Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O₂) with higher energy than the ground state triplet oxygen ^[1]. Metastability is a general scientific concept which describes states of delicate equilibrium Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Triplet oxygen is the Ground state of the Oxygen molecule The Electron configuration of the molecule has two unpaired electrons occupying two degenerate O₂ 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. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 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 reactions. The Diels-Alder reaction is an Organic chemical reaction (specifically a Cycloaddition) between a conjugated Diene and a substituted Alkene, The Ene reaction (also known as the Alder-ene reaction) is a Chemical reaction between an Alkene with an Allylic Hydrogen (the 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 ^[2]. Rose Bengal (4567-tetrachloro-2'4'5'7'-tetraiodofluorescein is a stain. Methylene blue is a heterocyclic aromatic Chemical compound with Molecular formula: C 16 H 18 Cl[[nitrogen A porphyrin is a heterocyclic Macrocycle derived from four Pyrroline subunits interconnected via their α carbon atoms via Methine bridges (=CH- Trioxidane, hydrogen trioxide or dihydrogen trioxide (H2O3 or HOOOH is an unstable molecule a type of hydrogen polyoxide which in Hydrogen peroxide (H2O2 is a very pale blue liquid which appears colorless in a dilute solution slightly more Viscous than water The hypochlorite Ion is Cl[[Oxygen O]]− A hypochlorite compound is a Chemical compound containing this group with chlorine in Oxidation 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. An allyl group is an Alkene Hydrocarbon group with the formula H2C=CH-CH2- The Ene reaction (also known as the Alder-ene reaction) is a Chemical reaction between an Alkene with an Allylic Hydrogen (the Organic peroxides are Organic compounds containing the Peroxide Functional group (ROOR' It can then be reduced to the allyl alcohol. Allyl alcohol or 2-propen-1-ol is an Organic compound with the formula CH2=CHCH2OH With some substrates dioxetanes are formed and cyclic dienes such as 1,3-Cyclohexadiene form [4+2]cycloaddition adducts. A cycloaddition is a Pericyclic Chemical reaction, in which two π bonds are lost and two σ bonds are gained the resulting reaction is a ^[3].

Biochemistry

In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. Chlorophyll is a green Pigment found in most Plants Algae and Cyanobacteria. 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. Carotenoids are organic Pigments that are naturally occurring in Chromoplasts of plants and some other photosynthetic Organisms Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Chlorophyll is a green Pigment found in most Plants Algae and Cyanobacteria.

In mammalian biology, singlet oxygen is a form of reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Foundations of modern biology There are five unifying principles Reactive oxygen species (ROS are ions or very small molecules that include Oxygen Ions free radicals, and Peroxides both inorganic and Low-density lipoprotein ( LDL) is a type of Lipoprotein that transports Cholesterol and Triglycerides from the Liver to peripheral Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian This is an article about the rock music band "Circulatory System" Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects ^[4]. A polyphenol antioxidant is a type of Antioxidant containing a Polyphenolic substructure

Singlet oxygen is the active species photodynamic therapy. Photodynamic therapy (PDT matured as a feasible medical technology in the 1980s at several institutions throughout the world is a third-level treatment for Cancer involving

Orbital states

Molecular orbital theory predicts two low-lying excited singlet states O₂(a¹Δ_g) and O₂(b¹Σ_g⁺) (for nomenclature see article on Molecular term symbol). In Chemistry, molecular orbital theory ( MO theory) is a method for determining molecular structure in which Electrons are not assigned to individual In Molecular physics, the molecular term symbol is a shorthand expression of the Group representation and angular momenta that characterize the state of These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding π_g-orbitals (see degenerate energy level). Antibonding (or anti-bonding) is a type of chemical bonding. An antibonding orbital is a form of Molecular orbital (MO that is located outside the region This article refers to physical states having the same energy The O₂(b¹Σ_g⁺)-state is very short lived and relaxes quickly to the lowest lying excited state, O₂(a¹Δ_g). Excitation is an elevation in energy level above an arbitrary baseline energy state Thus, the O₂(a¹Δ_g)-state is commonly referred to as singlet oxygen.

The energy difference between the lowest energy of O₂ in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (T_e (a¹Δ_g <- X³Σ_g^-) = 7918. 1 cm^-1. ) Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O₂(X³Σ_g^-). Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

Physics

Molecular orbital diagrams for the three electronic configurations of molecular oxygen, O₂. A molecular orbital diagram or MO diagram for short is a qualitative descriptive tool explaining Chemical bonding in Molecules in terms of Molecular Shown from the left are: The triplet ground state, the singlet oxygen a¹Δ_g excited state, and the singlet oxygen b¹Σ_g⁺ excited state. Note that the states only differ in the spin and the occupancy of oxygen's two degenerate antibonding π_g-orbitals. Antibonding (or anti-bonding) is a type of chemical bonding. An antibonding orbital is a form of Molecular orbital (MO that is located outside the region

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. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of 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 Physics and Chemistry, especially in the context of Quantum mechanics, a selection rule is a condition constraining the physical properties of the initial 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.

Direct detection of singlet oxygen is possible through its extremely weak phosphorescence at 1270 nm, which is not visible to the eye. Phosphorescence is a specific type of Photoluminescence related to fluorescence. 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 ^[5]. Fluorescence is a Luminescence that is mostly found as an

External links

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

References