# Hydroperoxyl
**Hydroperoxyl**, also called the **perhydroxyl radical** and written HO2• or HOO•, is the neutral radical with the structure H–O–O•, the protonated form of the superoxide anion O2•−. It is a key radical in atmospheric oxidation chemistry, in combustion and autoignition, in radiation chemistry of water, and in the interstellar medium.[1](https://journals.sagepub.com/doi/pdf/10.1089/104454902753759672?cf-mal-redirected=true)
| Key fact | Detail |
|---|---|
| Identity | Protonated form of superoxide; protonation/deprotonation equilibrium has a pKa of about 4.8, so at pH 7.4 superoxide exists predominantly as the anion[1](https://journals.sagepub.com/doi/pdf/10.1089/104454902753759672?cf-mal-redirected=true) • [2](https://doi.org/10.1002/anse.202600003) |
| Structure | H–O bond 0.977 × 10−10 m, O–O bond 1.335 × 10−10 m, H–O–O angle 104.1° (from DO2 rotational spectroscopy); early gas-phase work gave 0.96 Å, 1.3 Å, and about 108°[3](https://tf.nist.gov/general/pdf/275.pdf) • [4](https://cir.nii.ac.jp/crid/1364233269705778432) |
| Vibrational frequencies | 1095, 1390, and 3410 cm−1; ultraviolet absorption maximum at 2100 Å[4](https://cir.nii.ac.jp/crid/1364233269705778432) |
| Thermochemistry | Enthalpy of formation ΔHf0(HO2) = 4.0 ± 0.8 kcal mol−1 (3.3 ± 0.8 kcal mol−1 at 298 K)[5](https://www.sciencedirect.com/science/article/pii/S0368204898002643) |
| Self-reaction | HO2 + HO2 → H2O2 + O2, rate constant 3.6 ± 0.5 × 10−12 cm3 molecule−1 s−1 • [4](https://cir.nii.ac.jp/crid/1364233269705778432) |
| Atmospheric abundance | 1.5–10 × 108 molecules cm−3 at ground level in polluted urban environments[6](https://acp.copernicus.org/articles/22/3693/2022/) |
| Interstellar detection | Five lines detected toward ρ Oph A at about 130 and 260 GHz; fractional abundance about 1 × 10−10, similar to hydrogen peroxide[7](https://arxiv.org/abs/1205.0361) |
## Acid–base relation to superoxide
HO2• is the conjugate acid of the superoxide radical anion. The protonation/deprotonation equilibrium has a pKa of around 4.8, so in a typical cell at pH 7.4 only about 0.3% of superoxide is in the protonated form.[1](https://journals.sagepub.com/doi/pdf/10.1089/104454902753759672?cf-mal-redirected=true) This small protonated fraction matters biologically because the charge states differ sharply in membrane behavior: the permeability coefficient of O2•− through pure lipid membranes is 7.6 × 10−8 cm s−1, while the protonated HOO• is about four orders of magnitude more permeable at 4.9 × 10−4 cm s−1.[2](https://doi.org/10.1002/anse.202600003) [Molecular dynamics](https://www.edgepedia.org/molecular-dynamics) simulations indicate that superoxide is excluded even from the headgroup region, whereas HOO• interacts favorably with carbonyl groups and penetrates deeper into the bilayer.[2](https://doi.org/10.1002/anse.202600003) [Superoxide](https://www.edgepedia.org/superoxide) generated inside an organelle is therefore confined there unless converted to H2O2 by superoxide dismutases or moved through specific anion channels.[2](https://doi.org/10.1002/anse.202600003)
## Molecular structure and spectroscopy
The radical is a light asymmetric rotor with an unpaired electronic spin, which splits each asymmetric rotor level (except 00,0) into a doublet of states.[7](https://arxiv.org/abs/1205.0361) Its gas-phase absorption spectrum was obtained by molecular-modulation spectroscopy, with the radical produced by photolysis of hydrogen peroxide at 2537 Å, of ozone in the presence of hydrogen peroxide, and of Cl2 in the presence of hydrogen peroxide at 3500 Å.[4](https://cir.nii.ac.jp/crid/1364233269705778432) The observed vibrational frequencies of 1095, 1390, and 3410 cm−1 are consistent with the bent geometry, and the ultraviolet absorption maximum lies at 2100 Å, with an absorption cross section there of 4.5 × 10−18 cm2 molecule−1.[4](https://cir.nii.ac.jp/crid/1364233269705778432) A later combination of DO2 measurements near 60 GHz with HO2 rotational constants refined the structure to r(H–O) = 0.977 × 10−10 m, r(O–O) = 1.335 × 10−10 m, and ∠H–O–O = 104.1°.[3](https://tf.nist.gov/general/pdf/275.pdf) A photoionization study of HO2 and H2O2 determined the O–H bond dissociation energy in hydrogen peroxide as D0(H–OOH) = 86.7 ± 0.8 kcal mol−1, yielding the enthalpy of formation of the radical.[5](https://www.sciencedirect.com/science/article/pii/S0368204898002643)
## Role in atmospheric chemistry
HO2 is one half of the HOx radical family (OH + HO2), which drives the photochemical oxidation of the troposphere.[8](https://acp.copernicus.org/articles/23/14393/2023/) Primary sources include photolysis of formaldehyde and other oxygenated volatile organic compounds, alkene ozonolysis, and the reaction sequence in which OH attacks a VOC to form an organic peroxy radical RO2, which in the presence of NO yields HO2 and a carbonyl compound.[8](https://acp.copernicus.org/articles/23/14393/2023/) • [6](https://acp.copernicus.org/articles/22/3693/2022/) HO2 then reacts with NO (HO2 + NO → OH + NO2) or with ozone (HO2 + O3 → OH + 2 O2) to regenerate OH, completing the HOx cycle.[8](https://acp.copernicus.org/articles/23/14393/2023/) The NO2 formed in these reactions photolyzes below 400 nm to give oxygen atoms that combine with O2 to produce tropospheric ozone, so the conversion of NO to NO2 by HO2 and RO2 without ozone destruction is what allows ozone to accumulate when volatile organic compounds are present.[8](https://acp.copernicus.org/articles/23/14393/2023/) • [9](https://www.nationalacademies.org/read/10350/chapter/9) Termination occurs when HO2 combines to form hydrogen peroxide or when OH reacts with NO2 to form HNO3.[9](https://www.nationalacademies.org/read/10350/chapter/9)
**Measured concentrations.** Field campaigns have measured OH and HO2 across forest, marine, remote, polar, rural, suburban, and urban environments; in polluted urban air, ground-level HO2 concentrations are 1.5–10 × 108 molecules cm−3.[6](https://acp.copernicus.org/articles/22/3693/2022/) • [10](https://acp.copernicus.org/articles/22/7005/2022/index.html) In the marine boundary layer, observed OH and HO2 concentrations are generally about 3–8 × 106 and 1–4 × 108 molecules cm−3, and models generally reproduce them to within about 30% or better.[8](https://acp.copernicus.org/articles/23/14393/2023/) Heterogeneous uptake of HO2 on aerosols can be a significant HOx loss route under low-NO conditions, though the parameterization of uptake coefficients carries considerable uncertainty.[8](https://acp.copernicus.org/articles/23/14393/2023/)
## Role in combustion and autoignition
In the oxidation of hydrogen, the standard model consists of 38 reaction steps (19 reversible) among eight species: H2, O2, H2O, H, O, OH, HO2, and H2O2.[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC7158352/) The HO2 radical is described as one of the most important radicals in hydrogen oxidation and in the combustion of all fuels, and its self-reaction to give H2O2 and O2 acts as a terminating step at low temperatures.[12](https://www.sciencedirect.com/science/article/abs/pii/S0010218021007185) Reactions that convert the more potent OH radical into HO2 are effectively chain terminating below about 1100 K.[13](https://www.osti.gov/servlets/purl/1343063)
In hydrocarbon autoignition, the fate of the alkyl radical R + O2 system controls the chemistry: at low temperatures and moderate pressures it forms stabilized alkylperoxy radicals RO2, but at higher temperatures thermal dissociation of RO2 becomes faster and formation of HO2 plus the conjugate alkene begins to dominate.[14](https://www.jstage.jst.go.jp/article/jcombsj/50/151/50_29/_article/-char/en) Internal isomerization of RO2 produces hydroperoxyalkyl radicals (QOOH), whose reactions are the key to low-temperature chain branching; work combining high-level theory with pulsed-photolysis experiments showed that HO2 elimination occurs directly from an alkylperoxy radical without first isomerizing to QOOH, and that the rate coefficients for RO2 isomerization to QOOH may be significantly larger than previously thought.[14](https://www.jstage.jst.go.jp/article/jcombsj/50/151/50_29/_article/-char/en) Hydroperoxides bearing the OOH group, the closed-shell relatives of HO2, are ubiquitous in the atmospheric oxidation of volatile organic compounds near 300 K and in fuel oxidation at roughly 400–1000 K, and they are responsible for two-stage ignition in internal combustion engines.[15](https://www.sciencedirect.com/science/article/pii/S0360128518301175)
## Production and detection
Beyond photolysis of hydrogen peroxide, a convenient non-radiation method of producing HO2 is the chemical reaction of the ceric ion with hydrogen peroxide, a route used in radiation chemistry of water where the radical arises in intraspur reactions.[16](https://www.wipp.energy.gov/library/cra/2009_cra/references/Others/Draganic_Draganic_1971_The_radiation_Chemistry_of_Water.pdf) Instrumentally, HO2 is detected by laser-induced fluorescence in field instruments (calibrated by photolysis of water vapor at 184.9 nm to generate OH and HO2 in a 1:1 ratio) and by near-infrared wavelength-modulation spectroscopy in kinetics reactors.[8](https://acp.copernicus.org/articles/23/14393/2023/) • [17](https://pubmed.ncbi.nlm.nih.gov/18259411/) Early evaluated kinetic data cover its gas-phase reactions with itself and with H2, CO, NO, SO2, O3, alkanes, alkenes, aldehydes, and the atoms H, O, and OH, with the radical's reactivity compared against OH, O, H, and the halogens for atmospheric implications.[18](https://onlinelibrary.wiley.com/doi/abs/10.1002/kin.550060202)
## In space
[Hydrogen peroxide](https://www.edgepedia.org/hydrogen-peroxide) was detected toward the ρ Oph A star-forming core, and a gas–grain chemical model built on that detection predicted its direct precursor HO2 should also be detectable. Observations with the IRAM 30 m and APEX telescopes at about 130 and 260 GHz then detected five lines of HO2 (seven individual transitions), with a fractional abundance of about 1 × 10−10, similar to that of HOOH.[7](https://arxiv.org/abs/1205.0361) The result validated the model's treatment of water formation on dust grains, in which grain-surface hydrogenation chemistry forms HOOH as an intermediate.[7](https://arxiv.org/abs/1205.0361)
## References
1. [HO2•: The Forgotten Radical (Aubrey D.N.J. de Grey, DNA and Cell Biology, 2002)](https://journals.sagepub.com/doi/pdf/10.1089/104454902753759672?cf-mal-redirected=true)
2. [Molecular Probes for Tracking Reactive Oxygen Species at the Organelle Interface (Analysis & Sensing, 2026)](https://doi.org/10.1002/anse.202600003)
3. [The spectrum of DO2 near 60 GHz and the structure of the hydroperoxyl radical (J. Chem. Phys.)](https://tf.nist.gov/general/pdf/275.pdf)
4. [Spectra and Kinetics of the Hydroperoxyl Free Radical in the Gas Phase (Paukert & Johnston, J. Chem. Phys. 56, 2824, 1972)](https://cir.nii.ac.jp/crid/1364233269705778432)
5. [A photoionization study of the hydroperoxyl radical, HO2, and hydrogen peroxide, H2O2 (J. Electron Spectrosc. Relat. Phenom.)](https://www.sciencedirect.com/science/article/pii/S0368204898002643)
6. [OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides (Atmos. Chem. Phys., 2022)](https://acp.copernicus.org/articles/22/3693/2022/)
7. [Detection of the hydroperoxyl radical HO2 toward ρ Oph A (Parise et al., A&A, 2012)](https://arxiv.org/abs/1205.0361)
8. [Radical chemistry and ozone production at a UK coastal receptor site (Atmos. Chem. Phys., 2023)](https://acp.copernicus.org/articles/23/14393/2023/)
9. [Note on the Formation of Ozone and Secondary Fine Particulate Matter (TRB Special Report 264, 2002)](https://www.nationalacademies.org/read/10350/chapter/9)
10. [OH and HO2 radical chemistry during the EXPLORE-YRD campaign (Atmos. Chem. Phys., 2022)](https://acp.copernicus.org/articles/22/7005/2022/index.html)
11. [Combustion, Reactive Hazard, and Bioprocess Safety (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7158352/)
12. [HO2 + HO2: High level theory and the role of singlet channels (Combustion and Flame)](https://www.sciencedirect.com/science/article/abs/pii/S0010218021007185)
13. [Reaction Mechanisms on Multi- (SAND2016-10085J, OSTI)](https://www.osti.gov/servlets/purl/1343063)
14. [Recent Developments in the Coupling of Theory and Experiment to Study the Elementary Chemistry of Autoignition (Combust. Sci. Technol. Japan)](https://www.jstage.jst.go.jp/article/jcombsj/50/151/50_29/_article/-char/en)
15. [Exploring hydroperoxides in combustion: History, recent advances and perspectives (Prog. Energy Combust. Sci., 2019)](https://www.sciencedirect.com/science/article/pii/S0360128518301175)
16. [The Radiation Chemistry of Water, Chapter IV: The Hydroperoxyl Radical (Draganic & Draganic, 1971)](https://www.wipp.energy.gov/library/cra/2009_cra/references/Others/Draganic_Draganic_1971_The_radiation_Chemistry_of_Water.pdf)
17. [Time-resolved wavelength modulation spectroscopy measurements of HO2 kinetics (Appl. Opt. / PubMed)](https://pubmed.ncbi.nlm.nih.gov/18259411/)
18. [Evaluated and estimated kinetic data for phase reactions of the hydroperoxyl radical (Lloyd, Int. J. Chem. Kinet., 1974)](https://onlinelibrary.wiley.com/doi/abs/10.1002/kin.550060202)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Elements and inorganic substances › Oxides and oxygen compounds › Inorganic peroxides and hydroperoxides › Peroxides and hydroperoxides (overview)*
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