last7=Miller |first7=R. J. Dwayne |date=2017-03-27 |title=Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation |url=https://www.nature.com/articles/nchem.2751 |journal=Nature Chemistry |language=en |volume=9 |issue=6 |pages=516–522 |doi=10.1038/nchem.2751 |pmid=28537597 |issn=1755-4349|hdl=20.500.11820/52dbea74-99b4-454b-aac2-56c7be20947b |hdl-access=free }}