Singlet oxygen

Singlet oxygen
Names
IUPAC name
Singlet oxygen
Identifiers
ChEBI CHEBI:26689
491
Jmol 3D model Interactive image
Properties
O2
Molar mass 32.00 g·mol−1
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.[1]:355–68

In spectroscopic notation, the singlet and triplet forms of O2 are labeled 1Δg and 3Σ−
g, respectively.[2][3][4] 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

There are two singlet states of oxygen, denoted 1Σ+
g and 1Δg. 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 1Δg singlet state;[2] 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.[2]

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).[5] [2]

Molecular orbital theory predicts two low-lying excited singlet states, denoted by the molecular term symbols 1Δg and 1Σ+
g. 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 3Σ−
g.

Two less stable, higher energy excited states are readily accessible from this ground state, again in accordance with Hund's first rule;[6] 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).[5] [2] 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.[5] [2] 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 diagrams are for: 1Δg singlet oxygen, 1Σ+
g singlet oxygen, and 3Σ−
g triplet oxygen. The lowest energy 1s molecular orbitals are uniformly filled in all three and are omitted for simplicity. The broad horizontal lines labelled π and π* each represent two molecular orbitals (for filling by up to 4 electrons in total). The three states only differ in the occupancy and spin states of electrons in the two degenerate π* antibonding orbitals.

The 1Σ+
g 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 3Σ−
g).[2][3][4][7] 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 ← 3Σ−
g), is reported to be precisely 94.3 kJ/mol (0.98 electron volts, approx. 11340 kelvins), corresponding to a frequency of 7882 cm−1.[7]

The 1Σ+
g 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. 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.[8]

Paramagnetism due to orbital angular momentum

Singlet oxygen has no unpaired electrons and therefore no net electron spin. The 1Δg is however paramagnetic as shown by the observation of an electron paramagnetic resonance (EPR) spectrum.[9][10][11] 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 1Δg 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.[12][7] 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:[12]

H2O2 + NaOCl → O2(1Δg) + NaCl + H2O

Another method liberates singlet oxygen via phosphite ozonides, which are, in turn, generated in situ.[13] Phosphite ozonides will decompose to give singlet oxygen:[14]

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

An advantage of this method is that it is amenable to non-aqueous conditions.[14]

Reactions

Because of differences in their electron shells, singlet and triplet oxygen differ in their chemical properties, singlet oxygen is highly reactive.[15] 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.[14]

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.[14] It oxidizes thioethers to sulfoxides and organometallic complexes.[16][17] With some substrates 1,2-dioxetanes are formed; cyclic dienes such as 1,3-cyclohexadiene form [4+2] cycloaddition adducts.[18]

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.[14][19][20][21]

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.[22]

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 [23] or through its extremely weak phosphorescence at 1270 nm, which is not visible.[24] 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.[25]

References

  1. ↑ Wayne RP (1969). Pitts JN, Hammond GS, Noyes WA, eds. "Singlet Molecular Oxygen". Advances in Photochemistry 7: 311–71. doi:10.1002/9780470133378.ch4.
  2. 1 2 3 4 5 6 7 Klán P, Wirz J (2009). Photochemistry of Organic Compounds: From Concepts to Practice (Repr. 2010 ed.). Chichester, West Sussex, U.K.: Wiley. ISBN 1405190884.
  3. 1 2 Atkins P, de Paula J, Friedman R (2009). Quanta, Matter, and Change: A Molecular Approach to Physical Chemistry. Oxford: Oxford University Press. ISBN 0199206066.
  4. 1 2 Hill C. "Molecular Term Symbols" (PDF). Retrieved 11 August 2015.
  5. 1 2 3 Daniel G. Nocera (date unknown) "Lecture 2, Mar 11: Oxygen," self-published lecture in MIT course 5.03 Principles of Inorganic Chemistry, Cambridge, MA, USA: MIT Department of Chemistry, see , accessed 11 August 2015.
  6. ↑ Frimer AA (1985). Singlet Oxygen: Volume I, Physical-Chemical Aspects. Boca Raton, Fla.: CRC Press. pp. 4–7. ISBN 9780849364396.
  7. 1 2 3 Schweitzer C, Schmidt R (2003). "Physical Mechanisms of Generation and Deactivation of Singlet Oxygen". Chemical Reviews 103 (5): 1685–757. doi:10.1021/cr010371d. PMID 12744692.
  8. ↑ Wilkinson F, Helman WP, Ross AB (1995). "Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution. An expanded and revised compilation.". J. Phys. Chem. Ref. Data 24 (2): 663–677. doi:10.1063/1.555965.
  9. ↑ Direct measurements of absolute concentration and lifetime of singlet oxygen in the gas phase by electron paramagnetic resonance K.Hasegawa et al. (2008), Chem. Phys. Letters 457, 312–314
  10. ↑ Time-Resolved EPR Study of Singlet Oxygen in the Gas Phase M.Ruzzi et al. (2013), J. Phys. Chem. A, 117 5232–5240
  11. ↑ Paramagnetic resonance spectrum of the 1Δg oxygen molecule Falick, AM et al.(1965), J. Chem. Phys. 42, 1837-1838
  12. 1 2 Greer A (2006). "Christopher Foote's Discovery of the Role of Singlet Oxygen [1O2 (1Δg)] in Photosensitized Oxidation Reactions". Acc. Chem. Res. 39 (11): 797–804. doi:10.1021/ar050191g. PMID 17115719.
  13. ↑ Housecroft CE, Sharpe AG (2008). "Chapter 15: The group 16 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 438f. ISBN 9780131755536.
  14. 1 2 3 4 5 Wasserman HH, DeSimone RW, Chia KR, Banwell MG (2001). "Singlet Oxygen". e-EROS Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rs035.
  15. ↑ Ho RY, Liebman JF, Valentine JS (1995). "Overview of the Energetics and Reactivity of Oxygen". In Foote CS. Active Oxygen in Chemistry. London: Blackie Academic & Professional. pp. 1–23. doi:10.1007/978-94-007-0874-7_1. ISBN 978-0-7514-0371-8.
  16. ↑ Clennan EL, Pace A (2005). "Advances in singlet oxygen chemistry". Tetrahedron 61 (28): 6665–6691. doi:10.1016/j.tet.2005.04.017.
  17. ↑ Ogilby PR (2010). "Singlet oxygen: there is indeed something new under the sun". Chem Soc Rev 39 (8): 3181–209. doi:10.1039/b926014p. PMID 20571680.
  18. ↑ Carey FA, Sundberg RJ (1985). Structure and mechanisms (2 ed.). New York: Plenum Press. ISBN 0306411989.
  19. ↑ Stephenson LM, Grdina MJ, Orfanopoulos M (November 1980). "Mechanism of the ene reaction between singlet oxygen and olefins". Accounts of Chemical Research 13 (11): 419–425. doi:10.1021/ar50155a006.
  20. ↑ 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.
  21. ↑ Alsters PL, Jary W, Nardello-Rataj V, Jean-Marie A (2009). "Dark Singlet Oxygenation of β-Citronellol: A Key Step in the Manufacture of Rose Oxide". Organic Process Research & Development 14: 259–262. doi:10.1021/op900076g.
  22. ↑ Karp G, van der Geer P (2004). Cell and molecular biology: concepts and experiments (4th ed., Wiley International ed.). New York: J. Wiley & Sons. p. 223. ISBN 978-0471656654.
  23. ↑ Földes T, Čermák P, Macko M, Veis P, Macko P (January 2009). "Cavity ring-down spectroscopy of singlet oxygen generated in microwave plasma". Chemical Physics Letters 467 (4–6): 233–236. Bibcode:2009CPL...467..233F. doi:10.1016/j.cplett.2008.11.040.
  24. ↑ Nosaka Y, Daimon T, Nosaka, AY, Murakami Y (2004). "Singlet oxygen formation in photocatalytic TiO₂ aqueous suspension". Phys. Chem. Chem. Phys 6 (11): 2917–2918. doi:10.1039/B405084C.
  25. ↑ Mulliken RS (1928). "Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule". Nature 122: 505. doi:10.1038/122505a0.

Further reading

External links

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