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Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with two oxygen atoms in a quantum state where all electrons are spin-paired, known as a singlet state. It is the lowest excited state of the dioxygen, which in general has the chemical structure O=O and chemical formula . Singlet oxygen can be written more specifically as or . The more prevalent ground state of is known as triplet oxygen. At room temperature, singlet oxygen will slowly decay into triplet oxygen, releasing the energy of excitation.
Singlet oxygen is a gas with physical properties differing only subtly from the ground state. In terms of its chemical reactivity, however, singlet oxygen is far more reactive toward organic compounds. It is responsible for the photodegradation of many materials but can be put to constructive use in preparative organic chemistry and photodynamic therapy. Trace amounts of singlet oxygen are found in the upper atmosphere and in polluted urban atmospheres where it contributes to the formation of lung-damaging nitrogen dioxide. (1969). 9780470133378 ISBN 9780470133378 It often appears and coexists confounded in environments that also generate ozone, such as pine forests with photodegradation of turpentine.
The terms "singlet oxygen" and "triplet oxygen" derive from each form's number of electron spins. The singlet has only one possible arrangement of electron spins with a total quantum spin of 0, while the triplet has three possible arrangements of electron spins with a total quantum spin of 1, corresponding to three degenerate states.
In spectroscopic notation, the lowest singlet and triplet forms of O2 are labeled 1Δg and 3Σ, respectively. (2025). 9781405190886, Wiley. ISBN 9781405190886
Molecular orbital theory predicts the electronic ground state denoted by the molecular term symbol 3Σ, and two low-lying excited with term symbols 1Δg and 1Σ. These three electronic states differ only in the spin and the occupancy of oxygen's two antibonding πg-orbitals, which are degenerate (equal in energy). These two orbitals are classified as antibonding and are of higher energy. Following Hund's first rule, in the ground state, these electrons are electron pair and have like (same) spin. This open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (1Σ) ground states. (1991). 9780205127702, Prentice-Hall. ISBN 9780205127702
Two less stable, higher energy are readily accessible from this ground state, again in accordance with Hund's first rule; (1985). 9780849364396, CRC Press. ISBN 9780849364396 the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the 1Δg state (a term symbol, where the preceding superscripted "1" indicates it as a singlet state). Alternatively, both electrons can remain in their degenerate ground state orbitals, but the spin of one can "flip" so that it is now opposite to the second (i.e., it is still in a separate degenerate orbital, but no longer of like spin); this also creates a new state, a singlet state referred to as the 1Σ state. The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below.For triplet ground state on right side of diagram, see C.E.Housecroft and A.G.Sharpe Inorganic Chemistry, 2nd ed. (Pearson Prentice-Hall 2005), p.35 For changes in singlet states on left and in centre, see F. Albert Cotton and Geoffrey Wilkinson. Advanced Inorganic Chemistry, 5th ed. (John Wiley 1988), p.452
The 1Δg singlet state is 7882.4 cm−1 above the triplet 3Σ ground state., which in other units corresponds to 94.29 kJ/mol or 0.9773 eV. The 1Σ singlet is 13 120.9 cm−1 (157.0 kJ/mol or 1.6268 eV) above the ground state.
Radiative transitions between the three low-lying electronic states of oxygen are formally forbidden as electric dipole processes. The two singlet-triplet transitions are forbidden both because of the spin selection rule ΔS = 0 and because of the parity rule that g-g transitions are forbidden.
The lower, O2(1Δg) state is commonly referred to as singlet oxygen. The energy difference of 94.3 kJ/mol between ground state and singlet oxygen corresponds to a forbidden singlet-triplet transition in the near-infrared at ~1270 nm. As a consequence, singlet oxygen in the gas phase is relatively long lived (54-86 milliseconds), Physical Mechanisms of Generation and Deactivation of Singlet Oxygen Claude Schweitzer although interaction with solvents reduces the lifetime to microseconds or even nanoseconds. In 2021, the lifetime of airborne singlet oxygen at air/solid interfaces was measured to be 550 microseconds.
The higher 1Σ state is moderately short lived. In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 seconds. However in solvents such as Carbon disulfide and CCl4, it relaxes to the lower singlet 1Δg in milliseconds due to radiationless decay channels.
Another method uses a reaction of hydrogen peroxide with sodium hypochlorite in aqueous solution:
A retro-Diels Alder reaction of the diphenylanthracene peroxide can also yield singlet oxygen, along with an diphenylanthracene:
A third method liberates singlet oxygen via phosphite ozonides, which are, in turn, generated in situ such as triphenyl phosphite ozonide. Phosphite ozonides will decompose to give singlet oxygen:
An advantage of this method is that it is amenable to non-aqueous conditions.
Unlike ground state oxygen, singlet oxygen participates in Diels–Alder 4+2- and 2+2-cycloaddition reactions and formal concerted (Schenck ene reaction), causing photooxygenation. It oxidizes thioethers to sulfoxides. Organometallic complexes are often degraded by singlet oxygen. With some substrates 1,2-dioxetanes are formed; cyclic dienes such as 1,3-cyclohexadiene form 4+2 cycloaddition adducts.
The 4+2-cycloaddition between singlet oxygen and furans is widely used in organic synthesis.
In singlet oxygen reactions with alkenic , e.g., citronella, shown, by abstraction of the allylic proton, in an ene reaction, yielding the allyl hydroperoxide, R–O–OH (R = alkyl), which can then be reduced to the corresponding allylic alcohol.This reaction is not a true ene reaction, because it is not concerted; singlet oxygen forms an "epoxide oxide" exciplex, which then abstracts the hydrogen. See Alberti et al, op. cit.
In reactions with water, trioxidane, an unusual molecule with three consecutive linked oxygen atoms, is formed.
In biology, singlet oxygen is one of the reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects. Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects.
Ingestion of pigments capable of producing singlet oxygen with activation by light can produce severe photosensitivity of skin (see phototoxicity, photosensitivity in humans, photodermatitis, phytophotodermatitis). This is especially a concern in herbivorous animals (see Photosensitivity in animals).
Singlet oxygen is the active species in photodynamic therapy.
(2025). 9780805338423, PEARSON Benjamin Cummings. ISBN 9780805338423 The singlet-singlet transition between the two excited states is spin-allowed but parity-forbidden.
Paramagnetism due to orbital angular momentum
Both singlet oxygen states have no unpaired electrons and therefore no net electron spin. The 1Δg is however paramagnetism as shown by the observation of an electron paramagnetic resonance (EPR) spectrum. The paramagnetism of the 1Δg state is due to a net orbital (and not spin) electronic angular momentum. In a magnetic field the degeneracy of the levels is split into two levels with z projections of angular momenta +1 ħ and −1 ħ around the molecular axis. The magnetic transition between these levels gives rise to the EPR transition.
Production
Various methods for the production of singlet oxygen exist. Irradiation of oxygen gas in the presence of an organic dye as a sensitizer, such as rose bengal, methylene blue, or porphyrins—a photochemistry—results in its production. Large steady state concentrations of singlet oxygen are reported from the reaction of triplet excited state pyruvic acid with dissolved oxygen in water. Singlet oxygen can also be produced by chemical procedures without irradiation. One chemical method involves the decomposition of triethylsilyl hydrotrioxide generated in situ from triethylsilane and ozone.
Reactions
Because of differences in their electron shells, singlet and triplet oxygen differ in their chemical properties; singlet oxygen is highly reactive. (1995). 9780751403718, Blackie Academic & Professional. ISBN 9780751403718 The lifetime of singlet oxygen depends on the medium and pressure. In normal organic solvents, the lifetime is only a few microseconds whereas in solvents lacking C-H bonds, the lifetime can be as long as seconds.Kuntner N (2018). " Modeling and simulation of electronic excitation in oxygen-helium discharges and plasma-assisted combustion". University of Stuttgart. doi = http://dx.doi.org/10.18419/opus-9925
(1985). 9780306411984, Plenum Press. ISBN 9780306411984
Biochemistry
In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules. One of the roles of in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess light energy from chlorophyll molecules or quenching the singlet oxygen molecules directly.
(2025). 9780471656654, J. Wiley & Sons. ISBN 9780471656654
Analytical and physical chemistry
Singlet oxygen luminesces concomitant with its decay to the triplet ground state. This phenomenon was first observed in the thermal degradation of the endo peroxide of rubrene. (2025). 9783527306732 ISBN 9783527306732
Further reading
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