Singlet oxygen 1898647 217544453 2008-06-06T14:52:55Z SmackBot 433328 Date the maintenance tags or general fixes '''Singlet oxygen''' is the common name used for the two [[metastable state]]s of molecular [[oxygen]] (O<sub>2</sub>) with higher energy than the ground state [[triplet oxygen]] <ref>{{cite journal | title = Physical and chemical properties of singlet molecular oxygen | author = David R. Kearns | journal = [[Chemical Reviews]] | year = 1971 | volume = 71 | issue = 4 | pages = 395–427 | doi = 10.1021/cr60272a004 }}</ref>. O<sub>2</sub> is a common gas in the Earth's atmosphere, but it is typically found with its electrons in the "triplet" state. The molecule can absorb energy in order to change its electron configuration to one of the singlet configurations. Usually, "excited" electron configurations result in the immediate discharge of the extra energy in the form of a [[photon]], but because of its unusual properties, singlet oxygen can persist for over an hour, depending on the environment. Because of the differences in their electron shells, singlet oxygen has different chemical properties than triplet oxygen, including absorbing and emitting light at different wavelengths. ==Chemistry== The chemistry of singlet oxygen is different from that of ground state oxygen. Singlet oxygen can participate in [[Diels-Alder]] reactions and [[ene reaction]]s. It can be generated in a photosensitized process by energy transfer from dye molecules such as [[rose bengal]], [[methylene blue]] or [[porphyrins]], or by chemical processes such as spontaneous decomposition of [[hydrogen trioxide]] in water or the reaction of [[hydrogen peroxide]] with [[hypochlorite]] <ref>''Physical Mechanisms of Generation and Deactivation of Singlet Oxygen'' C. Schweitzer, R. Schmidt [[Chemical Reviews]] Volume 103, Pages 1685-1757 '''2003'''</ref>. Singlet oxygen reacts with an alkene -C=C-CH- by abstraction of the [[allylic]] proton in an [[ene reaction]] type reaction to the allyl [[hydroperoxide]] HO-O-C-C=C. It can then be reduced to the [[allyl alcohol]]. With some substrates [[dioxetane]]s are formed and cyclic dienes such as [[1,3-Cyclohexadiene]] form [4+2][[cycloaddition]] adducts. <ref>Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. ISBN 0-306-41198-9.</ref>. ==Biochemistry== In [[photosynthesis]], singlet oxygen can be produced from the light-harvesting [[chlorophyll]] molecules. One of the roles of [[carotenoid]]s 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 [[mammal]]ian [[biology]], singlet oxygen is a form of [[reactive oxygen species]], which is linked to oxidation of [[LDL]] [[cholesterol]] and resultant [[cardiovascular]] effects. [[Polyphenol antioxidant]]s can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects <ref>''Cell and Molecular Cell Biology concepts and experiments'' Fourth Edition. Gerald Karp. Page 223 '''2005'''</ref>. Singlet oxygen is the active species [[photodynamic therapy]]. ==Orbital states== [[Molecular orbital theory]] predicts two low-lying excited [[singlet]] states O<sub>2</sub>(a¹Δ<sub>g</sub>) and O<sub>2</sub>(b¹Σ<sub>g</sub><sup>+</sup>) (for nomenclature see article on [[Molecular term symbol]]). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate [[antibonding]] π<sub>g</sub>-orbitals (see [[degenerate energy level]]). The O<sub>2</sub>(b¹Σ<sub>g</sub><sup>+</sup>)-state is very short lived and relaxes quickly to the lowest lying [[excited state]], O<sub>2</sub>(a¹Δ<sub>g</sub>). Thus, the O<sub>2</sub>(a¹Δ<sub>g</sub>)-state is commonly referred to as singlet oxygen. The energy difference between the lowest energy of O<sub>2</sub> in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (T<sub>e</sub> (a¹Δ<sub>g</sub> <- X³Σ<sub>g</sub><sup>-</sup>) = 7918.1 cm<sup>-1</sup>.) Molecular [[oxygen]] differs from most molecules in having an open-shell triplet ground state, O<sub>2</sub>(X³Σ<sub>g</sub><sup>-</sup>). ==Physics== [[Image:Molekülorbital-Sauerstoff.png|500px|thumb|right|[[Molecular orbital diagrams]] for the three electronic configurations of molecular oxygen, O<sub>2</sub>. Shown from the left are: The triplet ground state, the singlet oxygen a¹Δ<sub>g</sub> excited state, and the singlet oxygen b¹Σ<sub>g</sub><sup>+</sup> excited state. Note that the states only differ in the spin and the occupancy of oxygen's two degenerate [[antibonding]] π<sub>g</sub>-orbitals.]] The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-[[infrared]] at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity [[selection rules]], making it one of nature's most forbidden transitions. In other words, 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). Interaction with solvents, however, reduces the lifetime to microseconds or even nanoseconds.{{Facts|date=June 2008}} Direct detection of singlet oxygen is possible through its extremely weak [[phosphorescence]] at 1270 nm, which is not visible to the eye. However, at high singlet oxygen concentrations, the [[fluorescence]] of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm <ref>''Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule'' R.S. [[Mulliken]] [[Nature (journal)]] Volume 122, Page 505 '''1928'''</ref>. ==External links== * [http://webbook.nist.gov/cgi/cbook.cgi?ID=C7782447&Units=SI&Mask=1000#Diatomic The NIST webbook on oxygen] * [http://www.photobiology.com/educational/len2/singox.html Photochemistry & Photobiology tutorial on Singlet Oxygen] * [http://chemed.chem.purdue.edu/demos/main_pages/5.12.html Demonstration of the Red Singlet Oxygen Dimol Emission (Purdue University)] == References == {{reflist}} {{oxygenallotropes}} [[Category:Oxidizing agents]] [[Category:Spectroscopy]] [[Category:Physical chemistry]] [[Category:Oxygen]] [[de:Sauerstoff#Molekülorbitale]] [[nl:Singlet zuurstof]] [[ja:一重項酸素]] [[pl:Tlen singletowy]] [[ru:Синглетный кислород]]