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Hydroxyl radical

Names
IUPAC name Hydroxyl radical
Systematic IUPAC name Oxidanyl (substitutive) Hydridooxygen(•) (additive)
Other names Hydroxy Hydroxyl λ-Oxidanyl
Identifiers
CAS Number	3352-57-6
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29191
ChemSpider	138477
Gmelin Reference	105
KEGG	C16844
PubChem CID	157350
CompTox Dashboard (EPA)	DTXSID901318744
InChI InChI=1S/HO/h1HKey: TUJKJAMUKRIRHC-UHFFFAOYSA-N
SMILES
Properties
Chemical formula	HO
Molar mass	17.007 g·mol
Acidity (pKa)	11.8 to 11.9
Thermochemistry
Std molar entropy (S298)	183.71 J K mol
Std enthalpy of formation (ΔfH298)	38.99 kJ mol
Related compounds
Related compounds	O2H OH O2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

The hydroxyl radical, HO, is the neutral form of the hydroxide ion (HO). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and stress corrosion cracking in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H₂O₂ (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

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. 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):

HO + 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)

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.

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 seconds and a high reactivity. This makes it a very dangerous compound to the organism.

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.

Importance in the Earth's atmosphere

The hydroxyl HO radicals 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 the troposphere, the lowest part of the atmosphere. Understanding HO variability is important to evaluating human impacts on the atmosphere and climate. The HO species has a lifetime in the Earth atmosphere of less than one second. Understanding the role of HO 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 HO radicals in the Earth atmosphere is very short, therefore HO concentrations in the air are very low and very sensitive techniques are required for its direct detection. 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) shows that the interannual variability in HO estimated from CH₃CCl₃ measurements is small, indicating that global HO is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases primarily oxidized by HO, 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.

Important subsequent reports of HO astronomical detections

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 Π_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, and dense clouds with T=10-30K and density n=10-10 cm. 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) suggest that in dense gas, HO is mainly formed by dissociative recombination of 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:

H₃O + e → HO + H₂ (1a) Dissociative recombination

H₃O + e → HO + H + H (1b) Dissociative recombination

HCO₂ + e → HO + CO (2a) Dissociative recombination

O + HCO → HO + CO (3a) Neutral-neutral

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. 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.

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 H+3 abundance. 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 and relative rates for important formation and destruction mechanisms

Rate constants can be derived from the dataset published on the website . Rate constants have the form:

k(T) = alpha*(T/300)*exp(-gamma/T)cms

The following table has the rate constants calculated for a typical temperature in a dense cloud T=10 K.

Formation rates r_ix can be obtained using the rate constants k(T) and the abundances of the reactants species C and D:

r_ix=k(T)_ix

where represents the abundance of the specie Y. In this approach, abundances were taken from The UMIST database for astrochemistry 2006, and the values are relatives to the H₂ density. Following table shows the ratio r_ix/r_1a in order to get a view of the most important reactions.

The results suggest that (1a) reaction is the most prominent reaction in dense clouds. It is in concordance with Harju et al. 2000.

Next table shows the results by doing the same procedure for destruction reaction:

Results shows that, 1A reaction is the main sink for HO 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 provides the possibility to study dense clouds, which are obscured by the dust they contain. The HO molecule has been observed in the interstellar medium since 1963 through its 18-cm transitions. 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 in 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, so it is difficult to detect far-infrared emission lines from a quiescent molecular cloud. Even at H₂ densities of 10 cm, 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 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 are directed to low mass regions of the hydrogen nucleus, as the center part of a diffuse cloud: Thermal width of hydrogen lines are the same order as the internal velocities' structures of interest, so clouds components of various temperatures and central velocities are indistinguishable in the spectrum. Molecular lines observations in principle doesn't suffer from these problems. Unlike HI, molecules generally have 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. Observation studies provide the most sensitive means of detections of 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. 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 the 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.

See also

• Hydroxyl ion absorption
• Hydrogen darkening

References

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26. 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

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External links

• Hydroxyl found in atmosphere of Venus.
• University lecture notes from the University of Colorado on Atmospheric Chemistry.

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