Hydroxyl radical

{{Short description|Neutral form of the hydroxide ion}} {{More citations needed|date = May 2010}}

{{Chembox | ImageFile = Hydroxyl radical.svg | ImageFile1 = OH orb5.jpg | ImageFileL1 = Hydroxide anion or hydroxyl radical spacefill.png | ImageFileR1 = Hydroxide anion or hydroxyl radical ball.png | ImageFile_Ref = {{chemboximage|correct|??}} | ImageSize1 = 180 | ImageSize = 180 | ImageName = Stick model of the hydroxyl radical with molecular orbitals | IUPACName = Hydroxyl radical | SystematicName = {{ubl|Oxidanyl<ref name = "Hydroxyl (CHEBI:29191)">{{Cite web|title = Hydroxyl (CHEBI:29191)|url = http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:29191|work = Chemical Entities of Biological Interest (ChEBI)|location = UK|publisher = European Bioinformatics Institute}}</ref> (substitutive)|Hydridooxygen(•)<ref name = "Hydroxyl (CHEBI:29191)" /> (additive)}} | OtherNames = {{ubl|Hydroxy|Hydroxyl|λ¹-Oxidanyl}} | Section1 = {{Chembox Identifiers | CASNo = 3352-57-6 | PubChem = 157350 | ChemSpiderID = 138477 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | KEGG = C16844 | KEGG_Ref = {{keggcite|correct|kegg}} | ChEBI = 29191 | Gmelin = 105 | SMILES = [OH] | StdInChI = 1S/HO/h1H | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = TUJKJAMUKRIRHC-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} }} | Section2 = {{Chembox Properties | H=1 | O=1 | pKa = 11.8 to 11.9<ref name=P82db>{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D. D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry 32}}</ref> }} | Section3 = {{Chembox Thermochemistry | DeltaHf = 38.99 kJ mol⁻¹ | Entropy = 183.71 J K⁻¹ mol⁻¹ }} | Section8 = {{Chembox Related |OtherCompounds = O₂H⁺<br />OH⁻<br />O₂²⁻ }} }}

[[Image:Pyrithione-tautomerism-2D-skeletal.png|thumb|Skeletal formulae of 1-hydroxy-2(''1H'')-pyridinethione and its tautomer]] The '''hydroxyl radical''', denoted as '''•OH''' or '''HO•''',{{efn|The dot (•) indicates a free radical, an atom or molecule with an unpaired electron. The notations '''•OH''' and '''•HO''' are chemically identical and used interchangeably. The '''•HO''' order is often used in reaction chemistry (such as astrochemistry) to visually emphasize the roles of the hydrogen and oxygen atoms.}} is the neutral form of the hydroxide ion (OH^–). As a free radical, it is highly reactive and consequently short-lived, making it a pivotal species in radical chemistry.<ref>{{cite book |last1=Finlayson-Pitts |first1=Barbara J. |last2=Pitts |first2=James N. |title=Chemistry of the Upper and Lower Atmosphere |publisher=Academic Press |year=2000 |isbn=978-0-12-257060-5}}</ref>

In nature, hydroxyl radicals are most notably produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. They are also significant in radiation chemistry, where their formation can lead to hydrogen peroxide and oxygen, which in turn can accelerate corrosion and stress corrosion cracking in environments such as nuclear reactor coolant systems. Other important formation pathways include the UV-light dissociation of hydrogen peroxide (H₂O₂) and the Fenton reaction, where trace amounts of reduced transition metals catalyze the breakdown of peroxide.

In organic synthesis, hydroxyl radicals are most commonly generated by photolysis of ''1-Hydroxy-2(1H)-pyridinethione''.

The hydroxyl radical is often referred to as the "detergent" of the troposphere because it reacts with many pollutants, often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone.<ref>{{cite book |author=Forster, P. |author2=V. Ramaswamy |author3=P. Artaxo |author4=T. Berntsen |author5=R. Betts |author6=D.W. Fahey |author7=J. Haywood |author8=J. Lean |author9=D.C. Lowe |author10=G. Myhre |author11=J. Nganga |author12=R. Prinn |author13=G. Raga |author14=M. Schulz |author15=R. Van Dorland |date=2007 |publisher=Cambridge University Press |chapter=Changes in Atmospheric Constituents and in Radiative Forcing |editor=Solomon, S. |editor2=D. Qin |editor3=M. Manning |editor4=Z. Chen |editor5=M. Marquis |editor6=K.B. Averyt |editor7=M.Tignor |editor8=H.L. Miller |title= Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |quote=The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere, destroying about 3.7 Gt of trace gases, including CH4 and all HFCs and HCFCs, each year (Ehhalt, 1999). |chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf }}</ref> The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergo photolysis or are rained out. For instance, methane, which reacts relatively slowly with hydroxyl radicals, has an average lifetime of >5 years and many CFCs have lifetimes of 50+ years. Pollutants, such as larger hydrocarbons, can have very short average lifetimes of less than a few hours.

The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom, forming water and an alkyl radical (R^•): :•OH + RH → H₂O + R• The alkyl radical will typically react rapidly with oxygen forming a peroxy radical: :R• + O₂ → RO₂• The fate of this radical in the troposphere is dependent on factors such as the amount of sunlight, pollution in the atmosphere and the nature of the alkyl radical that formed it (see chapters 12 & 13 in External Links "University Lecture notes on Atmospheric chemistry")<ref>{{cite web |last1=Jimenez |first1=Jose-Luis |title=CHEM-5151 / ATOC-5151 - Atmospheric Chemistry Graduate Course - Spring 2005; Lecture Notes |url=https://cires1.colorado.edu/jimenez/AtmChem/ |website=cires1.colorado.edu |access-date=11 March 2026}}</ref>.

== Biological significance == Hydroxyl radicals can occasionally be produced as a byproduct of immune action. Macrophages and microglia most frequently generate this compound when exposed to very specific pathogens, such as certain bacteria. The destructive action of hydroxyl radicals has been implicated in several neurological autoimmune diseases such as HIV-associated dementia, when immune cells become over-activated and toxic to neighboring healthy cells.<ref>{{cite journal|last=Kincaid-Colton|first=Carol|author2=Wolfgang Streit|title=The Brain's Immune System|journal=Scientific American|date=November 1995|volume=273 |issue=5 |pages=54–5, 58–61 |doi=10.1038/scientificamerican1195-54 |pmid=8966536 |bibcode=1995SciAm.273e..54S }}</ref>

The hydroxyl radical can damage virtually all types of macromolecules: carbohydrates, nucleic acids (mutations), lipids (lipid peroxidation) and amino acids (e.g. conversion of Phe to ''m''-tyrosine and ''o''-tyrosine). The hydroxyl radical has a very short ''in vivo'' half-life of approximately 10⁻⁹ seconds and a high reactivity.<ref>{{cite journal | author=Sies, Helmut | title=Strategies of antioxidant defense |date=March 1993 | journal=European Journal of Biochemistry | volume=215 | issue=2 | pages=213–219 | doi=10.1111/j.1432-1033.1993.tb18025.x | pmid=7688300 }}</ref> This makes it a very dangerous compound to the organism.<ref name="Reiter RJ, Melchiorri D, Sewerynek E, et al. 1995 1–11">{{cite journal | pmid = 7776173 | volume=18 | issue=1 | title=A review of the evidence supporting melatonin's role as an antioxidant |date=January 1995 |vauthors=Reiter RJ, Melchiorri D, Sewerynek E, ''et al.'' | journal=J. Pineal Res. | pages=1–11| doi=10.1111/j.1600-079x.1995.tb00133.x | bibcode=1995JPinR..18....1R | s2cid=24184946 }}</ref><ref>{{cite journal | pmid = 9288572 | doi=10.1055/s-2007-979057 | volume=29 | issue=8 | title=Melatonin in relation to cellular antioxidative defense mechanisms |date=August 1997 |vauthors=Reiter RJ, Carneiro RC, Oh CS | journal=Horm. Metab. Res. | pages=363–72| s2cid=22573377 }}</ref>

Unlike superoxide, which can be detoxified by superoxide dismutase, the hydroxyl radical cannot be eliminated by an enzymatic reaction. Mechanisms for scavenging peroxyl radicals for the protection of cellular structures include endogenous antioxidants such as melatonin and glutathione, and dietary antioxidants such as mannitol and vitamin E.<ref name="Reiter RJ, Melchiorri D, Sewerynek E, et al. 1995 1–11"/>

== Importance in the Earth's atmosphere == The hydroxyl radical (•OH) is one of the main chemical species controlling the oxidizing capacity of the Earth's atmosphere, having a major impact on the concentrations and distribution of greenhouse gases and most air pollutants. Even though •OH is of order 10⁻¹³ of the abundance of atmospheric oxygen,<ref name=":1">{{Citation |last=Crutzen |first=Paul J. |title=Global Tropospheric Chemistry |date=1994 |work=Low-Temperature Chemistry of the Atmosphere |pages=465–498 |editor-last=Moortgat |editor-first=Geert K. |url=http://link.springer.com/10.1007/978-3-642-79063-8_21 |access-date=2025-11-21 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-79063-8_21 |isbn=978-3-642-79065-2 |editor2-last=Barnes |editor2-first=Austin J. |editor3-last=Le Bras |editor3-first=Georges |editor4-last=Sodeau |editor4-first=John R.|url-access=subscription }}</ref> it is still the most widespread oxidizer in the troposphere, the lowest part of the atmosphere.<ref name=":0">{{Cite journal |last1=Prinn |first1=R. |last2=Cunnold |first2=D. |last3=Simmonds |first3=P. |last4=Alyea |first4=F. |last5=Boldi |first5=R. |last6=Crawford |first6=A. |last7=Fraser |first7=P. |last8=Gutzler |first8=D. |last9=Hartley |first9=D. |last10=Rosen |first10=R. |last11=Rasmussen |first11=R. |date=1992-02-20 |title=Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990 |url=https://doi.org/10.1029/91jd02755 |journal=Journal of Geophysical Research: Atmospheres |volume=97 |issue=D2 |pages=2445–2461 |doi=10.1029/91jd02755 |bibcode=1992JGR....97.2445P |issn=0148-0227|url-access=subscription }}</ref><ref name=":1" /> Understanding •OH variability is important to evaluating human impacts on the atmosphere and climate. Because •OH acts as the initiator of a number of photochemical chain reactions,<ref name=":12">{{Citation |last=Crutzen |first=Paul J. |title=Global Tropospheric Chemistry |date=1994 |work=Low-Temperature Chemistry of the Atmosphere |pages=465–498 |editor-last=Moortgat |editor-first=Geert K. |url=http://link.springer.com/10.1007/978-3-642-79063-8_21 |access-date=2025-11-21 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-79063-8_21 |isbn=978-3-642-79065-2 |editor2-last=Barnes |editor2-first=Austin J. |editor3-last=Le Bras |editor3-first=Georges |editor4-last=Sodeau |editor4-first=John R.|url-access=subscription }}</ref> it has a lifetime in the Earth's atmosphere of less than one second.<ref>{{Cite journal |last=Isaksen |first=I.S.A. |author2=S.B. Dalsøren |year=2011 |title=Getting a better estimate of an atmospheric radical |url=http://www.sciencemag.org/content/331/6013/38.short |journal=Science |volume=331 |issue=6013 |pages=38–39 |bibcode=2011Sci...331...38I |doi=10.1126/science.1199773 |pmid=21212344 |s2cid=206530807 |url-access=subscription |access-date=2011-01-09}}</ref> Understanding the role of •OH in the oxidation process of methane (CH₄) present in the atmosphere to first carbon monoxide (CO) and then carbon dioxide (CO₂) is important for assessing the residence time of this greenhouse gas, the overall carbon budget of the troposphere, and its influence on the process of global warming.

The main production pathway of the OH radical in the troposphere comes from the photolysis of ozone at wavelengths less than 320 nm.<ref name=":3">{{Cite journal |last1=Jones |first1=I. T. N. |last2=Wayne |first2=R. P. |date=1970-10-20 |title=The photolysis of ozone by ultraviolet radiation. IV. Effect of photolysis wavelength on primary step |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1970.0178 |journal=Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences |language=en |volume=319 |issue=1537 |pages=273–287 |doi=10.1098/rspa.1970.0178 |bibcode=1970RSPSA.319..273J |issn=0080-4630|url-access=subscription }}</ref><ref name=":4">{{Cite journal |last=Crutzen |first=Paul J. |date=1996-09-06 |title=My Life with O₃, NO_''x'', and Other YZO_''x'' Compounds (Nobel Lecture) |url=https://doi.org/10.1002/anie.199617581 |journal=Angewandte Chemie International Edition in English |volume=35 |issue=16 |pages=1758–1777 |doi=10.1002/anie.199617581 |issn=0570-0833|url-access=subscription }}</ref> The excited atomic oxygen formed as a result, O(¹D), reacts very quickly with water vapor, H₂O, forming <u>two</u> hydroxyl radicals.<ref name=":2">{{Cite journal |last1=Armerding |first1=W. |last2=Comes |first2=F. J. |last3=Schuelke |first3=B. |date=1995-03-01 |title=O(1D) Quantum Yields of Ozone Photolysis in the UV from 300 nm to Its Threshold and at 355 nm |url=https://doi.org/10.1021/j100010a025 |journal=The Journal of Physical Chemistry |volume=99 |issue=10 |pages=3137–3143 |doi=10.1021/j100010a025 |bibcode=1995JPhCh..99.3137A |issn=0022-3654|url-access=subscription }}</ref>

<chem>{O3} + h\nu \longrightarrow {O2} + {O(^1D)} \quad \lambda < 320 { nm}</chem>

<chem>{O(^1D)} + {H2O} \longrightarrow 2 {.}{OH} </chem>

Because •OH production in the troposphere relies on the abundance of UV-B radiation, its formation rate is maximized around the equator.<ref name=":5">{{Cite book |last=Warneck |first=Peter |title=Chemistry of the natural atmosphere |date=2000 |publisher=Academic Press |isbn=978-0-12-735632-7 |edition=2nd |series=This is volume 71 in the International geophysics series |location=San Diego}}</ref> The abundance of water vapor around the ITCZ, near the equator,<ref name=":6">{{Cite journal |last1=Sun |first1=De-Zheng |last2=Lindzen |first2=Richard S. |date=1993 |title=Distribution of Tropical Tropospheric Water Vapor |url=http://journals.ametsoc.org/doi/10.1175/1520-0469(1993)0502.0.CO;2 |journal=Journal of the Atmospheric Sciences |language=en |volume=50 |issue=12 |pages=1643–1660 |doi=10.1175/1520-0469(1993)050<1643:DOTTWV>2.0.CO;2 |bibcode=1993JAtS...50.1643S |issn=0022-4928}}</ref> helps the second reaction to evolve quickly.

The photolysis of hydrogen peroxide (H₂O₂) is another common way hydroxyl radicals are produced.<ref name=":7">{{Cite journal |last1=Aye |first1=Thin Thin |last2=Low |first2=Teck Yew |last3=Sze |first3=Siu Kwan |date=2005-09-01 |title=Nanosecond Laser-Induced Photochemical Oxidation Method for Protein Surface Mapping with Mass Spectrometry |url=https://pubs.acs.org/doi/10.1021/ac050353m |journal=Analytical Chemistry |language=en |volume=77 |issue=18 |pages=5814–5822 |doi=10.1021/ac050353m |pmid=16159110 |bibcode=2005AnaCh..77.5814A |issn=0003-2700|url-access=subscription }}</ref> This occurs at wavelengths less than 300 nm, maximizing its photolysis at 250 nm.<ref name=":7" />

<chem>{H2O2} + h\nu \longrightarrow 2 {.}{OH} \quad \lambda < {300 nm} </chem>

Because the lifetime of •OH radicals in the Earth's atmosphere is very short, •OH concentrations in the air are very low and very sensitive techniques are required for its direct detection.<ref>{{Cite journal |last=Heal |first=M.R. |author2=Heard, D.E. |author3=Pilling, M.J. |author4=Whitaker, B.J. |year=1995 |title=On the development and validation of FAGE for local measurement of tropospheric HO and HO2 |journal=Journal of the Atmospheric Sciences |volume=52 |issue=19 |pages=3428–3448 |bibcode=1995JAtS...52.3428H |doi=10.1175/1520-0469(1995)052<3428:OTDAVO>2.0.CO;2 |issn=1520-0469 |doi-access=free}}</ref> Global average hydroxyl radical concentrations have been measured indirectly by analyzing methyl chloroform (CH₃CCl₃) present in the air. The results obtained by Montzka ''et al.'' (2011)<ref>{{Cite journal | last = Montzka | first = S.A. |author2=M. Krol |author3=E. Dlugokencky |author4=B. Hall |author5=P. Jöckel |author6=J. Lelieveld | year = 2011 | title = Small interannual variability of global atmospheric hydroxyl | journal = Science | pmid = 21212353 | volume = 331 | issue = 6013 | pages = 67–69 | doi = 10.1126/science.1197640 | url = http://www.sciencemag.org/content/331/6013/67.abstract | access-date = 2011-01-09 |bibcode = 2011Sci...331...67M | s2cid = 11001130 | url-access = subscription }}</ref> show that the interannual variability in •OH estimated from CH₃CCl₃ measurements is small, less than 2%, indicating that global •OH is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases primarily oxidized by •OH, as well as global photochemical model calculations.

== Astronomical importance == === First detection of interstellar •HO === The first experimental evidence for the presence of 18 cm absorption lines of the hydroxyl (•HO) radical in the radio absorption spectrum of Cassiopeia A was obtained by Weinreb et al. (Nature, Vol. 200, pp. 829, 1963) based on observations made during the period October 15–29, 1963.<ref name="DieterEwen1964">{{cite journal |last1=Dieter |first1=N. H. |last2=Ewen |first2=H. I. |title=Radio Observations of the Interstellar OH Line at 1,667 Mc/s |journal=Nature |volume=201 |issue=4916 |year=1964 |pages=279–281 |issn=0028-0836 |doi=10.1038/201279b0|bibcode=1964Natur.201..279D |s2cid=4163406 }}</ref>

=== Important subsequent reports of •HO astronomical detections === {| class="wikitable" ! Year ! Description |- | 1967 | ''•HO Molecules in the Interstellar Medium''. Robinson and McGee. One of the first observational reviews of •HO observations. •HO had been observed in absorption and emission, but at this time the processes which populate the energy levels are not yet known with certainty, so the article does not give good estimates of •HO densities.<ref name="RobinsonMcGee1967">{{cite journal |last1=Robinson |first1=B J |last2=McGee |first2=R X |title=Oh Molecules in the Interestellar Medium |journal=Annual Review of Astronomy and Astrophysics |volume=5 |issue=1 |year=1967 |pages=183–212 |issn=0066-4146 |doi=10.1146/annurev.aa.05.090167.001151|bibcode=1967ARA&A...5..183R }}</ref> |- | 1967 | ''Normal •HO Emission and Interstellar Dust Clouds''. Heiles. First detection of normal emission from •HO in interstellar dust clouds.<ref name="Heiles1968">{{cite journal |last1=Heiles |first1=Carl E. |title=Normal OH Emission and Interstellar Dust Clouds |journal=The Astrophysical Journal |volume=151 |year=1968 |page=919 |issn=0004-637X |doi=10.1086/149493|bibcode=1968ApJ...151..919H }}</ref> |- | 1971 | ''Interstellar molecules and dense clouds''. D. M. Rank, C. H. Townes, and W. J. Welch. Review of the epoch about molecular line emission of molecules through dense clouds.<ref name="RankTownes1971">{{cite journal |last1=Rank |first1=D. M. |last2=Townes |first2=C. H. |last3=Welch |first3=W. J. |title=Interstellar Molecules and Dense Clouds |journal=Science |volume=174 |issue=4014 |year=1971 |pages=1083–1101 |issn=0036-8075 |doi=10.1126/science.174.4014.1083|pmid=17779392 |bibcode=1971Sci...174.1083R |s2cid=43499656 }}</ref> |- | 1980 | ''•HO observations of molecular complexes in Orion and Taurus''. Baud and Wouterloot. Map of •HO emission in molecular complexes Orion and Taurus. Derived column densities are in good agreement with previous CO results.<ref name="BaudWouterloot1980">{{citation |title=OH observations of molecular complexes in Orion and Taurus |bibcode=1980A&A....90..297B |last1=Baud |first1=B. |last2=Wouterloot |first2=J. G. A. |journal=Astronomy and Astrophysics |year=1980|volume=90 |page=297 }}</ref> |- | 1981 | ''Emission-absorption observations of •HO in diffuse interstellar clouds''. Dickey, Crovisier and Kazès. Observations of fifty-eight regions which show HI absorption were studied. Typical densities and excitation temperature for diffuse clouds are determined in this article.<ref name="DickeyCrovisier1981" /> |- | 1981 | ''Magnetic fields in molecular clouds—•HO Zeeman observations''. Crutcher, Troland and Heiles. •HO Zeeman observations of the absorption lines produced in interstellar dust clouds toward 3C 133, 3C 123, and W51.<ref name="CrutcherTroland1981">{{cite journal |last1=Crutcher |first1=R. M. |last2=Troland |first2=T. H. |last3=Heiles |first3=C. |title=Magnetic fields in molecular clouds - OH Zeeman observations |journal=The Astrophysical Journal |volume=249 |year=1981 |page=134 |issn=0004-637X |doi=10.1086/159268|bibcode=1981ApJ...249..134C }}</ref> |- | 1981 | ''Detection of interstellar •HO in the Far-Infrared''. J. Storey, D. Watson, C. Townes. Strong absorption lines of •HO were detected at wavelengths of 119.23 and 119.44 microns in the direction of Sgr B2.<ref name="StoreyWatson1981">{{cite journal |last1=Storey |first1=J. W. V. |last2=Watson |first2=D. M. |last3=Townes |first3=C. H. |title=Detection of interstellar OH in the far-infrared |journal=The Astrophysical Journal |volume=244 |year=1981 |pages=L27 |issn=0004-637X |doi=10.1086/183472|bibcode=1981ApJ...244L..27S }}</ref> |- | 1989 | ''Molecular outflows in powerful •HO megamasers''. Baan, Haschick and Henkel. Observations of •H and •HO molecular emission through •HO megamasers galaxies, in order to get a FIR luminosity and maser activity relation.<ref name="BaanHaschick1989">{{cite journal |last1=Baan |first1=Willem A. |last2=Haschick |first2=Aubrey D. |last3=Henkel |first3=Christian |title=Molecular outflows in powerful OH megamasers |journal=The Astrophysical Journal |volume=346 |year=1989 |page=680 |issn=0004-637X |doi=10.1086/168050|bibcode=1989ApJ...346..680B }}</ref> |}

=== Energy levels === •HO is a diatomic molecule. The electronic angular momentum along the molecular axis is +1 or −1, and the electronic spin angular momentum S=1/2. Because of the orbit-spin coupling, the spin angular momentum can be oriented in parallel or anti-parallel directions to the orbital angular momentum, producing the splitting into Π_1/2 and Π_3/2 states. The ²Π_3/2 ground state of •HO is split by lambda doubling interaction (an interaction between the nuclei rotation and the unpaired electron motion around its orbit). Hyperfine interaction with the unpaired spin of the proton further splits the levels.

=== Chemistry of the molecule •HO === In order to study gas phase interstellar chemistry, it is convenient to distinguish two types of interstellar clouds: diffuse clouds, with T=30–100 K, and n=10–1000 cm⁻³, and dense clouds with T=10–30K and density n={{val|e=4}}–{{val|e=3|u=cm⁻³}}. Ion-chemical routes in both dense and diffuse clouds have been established for some works (Hartquist 1990).

==== •HO production pathways ==== The •HO radical is linked with the production of H₂O in molecular clouds. Studies of •HO distribution in Taurus Molecular Cloud-1 (TMC-1)<ref name="HarjuWinnberg2000">{{citation |title=The distribution of OH in Taurus Molecular Cloud-1 |bibcode=2000A&A...353.1065H |last1=Harju |first1=J. |last2=Winnberg |first2=A. |last3=Wouterloot |first3=J. G. A. |journal=Astronomy and Astrophysics |year=2000|volume=353 |page=1065 }}</ref> suggest that in dense gas, •HO is mainly formed by dissociative recombination of {{chem2|H3O+}}. Dissociative recombination is the reaction in which a molecular ion recombines with an electron and dissociates into neutral fragments. Important formation mechanisms for •HO are:

{{chem2|H3O+ + e- -> •HO + H2}} (1a) Dissociative recombination {{chem2|H3O+ + e- -> •HO + •H + •H}} (1b) Dissociative recombination {{chem2|HCO2+ + e- -> •HO + CO}} (2a) Dissociative recombination {{chem2|•O + HCO -> •HO + CO}} (3a) Neutral-neutral {{chem2|H- + H3O+ -> •HO + H2 + •H}} (4a) Ion-molecular ion neutralization

==== •HO destruction pathways ==== Experimental data on association reactions of •H and •HO suggest that radiative association involving atomic and diatomic neutral radicals may be considered as an effective mechanism for the production of small neutral molecules in the interstellar clouds.<ref name="FieldAdams1980">{{citation |title=Molecular synthesis in interstellar clouds - The radiative association reaction H + OH yields H2O + h/nu/ |bibcode=1980MNRAS.192....1F |last1=Field |first1=D. |last2=Adams |first2=N. G. |last3=Smith |first3=D. |journal=Monthly Notices of the Royal Astronomical Society |year=1980|volume=192 |page=1 |doi=10.1093/mnras/192.1.1 |doi-access=free }}</ref> The formation of O₂ occurs in the gas phase via the neutral exchange reaction between •O and •HO, which is also the main sink for •HO in dense regions.<ref name="HarjuWinnberg2000" />

We can see that atomic oxygen takes part both in the production and destruction of •HO, so the abundance of •HO depends mainly on the abundance of {{chem2|H3+|link=trihydrogen cation}}. Then, important chemical pathways leading from •HO radicals are:

{{chem2|•HO + •O -> O2 + •H}} (1A) Neutral-neutral

{{chem2|•HO + C+ -> CO+ + •H}} (2A) Ion-neutral

{{chem2|•HO + •N -> NO + •H}} (3A) Neutral-neutral

{{chem2|•HO + C -> CO + •H}} (4A) Neutral-neutral

{{chem2|•HO + •H -> H2O + photon}} (5A) Neutral-neutral

==== Rate constants and relative rates for important formation and destruction mechanisms ==== Rate constants can be derived from the UMIST Database for Astrochemistry.<ref>{{cite web |title=The UMIST Database for Astrochemistry |url=https://udfa.ajmarkwick.net/ |website=udfa.ajmarkwick.net |access-date=2023-10-27}}</ref> Rate constants have the form: :<math>k(T) = \alpha\left(\frac{T}{300}\right)^\beta e^{-\frac{\gamma}{T}}</math> The following table has the rate constants calculated for a typical temperature in a dense cloud (10 K).

{| class="wikitable" |- ! Reaction ! <math>k(T=10\ \text{K})</math> / cm³s⁻¹ |- | 1a | <math>3.29\times 10^{-6}</math> |- | 1b | <math>1.41\times 10^{-7}</math> |- | 2a | <math>4.71\times 10^{-7}</math> |- | 3a | <math>5.0\times 10^{-11}</math> |- | 4a | <math>1.26\times 10^{-6}</math> |- | 5a | <math>2.82\times 10^{-6}</math> |- | 1A | <math>7.7\times 10^{-10}</math> |- | 2A | <math>3.5\times 10^{-11}</math> |- | 3A | <math>1.38\times 10^{-10}</math> |- | 4A | <math>1.0\times 10^{-10}</math> |- | 5A | <math>3.33\times 10^{-14}</math> |}

Formation rates (''r''_ix) can be obtained using the rate constants ''k''(''T'') and the abundances of the reactant species C and D: :''r''_ix = ''k''(''T'')_ix[''C''] [''D''] where [''Y''] represents the abundance of the species ''Y''. In this approach, abundances were taken from the 2006 UMIST database, and the values are relative to the H₂ density. The following table shows rates for each pathway relative to pathway 1a (as the ratio ''r''_ix/''r''_1a) in order to compare the contributions of each to hydroxyl formation.

{| class="wikitable" |- ! ! ''r''_1a ! ''r''_1b ! ''r''_2a ! ''r''_3a ! ''r''_4a ! ''r''_5a |- | '''Relative Rate''' | <math>1.0</math> | <math>4.3\times 10^{-2}</math> | <math>1.3\times 10^{-2}</math> | <math>3.5\times 10^{-2}</math> | <math>3.6\times 10^{-5}</math> | <math>6.79\times 10^{-1}</math> |}

The results suggest that pathway 1a is the most prominent mode of hydroxyl formation in dense clouds, which is consistent with the report from Harju ''et al.''.<ref name="HarjuWinnberg2000" />

The contributions of different pathways to hydroxyl destruction can be similarly compared:

{| class="wikitable" |- ! ! ''r''_1A ! ''r''_2A ! ''r''_3A ! ''r''_4A ! ''r''_5A |- | '''Relative Rate''' | <math>1.0</math> | <math>6.14\times 10^{-3}</math> | <math>1.52\times 10^{-1}</math> | <math>3.6\times 10^{-5}</math> | <math>4.29\times 10^{-3}</math> |}

These results demonstrate that reaction 1A is the main hydroxyl sink in dense clouds.

=== Importance of interstellar •HO observations === Discoveries of the microwave spectra of a considerable number of molecules prove the existence of rather complex molecules in the interstellar clouds and provide the possibility to study dense clouds, which are obscured by the dust they contain.<ref>{{cite journal |vauthors=Rank DM, Townes CH, Welch WJ | title=Interstellar Molecules and Dense Clouds | journal=Science | date=1971-12-01 | volume=174 | issue=4014 | pages=1083–1101 | url=http://www.sciencemag.org/cgi/content/refs/174/4014/1083 | access-date=2009-01-13 | doi=10.1126/science.174.4014.1083 | pmid=17779392 |bibcode = 1971Sci...174.1083R | s2cid=43499656 | url-access=subscription }}</ref> The •HO molecule has been observed in the interstellar medium since 1963 through its 18-cm transitions.<ref>{{cite journal |vauthors=Dieter NH, Ewen HI | title=Radio Observations of the Interstellar HO Line at 1,667 Mc/s | journal=Nature | volume=201 | issue=4916 | pages=279–281 | date=1964-01-18 | doi=10.1038/201279b0 | url=http://www.nature.com/nature/journal/v201/n4916/abs/201279b0.html | access-date=2009-01-13 |bibcode = 1964Natur.201..279D | s2cid=4163406 | url-access=subscription }}</ref> In the subsequent years, •HO was observed by its rotational transitions at far-infrared wavelengths, mainly in the Orion region. Because each rotational level of •HO is split by lambda doubling, astronomers can observe a wide variety of energy states from the ground state.

==== •HO as a tracer of shock conditions ==== Very high densities are required to thermalize the rotational transitions of •HO,<ref>{{cite journal |vauthors=Storey JW, Watson DM, Townes CH | title=Detection of interstellar HO in the far-infrared | journal=Astrophysical Journal, Part 2 - Letters to the Editor | volume=244 | date=1981-02-15 | pages=L27–L30 | doi=10.1086/183472 | bibcode=1981ApJ...244L..27S }}</ref> so it is difficult to detect far-infrared emission lines from a quiescent molecular cloud. Even at H₂ densities of 10⁶ cm⁻³, dust must be optically thick at infrared wavelengths. But the passage of a shock wave through a molecular cloud is precisely the process which can bring the molecular gas out of equilibrium with the dust, making observations of far-infrared emission lines possible. A moderately fast shock may produce a transient raise in the •HO abundance relative to hydrogen. So, it is possible that far-infrared emission lines of •HO can be a good diagnostic of shock conditions.

==== In diffuse clouds ==== Diffuse clouds are of astronomical interest because they play a primary role in the evolution and thermodynamics of the ISM. Observation of the abundant atomic hydrogen in 21 cm has shown good signal-to-noise ratio in both emission and absorption. Nevertheless, HI observations have a fundamental difficulty when they are directed to low-mass regions of the hydrogen nucleus, such as the center part of a diffuse cloud: the thermal width of hydrogen lines are of the same order as the internal velocity structures of interest, so cloud components of various temperatures and central velocities are indistinguishable in the spectrum. Molecular line observations in principle do not suffer from these problems. Unlike HI, molecules generally have an excitation temperature T_ex << T_kin, so that emission is very weak even from abundant species. CO and •HO are considered to be the most easily studied candidate molecules. CO has transitions in a region of the spectrum (wavelength < 3 mm) where there are not strong background continuum sources, but •HO has the 18 cm emission line, convenient for absorption observations.<ref name="DickeyCrovisier1981">{{cite journal |vauthors=Dickey JM, Crovisier J, Kazes I | title=Emission-absorption observations of •HO in diffuse interstellar clouds | journal=Astronomy and Astrophysics | volume=98 | issue=2 |date=May 1981 | pages=271–285 | bibcode=1981A&A....98..271D}}</ref> Observation studies provide the most sensitive means of detection for molecules with sub-thermal excitation, and can give the opacity of the spectral line, which is a central issue to model the molecular region.

Studies based in the kinematic comparison of •HO and HI absorption lines from diffuse clouds are useful in determining their physical conditions, especially because heavier elements provide higher velocity resolution.

==== •HO masers ==== •HO masers, a type of astrophysical maser, were the first masers to be discovered in space and have been observed in more environments than any other type of maser.

In the Milky Way, •HO masers are found in stellar masers (evolved stars), interstellar masers (regions of massive star formation), or in the interface between supernova remnants and molecular material. Interstellar HO masers are often observed from molecular material surrounding ultracompact H II regions (UC H II). But there are masers associated with very young stars that have yet to create UC H II regions.<ref>{{cite journal |vauthors=Argon AL, Reid MJ, Menten KM | title=A class of interstellar •HO masers associated with protostellar outflows |date=August 2003 | journal=The Astrophysical Journal | volume=593 | issue=2 | pages=925–930 | doi=10.1086/376592 | bibcode=2003ApJ...593..925A |arxiv = astro-ph/0304565 }}</ref> This class of •HO masers appears to form near the edges of very dense material, places where H₂O masers form, and where total densities drop rapidly and UV radiation from young stars can dissociate H₂O molecules. So, observations of •HO masers in these regions can be an important way to probe the distribution of the important H₂O molecule in interstellar shocks at high spatial resolutions.

== Application in water purification == Hydroxyl radicals also play a key role in the oxidative destruction of organic pollutants.<ref>[https://theconversation.com/la-materia-de-la-que-estan-hechos-los-rayos-puede-ayudarnos-a-depurar-el-agua-y-a-afrontar-la-sequia-225516 The Conversation (Spanish Edition): The material that rays are made of can help us purify water and deal with drought Published: March 21, 2024 22:42 CET]</ref>

== See also == * Hydroxyl ion absorption * Hydrogen darkening

== Notes == {{notelist}}

== References == {{reflist|2}} {{refbegin}} * {{cite journal |vauthors=Downes A, Blunt TP | year = 1879 | title = The effect of sunlight upon hydrogen peroxide | url = | journal = Nature | volume = 20 | issue = 517| page = 521 | doi = 10.1038/020521a0 | bibcode = 1879Natur..20Q.521. }} {{refend}}

== External links == * [http://www.physorg.com/news130065276.html Hydroxyl found in atmosphere of Venus.] * [http://cires.colorado.edu/jimenez/AtmChem/ University lecture notes from the University of Colorado on Atmospheric Chemistry.]

{{Molecules detected in outer space}} {{DEFAULTSORT:Hydroxyl Radical}} Category:Alcohols Category:Biological processes Category:Environmental chemistry Category:Free radicals Category:Hydroxides Category:Reactive intermediates