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| Names | |||
|---|---|---|---|
| IUPAC name Hydroxyl radical | |||
| Systematic IUPAC name | |||
Other names
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| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| 105 | |||
| KEGG |
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| Properties | |||
| HO | |||
| Molar mass | 17.007 g·mol−1 | ||
| Acidity (pKa) | 11.8 to 11.9[2] | ||
| Thermochemistry | |||
Std molar entropy(S⦵298) | 183.71 J K−1 mol−1 | ||
Std enthalpy of formation(ΔfH⦵298) | 38.99 kJ mol−1 | ||
| Related compounds | |||
Related compounds | O2H+ OH− O22− | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Thehydroxyl radical, denoted as•OH orHO•,[a] 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 inradical chemistry.[3]
In nature, hydroxyl radicals are most notably produced from the decomposition ofhydroperoxides (ROOH) or, inatmospheric chemistry, by the reaction ofexcitedatomic oxygen with water. They are also significant in radiation chemistry, where their formation can lead to hydrogen peroxide and oxygen, which in turn can acceleratecorrosion andstress corrosion cracking in environments such as nuclear reactor coolant systems. Other important formation pathways include the UV-light dissociation ofhydrogen peroxide (H2O2) and theFenton reaction, where trace amounts of reduced transition metals catalyze the breakdown of peroxide.
Inorganic synthesis, hydroxyl radicals are most commonly generated byphotolysis of1-Hydroxy-2(1H)-pyridinethione.
The hydroxyl radical is often referred to as the "detergent" of thetroposphere because it reacts with many pollutants, often acting as the first step to their removal. It also has an important role in eliminating somegreenhouse gases likemethane andozone.[4] The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergophotolysis or arerained out. For instance, methane, which reacts relatively slowly with hydroxyl radicals, has an average lifetime of >5 years and manyCFCs have lifetimes of 50+ years. Pollutants, such as largerhydrocarbons, can have very short average lifetimes of less than a few hours.
The first reaction with manyvolatile organic compounds (VOCs) is the removal of a hydrogen atom, forming water and analkyl radical (R•):
The alkyl radical will typically react rapidly withoxygen forming aperoxy radical:
The fate of this radical in thetroposphere is dependent on factors such as the amount of sunlight, pollution in the atmosphere and the nature of thealkyl radical that formed it (see chapters 12 & 13 in External Links "University Lecture notes on Atmospheric chemistry").
Hydroxyl radicals can occasionally be produced as a byproduct ofimmune action.Macrophages andmicroglia most frequently generate this compound when exposed to very specificpathogens, such as certain bacteria. The destructive action of hydroxyl radicals has been implicated in severalneurologicalautoimmune diseases such asHIV-associated dementia, when immune cells become over-activated and toxic to neighboring healthy cells.[5]
The hydroxyl radical can damage virtually all types of macromolecules: carbohydrates, nucleic acids (mutations), lipids (lipid peroxidation) and amino acids (e.g. conversion ofPhe tom-tyrosine ando-tyrosine). The hydroxyl radical has a very shortin vivohalf-life of approximately 10−9 seconds and a high reactivity.[6] This makes it a very dangerous compound to the organism.[7][8]
Unlikesuperoxide, which can be detoxified bysuperoxide dismutase, the hydroxyl radical cannot be eliminated by anenzymatic reaction. Mechanisms for scavenging peroxyl radicals for the protection ofcellular structures include endogenousantioxidants such asmelatonin andglutathione, and dietaryantioxidants such asmannitol andvitamin E.[7]
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 pollutants. It is the most widespread oxidizer in thetroposphere, the lowest part of the atmosphere. Understanding •OH variability is important to evaluating human impacts on the atmosphere and climate. The •OH species has a lifetime in the Earth's atmosphere of less than one second.[9] 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 lifetime of •OH radicals in the Earth's atmosphere is very short; therefore, •OH concentrations in the air are very low and very sensitive techniques are required for its direct detection.[10] Global average hydroxyl radical concentrations have been measured indirectly by analyzingmethyl chloroform (CH3CCl3) present in the air. The results obtained by Montzkaet al. (2011)[11] show that the interannual variability in •OH estimated from CH3CCl3 measurements is small, indicating that global •OH is generally well buffered against perturbations. This small variability is consistent with measurements ofmethane and other trace gases primarily oxidized by •OH, as well as global photochemical model calculations.
The first experimental evidence for the presence of 18 cm absorption lines of the hydroxyl (•HO) radical in the radio absorption spectrum ofCassiopeia A was obtained by Weinreb et al. (Nature, Vol. 200, pp. 829, 1963) based on observations made during the period October 15–29, 1963.[12]
| 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.[13] |
| 1967 | Normal •HO Emission and Interstellar Dust Clouds. Heiles. First detection of normal emission from •HO in interstellar dust clouds.[14] |
| 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.[15] |
| 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.[16] |
| 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.[17] |
| 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.[18] |
| 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.[19] |
| 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.[20] |
•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 Π1/2 and Π3/2 states. The2Π3/2 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.
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=104–103 cm−3. Ion-chemical routes in both dense and diffuse clouds have been established for some works (Hartquist 1990).
The •HO radical is linked with the production of H2O in molecular clouds. Studies of •HO distribution in Taurus Molecular Cloud-1 (TMC-1)[21] suggest that in dense gas, •HO is mainly formed by dissociative recombination ofH3O+. 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:
H3O+ + e− → •HO + H2 (1a) Dissociative recombinationH3O+ + e− → •HO + •H + •H (1b) Dissociative recombinationHCO+2 + e− → •HO + CO (2a) Dissociative recombination•O + HCO → •HO + CO (3a) Neutral-neutralH− + H3O+ → •HO + H2 + •H (4a) Ion-molecular ion neutralization
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.[22] 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.[21]
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 ofH+3. Then, important chemical pathways leading from •HO radicals are:
•HO + •O → O2 + •H (1A) Neutral-neutral
•HO + C+ → CO+ + •H (2A) Ion-neutral
•HO + •N → NO + •H (3A) Neutral-neutral
•HO + C → CO + •H (4A) Neutral-neutral
•HO + •H → H2O + photon (5A) Neutral-neutral
Rate constants can be derived from the UMIST Database for Astrochemistry.[23] Rate constants have the form:
The following table has the rate constants calculated for a typical temperature in a dense cloud (10 K).
| Reaction | / cm3s−1 |
|---|---|
Formation rates (rix) can be obtained using the rate constantsk(T) and the abundances of the reactant species C and D:
where [Y] represents the abundance of the speciesY. 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 ratiorix/r1a) in order to compare the contributions of each to hydroxyl formation.
| r1a | r1b | r2a | r3a | r4a | r5a | |
|---|---|---|---|---|---|---|
| Relative Rate |
The results suggest that pathway 1a is the most prominent mode of hydroxyl formation in dense clouds, which is consistent with the report from Harjuet al..[21]
The contributions of different pathways to hydroxyl destruction can be similarly compared:
| r1A | r2A | r3A | r4A | r5A | |
|---|---|---|---|---|---|
| Relative Rate |
These results demonstrate that reaction 1A is the main hydroxyl sink in dense clouds.
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.[24] The •HO molecule has been observed in the interstellar medium since 1963 through its 18-cm transitions.[25] 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.
Very high densities are required to thermalize the rotational transitions of •HO,[26] 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.
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 anexcitation temperature Tex << Tkin, 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.[17] 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.
•HOmasers, a type ofastrophysical maser, were the first masers to be discovered in space and have been observed in more environments than any other type of maser.
In theMilky 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 ultracompactH II regions (UC H II). But there are masers associated with very young stars that have yet to create UC H II regions.[27] 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 highspatial resolutions.
Hydroxyl radicals also play a key role in theoxidative destruction oforganic pollutants.[28]
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).
