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Oxygen with all of its electrons spin paired

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow. The lowest excited state of the diatomic oxygen molecule is a singlet state. It is a gas with physical properties differing only subtly from those of the more prevalent triplet ground state of O2. 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. 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.

Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted 1Σ and 1Δg (the preceding superscript "1" indicates a singlet state). The singlet states of oxygen are 158 and 95 kilojoules per mole higher in energy than the triplet ground state of oxygen. Under most common laboratory conditions, the higher energy 1Σ singlet state rapidly converts to the more stable, lower energy 1Δg singlet state. Two less stable, higher energy excited states are readily accessible from this ground state, again in accordance with Hund's first rule; 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). 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 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 singlet singlet transition between the two excited states is spin allowed but parity 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), 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 very short lived. In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 s. 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 +1ħ and −1ħ around the molecular axis. The magnetic transition between these levels gives rise to the EPR transition.

Unlike ground state oxygen, singlet oxygen participates in Diels–Alder [4+2] and [2+2] cycloaddition reactions and formal concerted ene reactions. 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 allyl groups, e. g., citronella, shown, by abstraction of the allylic proton, in an ene like reaction, yielding the allyl hydroperoxide, R–O–OH (R = alkyl), which can then be reduced to the corresponding allylic alcohol. In reactions with water trioxidane, an unusual molecule with three consecutive linked oxygen atoms, is formed.

In photosynthesis, singlet oxygen can be produced from the light harvesting chlorophyll molecules. One of the roles of carotenoids 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. In mammalian 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.

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.