Editing Quantum tunnelling (section)

=== Chemistry ===

==== Energetically forbidden reactions ====
Chemical reactions in the [[interstellar medium]] occur at extremely low energies. Probably the most fundamental ion-molecule reaction involves hydrogen ions with hydrogen molecules.  
The quantum mechanical tunnelling rate for the same reaction using the [[hydrogen]] isotope [[deuterium]], D<sup>−</sup> + H<sub>2</sub> → H<sup>−</sup> + HD, has been measured experimentally in an ion trap.
The deuterium was placed in an [[ion trap]] and cooled. The trap was then filled with hydrogen. At the temperatures used in the experiment, the energy barrier for reaction would not allow the reaction to succeed with classical dynamics alone. Quantum tunneling allowed reactions to happen in rare collisions. It was calculated from the experimental data that collisions happened one in every hundred billion.<ref>{{Cite journal |last1=Wild |first1=Robert |last2=Nötzold |first2=Markus |last3=Simpson |first3=Malcolm |last4=Tran |first4=Thuy Dung |last5=Wester |first5=Roland |date=2023-03-16 |title=Tunnelling measured in a very slow ion–molecule reaction |url=https://www.nature.com/articles/s41586-023-05727-z |journal=Nature |language=en |volume=615 |issue=7952 |pages=425–429 |doi=10.1038/s41586-023-05727-z |pmid=36859549 |bibcode=2023Natur.615..425W |issn=0028-0836|arxiv=2303.14948 }}</ref>

==== Kinetic isotope effect ====
{{Main|Kinetic isotope effect}}

In [[chemical kinetics]], the substitution of a light [[isotope]] of an element with a heavier one typically results in a slower reaction rate. This is generally attributed to differences in the zero-point vibrational energies for chemical bonds containing the lighter and heavier isotopes and is generally modeled using [[transition state theory]]. However, in certain cases, large isotopic effects are observed that cannot be accounted for by a semi-classical treatment, and quantum tunnelling is required. [[Ronnie Bell (chemist)|R. P. Bell]] developed a modified treatment of Arrhenius kinetics that is commonly used to model this phenomenon.<ref>{{Cite book |last=Bell |first=Ronald Percy |title=The tunnel effect in chemistry |date=1980 |publisher=Chapman and Hall |isbn=0412213400 |location=London |oclc=6854792}}</ref>

==== Astrochemistry in interstellar clouds ====
By including quantum tunnelling, the [[Astrochemistry|astrochemical]] syntheses of various molecules in [[interstellar cloud]]s can be explained, such as the synthesis of [[Hydrogen#Spin isomers|molecular hydrogen]], [[water]] ([[ice]]) and the [[Abiogenesis|prebiotic]] important [[formaldehyde]].<ref name="Trixler2013" /> Tunnelling of molecular hydrogen has been observed in the lab.<ref>{{Cite journal |last1=Wild |first1=Robert |last2=Nötzold |first2=Markus |last3=Simpson |first3=Malcolm |last4=Tran |first4=Thuy Dung |last5=Wester |first5=Roland |date=2023-03-01 |title=Tunnelling measured in a very slow ion–molecule reaction |url=https://www.nature.com/articles/s41586-023-05727-z |journal=Nature |volume=615 |issue=7952 |language=en |pages=425–429 |doi=10.1038/s41586-023-05727-z |pmid=36859549 |arxiv=2303.14948 |bibcode=2023Natur.615..425W |s2cid=257282176 |issn=1476-4687}}</ref>

==== Quantum biology ====
Quantum tunnelling is among the central non-trivial quantum effects in [[quantum biology]].<ref name="life">{{cite journal |title=Quantum Tunnelling to the Origin and Evolution of Life |year=2013 |pmc=3768233 |last1=Trixler |first1=F. |journal=Current Organic Chemistry |volume=17 |issue=16 |pages=1758–1770 |doi=10.2174/13852728113179990083 |pmid=24039543 }}</ref> Here it is important both as electron tunnelling and [[proton tunneling|proton tunnelling]]. Electron tunnelling is a key factor in many biochemical [[redox reactions]] ([[photosynthesis]], [[cellular respiration]]) as well as enzymatic catalysis. Proton tunnelling is a key factor in spontaneous [[DNA]] mutation.<ref name="Trixler2013" />

Spontaneous mutation occurs when normal DNA replication takes place after a particularly significant proton has tunnelled.<ref>{{cite book |last=Matta |first=Cherif F. |title=Quantum Biochemistry: Electronic Structure and Biological Activity |location=Weinheim |publisher=Wiley-VCH |year=2014 |url=https://books.google.com/books?id=a4JhVFaUOjgC |isbn=978-3-527-62922-0 }}</ref> A hydrogen bond joins DNA base pairs. A double well potential along a hydrogen bond separates a potential energy barrier. It is believed that the double well potential is asymmetric, with one well deeper than the other such that the proton normally rests in the deeper well. For a mutation to occur, the proton must have tunnelled into the shallower well. The proton's movement from its regular position is called a [[Tautomer|tautomeric transition]]. If DNA replication takes place in this state, the base pairing rule for DNA may be jeopardised, causing a mutation.<ref>{{cite book |last=Majumdar |first=Rabi |title=Quantum Mechanics: In Physics and Chemistry with Applications to Bioloty |location=Newi |publisher=PHI Learning |year=2011 |url=https://books.google.com/books?id=IJDvyNVeBiYC |isbn=9788120343047 }}</ref> [[Per-Olov Lowdin]] was the first to develop this theory of spontaneous mutation within the [[Nucleic acid double helix|double helix]]. Other instances of quantum tunnelling-induced mutations in biology are believed to be a cause of ageing and cancer.<ref>{{cite journal |last=Cooper |first=W. G. |date=June 1993 |title=Roles of Evolution, Quantum Mechanics and Point Mutations in Origins of Cancer |journal=Cancer Biochemistry Biophysics |volume=13 |issue=3 |pages=147–170 |pmid=8111728}}</ref>