The hydroxyl radical, •HO, is the neutral form of the hydroxide ion. 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
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 SCC in coolant systems
subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light
dissociation of H2O2 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 radical, 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 is the removal of a hydrogen atom,
forming water and an alkyl radical. •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
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 has HAND 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, lipids and amino acids. PMID 7776173. The hydroxyl radical has a very short
in vivo half-life of approximately 10−9 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
includes endogenous antioxidants such as melatonin and glutathione, and dietary antioxidants
such as mannitol and vitamin E.
Importance in the Earth atmosphere The hydroxyl •HO radicals is one of the
main chemical species controlling the oxidizing capacity of the global Earth atmosphere. This
oxidizing reactive species has a major impact on the concentrations and distribution of
greenhouse gases and pollutants in the Earth atmosphere. 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 present in the atmosphere to first carbon
monoxide and then carbon dioxide 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 present in the air. The results obtained by
Montzka et al. 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.
In 2014, researchers reported their discovery of a "hole" or absence of hydroxyl throughout
the entire depth of the troposphere across a large region of the tropical West Pacific.
They suggested that this hole is permitting large quantities of ozone-degrading chemicals
to reach the stratosphere, and that this may be significantly reinforcing ozone depletion
in the polar regions with potential consequences for the climate of the Earth.
Astronomical importance First detection of interstellar •HO
The first experimental evidence for the presence of 18 cm absorption lines of the hydroxyl
radical in the radio absorption spectrum of Cassiopeia A was obtained by Weinreb et al.
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 Π1/2 and Π3/2 states. The 2Π3/2 ground state of •HO is split
by lambda doubling interaction. 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=104-103 cm−3. Ion chemical routes in both dense and diffuse
clouds have been established for some works. •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 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 Dissociative recombination
H3O+ + e− → •HO + •H + •H Dissociative recombination
HCO2+ + e− → •HO + CO Dissociative recombination •O + HCO → •HO + CO Neutral-neutral
H− + H3O+ → •HO + H2 + •H 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 H3+ abundance. Then, important chemical pathways leading from •HO radicals are:
•HO + •O → O2 + •H Neutral-neutral •HO + C+ → CO+ + •H Ion-neutral
•HO + •N → NO + •H Neutral-neutral •HO + C → CO + •H Neutral-neutral
•HO + •H → H2O + photon Neutral-neutral Rate constants and relative rates for important
formation and destruction mechanisms Rate constants can be derived from the dataset
published in a website. Rate constants have the form:
k(T) = alpha*(TT)cm3s−1 The following table has the rate constants
calculated for a typical temperature in a dense cloud T=10 K.
Formation rates rix can be obtained using the rate constants k(T) and the abundances
of the reactants species C and D: rix=k(T)ix[C][D]
where [Y] 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 rix/r1a in order to get a view of the most
important reactions. The results suggest that 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 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 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 of this problems. Unlike HI, molecules generally have excitation
temperature Tex << Tkin, so that emission is very weak even from abundant species. CO
and •HO are the most easily studied candidates molecules. CO has transitions in a region
of the spectrum where there is 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 subthermal 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, specially 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, interstellar masers, or in the interface between supernova remnants and
molecular material. Interstellar HO masers are often observed from molecular material
surrounding ultracompact H II regions. 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, place where H2O masers form, and where total densities
drop rapidly and UV radiation form 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 spacial resolutions.
Application in water purification Hydroxyl radicals play a key role in the oxidative
destruction of organic pollutant using a series of methodologies collectively known as advanced
oxidation processes. The destruction of pollutants in AOPs is based on the non-selective reaction
of hydroxyl radicals on organic compounds. It is highly effective against a series of
pollutants including pesticides, pharmaceutical compounds, dyes etc.
See also Hydroxyl ion absorption
Hydrogen darkening References
External links Hydroxyl found in atmosphere of Venus.
University lecture notes from the University of Colorado on Atmospheric Chemistry.