Singlet oxygen

Singlet oxygen

Names
IUPAC name Singlet oxygen
Identifiers
ChEBI	CHEBI:26689
Gmelin Reference	491
Jmol 3D model	Interactive image
InChI InChI=1S/O2/c1-2 Key: MYMOFIZGZYHOMD-UHFFFAOYSA-N
SMILES O=O
Properties
Chemical formula	O2
Molar mass	32.00 g·mol−1
Solubility in water	Reacts
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

In spectroscopic notation, the singlet and triplet forms of O₂ are labeled ¹Δ_g and ³Σ_g⁻, respectively. 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.

Electronic structure

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_g⁺ singlet state rapidly converts to the more stable, lower energy ¹Δ_g 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 ¹Δ_g (where the preceding superscripted "1" indicates it as a singlet state).

Molecular orbital theory predicts two low-lying excited singlet states, denoted by the molecular term symbols ¹Δ_g and ¹Σ_g⁺. These electronic states differ only in the spin and the occupancy of oxygen's two antibonding π_g-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 ³Σ_g⁻.

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 ¹Δ_g 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_g⁺ state. 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).

MO diagram, triplet ground state and two singlet excited states of molecular dioxygen. Shown are three electronic configurations of the molecular orbitals (MOs) of molecular oxygen, O₂. From left to right, the MOs are for: ¹Δ_g singlet oxygen, ¹Σ_g⁺ singlet oxygen, and ³Σ_g^– triplet oxygen. The lowest energy 1s molecular orbital uniformly filled in all three is omitted for simplicity. Note, note the broad horizontal line with the π and π* labels represent two molecular orbitals (for filling by up to 4 electrons in total). Critically, note that the three states only differ in the occupancy and spin states of electrons in the two degenerate π* antibonding orbitals.

The ¹Σ_g⁺ and ¹Δ_g 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 ³Σ_g⁻). 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, (T_e: ¹Δ_g ← ³Σ_g⁻), is reported to be precisely 94.3 kJ/mol (0.98 electron volts, approx. 11340 Kelvin), corresponding to a frequency of 7882 cm⁻¹.

The ¹Σ_g⁺ state is very short lived and relaxes quickly to the lowest lying ¹Δ_g excited state; it is this lower, O₂(¹Δ_g) 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. 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.

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 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(¹Δ_g) + NaCl + H₂O

Another method liberates singlet oxygen via phosphite ozonides, which are, in turn, generated in situ. Phosphite ozonides are decompose to give singlet oxygen:

(RO)₃P + O₃ → (RO)₃PO₃

(RO)₃PO₃ → (RO)₃PO + ¹O₂

An advantage of this method is that it is amenable to non-aqueous conditions.

Reactions

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.

Organic chemistry

Singlet oxygen-based oxidation of citronellol. This is a net, but not a true ene reaction. Abbreviations, step 1: H₂O₂, hydrogen peroxide; Na₂MoO₄ (catalyst), sodium molybdate. Step 2: Na₂SO₃ (reducing agent), sodium sulfite.

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.

In reactions with water trioxidane, an unusual molecule with three consecutive linked oxygen atoms, is formed.

Biochemistry

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

Red glow of singlet oxygen passing into triplet state.

Direct detection of singlet oxygen is possible using sensitive laser spectroscopy 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.

Further reading

• 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 [2] and [3], 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.]

External links

• The NIST webbook on oxygen
• Photochemistry & Photobiology tutorial on Singlet Oxygen
• Demonstration of the Red Singlet Oxygen Dimol Emission (Purdue University)