Singlet oxygen

Electronic structure

Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted ¹Σ+
g and ¹Δ_g (the preceding superscripted "1" indicates it as 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+
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.

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 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.

The ¹Δ_g singlet state is 7882.4 cm⁻¹ above the triplet ³Σ−
g ground state., which in other units corresponds to 94.29 kJ/mol or 0.9773 eV. The ¹Σ+
g singlet is 13 120.9 cm⁻¹ (157.0 kJ/mol or 1.6268 eV) above the ground state. The open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (¹Σ+
g) ground states.

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.

Paramagnetism due to orbital angular momentum

Singlet oxygen has no unpaired electrons and therefore no net electron spin. The ¹Δ_g is however paramagnetic as shown by the observation of an electron paramagnetic resonance (EPR) spectrum. The paramagnetism is due to a net orbital (and not spin) electronic angular momentum. In a magnetic field the degeneracy of the π* level is split into two levels corresponding to molecular orbitals with angular momenta +1ħ and −1ħ around the molecular axis. In the ¹Δ_g state one of these orbitals is doubly occupied and the other empty, so that transitions are possible between the two.

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. One chemical method involves the decomposition of triethylsilyl hydrotrioxide generated in situ from triethylsilane and ozone.

(C₂H₅)₃SiH + O₃ → (C₂H₅)₃SiOOOH → (C₂H₅)₃SiOH + O₂(¹Δ_g)

Another method uses the aqueous reaction of hydrogen peroxide with sodium hypochlorite:

H₂O₂ + NaOCl → O₂(¹Δ_g) + NaCl + H₂O

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

(RO)₃P + O₃ → (RO)₃PO₃(RO)₃PO₃ → (RO)₃PO + O₂(¹Δ_g)

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

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

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.