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Chlorine monoxide
Chlorine monoxide
Chlorine monoxide
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Chlorine monoxide
Names
Preferred IUPAC name
Chlorine monoxide
Systematic IUPAC name
Chlorooxidanyl
Other names
Chlorine(II) oxide
Identifiers
3D model (JSmol)
Abbreviations ClO•
ChEBI
ChemSpider
MeSH Chlorosyl
UNII
  • InChI=1S/ClO/c1-2
    Key: NHYCGSASNAIGLD-UHFFFAOYSA-N
  • [O]Cl
Properties
ClO
Molar mass 51.45 g·mol−1
Thermochemistry
101.8 kJ/mol[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Chlorine monoxide is a chemical radical with the chemical formula ClO•. It plays an important role in the process of ozone depletion. In the stratosphere, chlorine atoms react with ozone molecules to form chlorine monoxide and oxygen.

Cl• + O3 → ClO• + O2

This reaction causes the depletion of the ozone layer.[1] The resulting ClO• radicals can further react:

ClO• + O• → Cl• + O2

regenerating the chlorine radical. In this way, the overall reaction for the decomposition of ozone is catalyzed by chlorine, as ultimately chlorine remains unchanged. The overall reaction is:

O• + O3 → 2 O2

There has been a significant impact of the use of CFCs on the upper stratosphere, although many countries have agreed to ban the use of CFCs. The nonreactive nature of CFCs allows them to pass into the stratosphere, where they undergo photo-dissociation to form Cl radicals. These then readily form chlorine monoxide, and this cycle can continue until two radicals react to form dichlorine monoxide, terminating the radical reaction. Because the concentration of CFCs in atmosphere is very low, the probability of a terminating reaction is exceedingly low, meaning each radical can decompose many thousands of molecules of ozone.

Even though the use of CFCs has been banned in many countries, CFCs can stay in the atmosphere for 50 to 500 years. This causes many chlorine radicals to be produced and hence a significant amount of ozone molecules are decomposed before the chlorine radicals are able to react with chlorine monoxide to form dichlorine monoxide.

References

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Chlorine monoxide

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from Grokipedia

(ClO) is a diatomic inorganic radical consisting of a single atom covalently bonded to an oxygen atom.
This highly reactive species exists transiently in the Earth's atmosphere, where it acts as a key intermediate in catalytic cycles that destroy stratospheric , particularly over polar regions during periods of low .
In the primary chlorine cycle, atomic chlorine reacts with ozone to form ClO and molecular oxygen, followed by ClO reacting with atomic oxygen to regenerate chlorine, resulting in the net decomposition of two ozone molecules into three oxygen molecules without net consumption of chlorine.
ClO is paramagnetic due to its and adopts a , with a bond length of approximately 1.48 Å.
Its presence in enhanced concentrations correlates with observed ozone minima, as confirmed by spectroscopic measurements from satellites and ground-based instruments.

Structure and properties

Molecular geometry and bonding

Chlorine monoxide (ClO) is a diatomic radical with a , as expected for all diatomic molecules where the atomic nuclei are aligned along the internuclear axis. The experimental Cl–O bond length is 1.569 (156.9 pm), shorter than a typical single Cl–O bond (around 1.7 in compounds), indicating significant multiple bonding character. The molecule exhibits a dipole moment of 1.24 D, arising from the difference between oxygen (3.44) and (3.16), with the negative end at oxygen. In , the can be described by resonance structures, primarily Cl=O (with the on oxygen) and contributions from Cl–O forms, yielding a closer to 2 than to 1. Quantum chemical computations confirm this, showing the electronic structure resembles that of a more than a , consistent with the observed and strength. for ClO, with 13 valence electrons, places the in a π* antibonding orbital in the ^2Π , resulting in a formal of 2.5 when accounting for filled bonding (σ and π) and partially filled antibonding orbitals. The Cl–O bond dissociation energy, derived from thermochemical data, is approximately 269 kJ/mol at 0 , reflecting the robust nature of the bond despite the radical character. This energy is calculated as the difference in enthalpies of formation: D_0 = \Delta H_f^\circ (\ce{Cl}) + \Delta H_f^\circ (\ce{O}) - \Delta H_f^\circ (\ce{ClO}), using standard atomic values and the measured \Delta H_f^\circ (\ce{ClO}) = 101.7 \pm 0.04 kJ/mol.

Physical properties

Dichlorine monoxide exists as a yellowish-brown gas under standard conditions, exhibiting a disagreeable, suffocating similar to . It condenses to a dark red liquid at low temperatures and is highly unstable, decomposing readily above its or upon exposure to light and moisture. Key thermophysical parameters include a of −120.6 °C and a of approximately 2 °C, reflecting its low volatility and tendency to exist as a gas near ambient temperatures despite rapid . The gas is 3.89 g/L at 0 °C, consistent with its molecular weight of 86.90 g/mol.
PropertyValueConditions/Source Notes
−120.6 °CExperimental data
2 °CReported across multiple chemical databases
(gas)3.89 g/LAt 0 °C
Dichlorine monoxide demonstrates high reactivity with water, hydrolyzing exothermically to form and , with exceeding 1.43 g per gram of water before significant decomposition occurs. Due to its instability, it is typically handled at cryogenic temperatures or generated for applications.

Thermodynamic data

The (Δ_f H^°) of gaseous chlorine monoxide (ClO) at 298.15 is 101.7 ± 0.04 kJ/mol, as determined from active thermochemical tables (ATcT) that integrate high-level quantum chemical calculations with experimental for high precision. At 0 , this value is 101.1 kJ/mol. The (S^°) of ClO(g) at 298.15 K and 1 bar is 226.65 J mol^{-1} ^{-1}, based on JANAF thermochemical tables evaluated from spectroscopic and calorimetric measurements. The constant-pressure (C_p) of ClO(g) at 298.15 K is 31.55 J mol^{-1} ^{-1}. Shomate equation coefficients for C_p over 298–600 K (A = 18.72, B = 59.08, C = -73.82, D = 34.56, E = 0.076) enable computation of temperature-dependent values, derived from fitted spectroscopic data.
PropertySymbolValue at 298.15 KUnit
Enthalpy of formationΔ_f H^°101.7 ± 0.04kJ/mol
Molar entropyS^°226.65J mol^{-1} K^{-1}
Heat capacityC_p31.55J mol^{-1} K^{-1}

Synthesis

Laboratory preparation

Chlorine monoxide (Cl₂O) is commonly prepared in the laboratory by passing dry chlorine gas through a column or over heated yellow mercury(II) oxide (HgO), which yields gaseous Cl₂O along with mercury chloride byproducts. The reaction proceeds at elevated temperatures, typically around 100–150°C, to facilitate the oxidation and chlorination processes, producing relatively pure Cl₂O in the exit gas stream that can be collected or used directly. The spent HgO catalyst can be regenerated by treatment with aqueous sodium hydroxide, filtration, water washing, and drying at 110°C. An alternative method involves the reaction of chlorine gas with moist sodium carbonate, following the equation 2Cl₂ + 2Na₂CO₃ + H₂O → Cl₂O + 2NaCl + 2NaHCO₃, which generates Cl₂O in situ amid bicarbonate formation. Cl₂O can also be derived from hypochlorous acid (HOCl) solutions via dehydration, leveraging the equilibrium 2HOCl ⇌ Cl₂O + H₂O, often achieved by distillation under reduced pressure to shift the equilibrium toward the monoxide. This approach requires careful control to minimize decomposition, as Cl₂O is highly reactive and prone to explosive disproportionation above certain concentrations or temperatures. All preparations demand inert atmospheres, low temperatures for storage (often as a frozen hydrate), and rigorous safety measures due to the compound's instability and toxicity.

Atmospheric formation

Chlorine monoxide (ClO) forms in the primarily via the reaction of atomic chlorine with : Cl + O₃ → ClO + O₂. This , with a rate constant of approximately 1.1 × 10^{-11} cm³ ⁻¹ s⁻¹ at 298 K, serves as the initial step in catalytic ozone destruction cycles, where ClO acts as a transient for reactive chlorine. Atomic chlorine atoms, necessary for ClO production, originate from the photolysis of chlorine reservoir species transported from the troposphere, predominantly chlorofluorocarbons (CFCs) that photodissociate above ~30 km altitude to release chlorine. Key reservoirs include hydrogen chloride (HCl) and chlorine nitrate (ClONO₂), with photolysis reactions such as ClONO₂ + hν → Cl + NO₃ (quantum yield near unity for wavelengths <400 nm) and minor contributions from HCl + hν → Cl + H (peaking at Lyman-α UV). These processes maintain ClO mixing ratios up to several ppbv in the lower stratosphere, with peak concentrations during winter in polar vortices due to enhanced reservoir conversion. In polar regions, heterogeneous reactions on polar stratospheric clouds (PSCs) amplify ClO formation by converting stable reservoirs into photolabile species. The dominant pathway is the surface reaction ClONO₂ + HCl → Cl₂ + HNO₃, occurring efficiently on particles at temperatures below 198 K, followed by photolysis of Cl₂: Cl₂ + hν → 2Cl. This "chlorine activation" mechanism, observed during spring, elevates ClO levels to 1-2 ppmv locally, far exceeding mid-latitude values of ~10-100 pptv, and is responsible for rapid loss episodes. Additional minor sources include reactions like HOCl + HCl → Cl₂ + H₂O on type I PSCs, but these contribute less to overall ClO production compared to ClONO₂-HCl processing.

Chemical reactions

Radical recombination and disproportionation

The self-reaction of chlorine monoxide radicals (ClO) proceeds via two primary channels: termolecular recombination to form the chlorine dioxide dimer and bimolecular producing atomic chlorine and . The recombination channel, ClO + ClO + M → Cl₂O₂ + M (where M is a third-body collider such as N₂ or O₂), exhibits a termolecular rate constant of approximately 1.1 × 10⁻³¹ cm⁶ ⁻² s⁻¹ at 298 K and 1 atm, with the equilibrium shifting toward dissociation at higher temperatures relevant to the (e.g., above 220 K). This process forms Cl₂O₂ (often denoted as ClOOCl), a transient that plays a key role in modulating ClO concentrations by sequestering radicals until photolysis or further reaction releases them. In parallel, the disproportionation channel, 2 ClO → Cl + ClO₂, occurs bimolecularly with a rate constant around 7.5 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at 298 K, involving direct abstraction or rearrangement where one ClO is reduced to Cl (oxidation state 0) and the other oxidized to ClO₂ (Cl +4). This pathway regenerates Cl atoms, which can propagate destruction cycles (e.g., Cl + O₃ → ClO + O₂), while ClO₂ serves as a sink or further reactant, though its atmospheric yield from this channel remains minor compared to recombination under typical stratospheric pressures (branching ratio <10%). Both channels' kinetics are pressure- and temperature-dependent, with recombination dominating at lower temperatures and higher pressures, as verified by modulated photolysis and studies. A minor channel, 2 ClO → Cl₂ + O₂, contributes negligibly due to its higher activation barrier. These reactions underscore ClO's role in radical chain processes, with experimental data from confirming the channels' competition in gas-phase conditions mimicking the upper atmosphere.

Reactions with oxygen species

ClO reacts rapidly with ground-state atomic oxygen (O(^3P)) to form chlorine atoms and molecular oxygen via the exothermic reaction ClO + O → Cl + O₂. This bimolecular process proceeds at near the gas-kinetic collision rate, with a temperature-independent rate constant of (3.0 ± 0.5) × 10^{-11} cm³ molecule^{-1} s^{-1} over 220–400 K, measured using discharge-flow techniques with mass spectrometric detection. The reaction's low (effectively zero) facilitates its role in propagating catalytic cycles involving chlorine radicals. The reaction of ClO with (O₃) is negligible under stratospheric conditions, with the proposed channel ClO + O₃ → + O₂ exhibiting an upper limit rate constant below 1.5 × 10^{-17} cm³ ^{-1} s^{-1} at 298 , determined via discharge-flow modulation and resonance fluorescence. No significant products were observed, indicating this pathway does not contribute meaningfully to ClO loss or processing. ClO reacts with the (OH) primarily through OH + ClO → Cl + HO₂, with a branching ratio exceeding 65%, alongside a minor channel yielding HCl + O₂. The total rate constant is (1.1 ± 0.2) × 10^{-11} cm³ ^{-1} s^{-1} at 298 , measured over 220–400 K using discharge-flow systems with resonance fluorescence detection of OH decay. This abstraction reaction interconverts radical families without net loss. The dominant interaction of ClO with (HO₂) yields HOCl + O₂, with branching ratios approaching 100% at low pressures and temperatures of 210–300 , as confirmed by product studies using matrix isolation and flow tube . The effective bimolecular rate constant is (5.2 ± 1.0) × 10^{-11} cm³ ^{-1} s^{-1} at 298 , exhibiting weak temperature dependence, and serves as a temporary chlorine reservoir in oxygenated environments. Minor channels to HCl + O₃ are suppressed below 1%. ClO shows no measurable reaction with molecular oxygen (O₂) at atmospheric temperatures, consistent with its thermodynamic stability as a radical lacking facile addition or abstraction pathways.

Atmospheric role

Involvement in stratospheric chemistry

Chlorine monoxide (ClO) serves as a key intermediate in catalytic cycles that deplete stratospheric ozone, primarily through reactions that convert ozone (O₃) and atomic oxygen (O) into molecular oxygen (O₂) without net consumption of ClO. In the basic cycle, ClO reacts with O to regenerate chlorine atoms (Cl), which then react with O₃: ClO + O → Cl + O₂, followed by Cl + O₃ → ClO + O₂, resulting in net destruction of O₃ + O → 2O₂. This cycle accounts for ongoing ozone loss in the stratosphere, with ClO densities observed to vary but contribute significantly to the chlorine-driven catalytic efficiency. In polar regions, particularly during winter in the vortex, ClO concentrations increase dramatically due to heterogeneous reactions on polar stratospheric clouds (PSCs) that convert reservoir species like chlorine nitrate (ClONO₂) and (HCl) into active chlorine, including Cl₂, which photolyzes to Cl and subsequently forms ClO via Cl + O₃. Elevated ClO then undergoes self-reaction to form (Cl₂O₂): 2 ClO + M → Cl₂O₂ + M (where M is a third body), followed by photolysis of Cl₂O₂ yielding two Cl atoms and O₂, enabling each Cl to destroy another O₃ molecule and amplifying loss rates up to 10⁶ times faster than in non-polar conditions. This ClO dimer mechanism dominates polar ozone hole formation, with ClO levels reaching 10¹²–10¹³ molecules cm⁻³ in the lower stratosphere during activation events. ClO also participates in cross-halogen cycles, such as with monoxide (BrO): ClO + BrO → Br + ClOO, followed by ClOO photolysis to Cl + O₂ and Br + O₃ → BrO + O₂, netting O₃ + O → 2O₂ while recycling both radicals; this enhances total halogen-mediated depletion, as bromine atoms efficiently convert ClO to Cl. Measurements confirm ClO's pivotal role, with its abundance correlating inversely with column densities, particularly in the 15–20 km altitude range where PSCs form at temperatures below 195 . Reservoir reformation, such as ClO + NO₂ → ClONO₂, limits ClO's persistence in sunlit conditions, tying its impact to seasonal sunlight return in polar spring.

Contribution to ozone depletion

Chlorine monoxide (ClO) serves as a key intermediate in catalytic cycles that deplete , enabling a single atom to destroy thousands of molecules before being sequestered. In the primary cycle, atomic (Cl) reacts with to form ClO and molecular oxygen: Cl + O₃ → ClO + O₂. Subsequently, ClO reacts with atomic oxygen: ClO + O → Cl + O₂. The net result is the destruction of one molecule and one atomic oxygen atom, yielding two molecules of O₂, with regenerated as Cl to perpetuate the cycle. This mechanism, first proposed in theoretical models in the 1970s, operates globally but is limited by the scarcity of atomic oxygen in the lower . In polar regions, particularly during the Antarctic spring, ClO concentrations elevate dramatically to 1–2 due to activation of chlorine reservoirs on polar stratospheric clouds (PSCs), amplifying loss through the ClO dimer cycle. Two ClO radicals form the dimer chlorine peroxide (Cl₂O₂): 2 ClO + M → Cl₂O₂ + M (where M is a third body). Photolysis of Cl₂O₂ yields Cl and chlorine (ClOO): Cl₂O₂ + hν → Cl + ClOO, followed by ClOO decomposition: ClOO + M → Cl + O₂. Each Cl then re-enters the basic cycle, resulting in net destruction of two molecules per dimer cycle: 2 O₃ → 3 O₂. This pathway accounts for the majority of loss in the ozone hole, with models indicating it can explain up to 74% of observed depletion in winters under cold conditions. The efficiency of ClO-mediated depletion stems from its radical nature and the catalytic regeneration of active chlorine , with one chlorine atom capable of destroying approximately 100,000 molecules over its atmospheric lifetime. Measurements confirm ClO's dominance in polar vortices, where heterogeneous reactions on PSCs convert inert chlorine nitrates (ClONO₂) and HCl to photolabile Cl₂, which photodissociates to Cl and subsequently forms ClO. Cross-reactions with bromine monoxide (BrO), such as ClO + BrO → Br + ClOO, further enhance loss by coupling chlorine and bromine cycles, contributing an additional 20–30% to polar reduction. Overall, ClO-driven processes are responsible for the bulk of anthropogenic attributed to chlorofluorocarbons (CFCs), as evidenced by correlations between stratospheric chlorine levels and minima observed since the 1980s.

Detection and spectroscopy

Spectroscopic identification

The chlorine monoxide radical (ClO) was first identified spectroscopically in 1950 by using of chlorine monoxide or related precursors, followed by time-resolved UV , which revealed characteristic transient absorption bands attributable to the ClO species. This electronic transition, corresponding to the A²Π ← X²Π system, exhibits structured bands in the near-ultraviolet region, typically between 250 and 350 nm, with a prominent progression peaking around 270-310 nm, enabling unambiguous identification of the radical in laboratory gas-phase experiments. Infrared spectroscopy has provided detailed vibrational characterization, with the fundamental ν=1←0 band of ³⁵ClO centered near 850 cm⁻¹ (ranging from approximately 829 to 881 cm⁻¹ for rotational lines), observed using tunable diode lasers and Fourier-transform infrared techniques in discharge-flow systems. High-resolution measurements of this band, including both spin-orbit components (²Π_{3/2} and ²Π_{1/2}), have yielded precise rotational constants and confirmed the X²Π ground state, with line intensities consistent with theoretical predictions for a diatomic radical. Overtone bands (e.g., ν=2←0) have also been resolved, supporting structural assignments. Microwave spectroscopy complements these by probing the pure rotational spectrum in the ground electronic and vibrational states, revealing due to chlorine nuclear spin and confirming bond lengths around 1.48 from derived constants B_e ≈ 0.59 cm⁻¹. These spectroscopic signatures—UV for rapid detection of transients, IR for vibrational analysis, and for ground-state precision—collectively enable definitive identification of ClO, distinguishing it from related species like Cl₂O or OClO through wavelength-specific absorption features and isotopic shifts (e.g., ³⁵ClO vs. ³⁷ClO).

Measurement techniques

Chlorine monoxide (ClO) concentrations in the are primarily measured using techniques such as ground-based millimeter-wave , which detects emission from ClO rotational lines near 204 GHz or 278 GHz to retrieve vertical profiles from the upper stratosphere down to about 20 km altitude. These instruments, deployed at sites like (19.8°N) and , , provide long-term monitoring data, with re-analyses confirming ClO trends consistent with declining stratospheric chlorine loading since the 1990s. Satellite-based measurements employ submillimeter radiometry, as in the Odin satellite's Sub-Millimetre Radiometer (SMR), which observes ClO emission at 649 GHz for nighttime profiles up to 50 km, enabling studies of ClO/ClOOCl equilibrium without photolysis interference. Complementary satellite methods include solar occultation spectroscopy by instruments like HALOE, which infer ClO indirectly from HCl and ClONO2 profiles, though direct ClO detection is limited by its short lifetime. In-situ techniques, such as the balloon-borne or aircraft-borne HALOX instrument, utilize chemical conversion resonance (CCRF), where ambient ClO reacts with injected (NO) to produce chlorine atoms (), detected via vacuum-ultraviolet resonance for high-sensitivity measurements (detection limits around 10^6 molecules cm^{-3}) in the lower . , including infrared (FTIR) from ground or platforms, quantifies ClO via absorption in the 11 μm vibrational band, aiding validation against data despite challenges from overlapping lines. Ultraviolet-visible spectroscopy supports both laboratory calibration and limited atmospheric remote sensing, fitting ClO absorption features around 275-300 nm to derive concentrations, often coupled with equilibrium modeling for ClOOCl interference. Laser-induced fluorescence (LIF) at 167-180 nm excites ClO for in-situ detection in controlled environments or high-altitude probes, offering sub-ppbv sensitivity but requiring vacuum-ultraviolet sources. Cross-validation across these methods, such as microwave with Aura MLS satellite data, confirms measurement accuracies within 10-20% for polar winter ClO enhancements exceeding 10 ppbv.

Historical context

Early observations

The chlorine monoxide radical (ClO) was first directly observed through its electronic emission spectrum in 1948 by Georges Pannetier and Alfred G. Gaydon, who identified bands of the A²Π–X²Π transition in oxy-hydrogen flames containing chlorine. This detection confirmed the radical's existence, which had been inferred earlier from kinetic studies of decompositions suggesting short-chain reactions involving ClO intermediates. Prior indirect evidence included proposals by Finkelnburg and colleagues in for ClO participation in the thermal breakdown of (Cl₂O), though without spectroscopic verification. Subsequent early laboratory observations in the late and refined ClO's spectroscopic properties using absorption and emission techniques in discharge tubes and flame environments, establishing key vibrational and rotational constants. These efforts, building on Pannetier and Gaydon's work, involved low-pressure systems to isolate the radical from recombination products like Cl₂ and O₂. By the mid-, methods enabled transient detection of ClO via UV absorption, providing initial insights into its reactivity with and other species.

Development in ozone research

In 1974, Mario J. Molina and proposed a catalytic mechanism for stratospheric driven by atoms released from chlorofluorocarbons (CFCs), with (ClO) serving as a critical intermediate. In this cycle, Cl reacts with to produce ClO and O₂, followed by ClO reacting with O to regenerate Cl and form O₂, yielding a net destruction of two molecules per cycle without net consumption of the catalyst. Their model predicted stratospheric ClO mixing ratios on the order of 10⁻¹⁰ to 10⁻⁹, prompting experimental searches to verify its presence and abundance. Initial detections of stratospheric ClO occurred in the early 1980s using ground-based millimeter-wave spectroscopy, confirming concentrations aligning with theoretical expectations from gas-phase chemistry alone. These observations, conducted at mid-latitudes, provided evidence for the Molina-Rowland cycle but showed ClO levels insufficient to explain observed global trends. The discovery of the Antarctic hole in 1985, reported by Joseph Farman and colleagues based on ground-based Dobson spectrophotometer data revealing springtime total columns below 220 Dobson units, intensified research into polar processes. A pivotal advancement came during the 1986-1987 Airborne Antarctic Experiment (AAOE), led by , where balloon-borne and aircraft-based microwave radiometers measured ClO mixing ratios exceeding 1 ppbv within the —over an order of magnitude higher than mid-latitude values. These elevated ClO levels correlated inversely with concentrations, supporting a heterogeneous mechanism on polar stratospheric clouds (PSCs) that converts like ClONO₂ and HCl into reactive , including ClO, via reactions such as ClONO₂ + HCl → Cl₂ + HNO₃ followed by photolysis. Further analysis revealed the ClO-ClO dimer (Cl₂O₂) as a enabling rapid catalytic loss through Cl₂O₂ + hν → 2 ClO, then 2 ClO → ClOO + Cl, and ClOO + M → Cl + ClOOM (with subsequent regeneration), amplifying destruction rates by up to 10⁶ molecules per chlorine atom. Subsequent satellite missions, such as the Upper Atmosphere Research Satellite (UARS) launched in 1991, provided global mapping of ClO distributions, validating seasonal and latitudinal variations tied to PSC formation and vortex dynamics. These measurements, combined with confirmation of key reaction rates, solidified ClO's central role in both mid-latitude and polar chemistry, influencing the 1987 Protocol's phase-out of ozone-depleting substances. Ongoing monitoring, including from the satellite's Microwave Limb Sounder since 2004, continues to track ClO declines paralleling reduced stratospheric loading.

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