|Molar mass||32.00 g·mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Singlet oxygen is a high energy form of oxygen. A gas with the formula O2, its physical properties differ only subtly from those of the more prevalent triplet form 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 also in polluted urban atmospheres where it contributes to the formation of lung damaging nitrogen dioxide.:355–68
In spectroscopic notation, the singlet and triplet forms of O2 are labeled 1Δg and 3Σg−, respectively.[page needed] The terms 'singlet oxygen' and 'triplet oxygen' refer to the quantum state of the molecules, with singlet oxygen existing in the singlet state with a total quantum spin of 0.
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 1Σg+ singlet state rapidly converts to the more stable, lower energy 1Δg singlet state; it is this, the more stable of the two excited states, the one with its electrons remaining in separate degenerate orbital but no longer with like spin, that is referred to by the title term, singlet oxygen, commonly abbreviated 1O2, to distinguish it from the triplet ground state molecule, 3O2.
Singlet oxygen refers commonly to one of several singlet electronic excited states, the state termed the ¹Δg (where the preceding superscripted "1" indicates it as a singlet state).[better source needed] 
Molecular orbital theory predicts two low-lying excited singlet states, denoted by the molecular term symbols ¹Δg) and ¹Σg+. These electronic states differ only in the spin and the occupancy of oxygen's two antibonding πg-orbitals, which are degenerate (equal in energy). Following Hund's first rule, these electrons are unpaired. These two orbitals are classified as antibonding and are of higher energy; the electrons occupying them are have like (same) spin, and this ground state is represented, as noted, by the term symbol 3Σg−.
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).[better source needed]  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 1Σg+ state.[better source needed]  The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below (a theoretical presentation and a simplification, as the experimentally observed excited states of oxygen are actually made up of combinations of electronic states).
The 1Σg+ and 1Δg singlet states are 158 and 94-95 kilojoules per mole (kJ/mol) higher in energy than the triplet ground state of oxygen (referred to as the 3Σg−).[better source needed] Hence, molecular oxygen differs from most molecules in having an open-shell triplet ground state. For instance, the energy difference for the transition between the lowest energy of O2 in the singlet state and the lowest energy in the triplet state, (Te: ¹Δg ← ³Σg−), is reported to be precisely 94.3 kJ/mol (0.98 electron volts, approx. 11340 Kelvin), corresponding to a frequency of 7882 cm−1.[original research?][verification needed]
The ¹Σg+ state is very short lived and relaxes quickly to the lowest lying ¹Δg excited state; it is this lower, O2(¹Δg) state that is commonly referred to as singlet oxygen. The energy difference between ground state and singlet oxygen referred to above (e.g., 94.3 kJ/mol corresponds to a transition in the near-infrared at ~1270 nm.[original research?][verification needed] This transition is strictly forbidden by spin, symmetry, and parity selection rules; hence, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes), although interaction with solvents reduces the lifetime to microseconds or even nanoseconds.[non-primary source needed]
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 photochemical method—results in its production. Singlet oxygen can also be in non-photochemical, preparative chemical procedures. Such chemical methods include decomposition of hydrogen trioxide, and aqueous reaction of hydrogen peroxide with sodium hypochlorite:
- H2O2 + NaOCl → O2(1Δg) + NaCl + H2O
- (RO)3P + O3 → (RO)3PO3
- (RO)3PO3 → (RO)3PO + 1O2
An advantage of this method is that it is amenable to non-aqueous conditions.
Because of differences in their electron shells, singlet and triplet oxygen differ in their chemical properties, singlet oxygen is highly reactive. The lifetime of singlet oxygen depends on the medium. 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.
Unlike ground state oxygen, singlet oxygen participates in Diels-Alder [4+2] and [2+2] cycloaddition reactions and formal concerted ene reactions. It oxidizes thioethers to sulfoxides and organometallic complexes. With some substrates 1,2-dioxetanes are formed; cyclic dienes such as 1,3-cyclohexadiene form [4+2] cycloaddition adducts.
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.
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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.
Analytical and physical chemistry
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Direct detection of singlet oxygen is possible using sensitive laser spectroscopy [non-primary source needed] or through its extremely weak phosphorescence at 1270 nm, which is not visible. However, at high singlet oxygen concentrations, the fluorescence of the singlet oxygen "dimol" species—simultaneous emission from two singlet oxygen molecules upon collision—can be observed as a red glow at 634 nm.[better source needed]
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- 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.
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- Mulliken RS (1928). "Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule". Nature 122: 505. doi:10.1038/122505a0.[better source needed]
- Bodner, G.M. (2002) Lecture Demonstration Movie Sheets: 8.4 Liquid Oxygen—Paramagnetism and Color, West Lafayette, IN, USA: Purdue University Department of Chemistry, see  and , accessed 11 August 2015; alternatively, see Bodner, G.M.; K. Keyes & T.J. Greenbowe (1995) Purdue University Lecture Demonstration Manual, 2nd Edn, p. TBD, New York, NY, USA: John Wiley and Sons. [Earlier appearing reference on magnetic properties of oxygen states.]
- The NIST webbook on oxygen
- Photochemistry & Photobiology tutorial on Singlet Oxygen
- Demonstration of the Red Singlet Oxygen Dimol Emission (Purdue University)