In chemistry, a nitrene is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and univalent, so it has only 6 electrons in its valence level—one covalent bond and four non-bonded electrons, it is therefore considered an electrophile due to the unsatisfied octet. A nitrene is involved in many chemical reactions; the simplest nitrene, HN, is called imidogen, that term is sometimes used as a synonym for the nitrene class. In the simplest case, the linear N–H molecule has its nitrogen atom sp hybridized, with two of its four non-bonded electrons as a lone pair in an sp orbital and the other two occupying a degenerate pair of p orbitals; the electron configuration is consistent with Hund's rule: the low energy form is a triplet with one electron in each of the p orbitals and the high energy form is the singlet with an electron pair filling one p orbital and the other p orbital one vacant. As with carbenes, a strong correlation exists between the spin density on the nitrogen atom which can be calculated in silico and the zero-field splitting parameter D which can be derived experimentally from electron spin resonance.
Small nitrenes such as NH or CF3N have D values around 1.8 cm−1 with spin densities close to a maximum value of 2. At the lower end of the scale are molecules with low D values and spin density of 1.2 to 1.4 such as 9-anthrylnitrene and 9-phenanthrylnitrene. Because nitrenes are so reactive, they are not isolated. Instead, they are formed as reactive intermediates during a reaction. There are two common ways to generate nitrenes: From azides by thermolysis or photolysis, with expulsion of nitrogen gas; this method is analogous to the formation of carbenes from diazo compounds. From isocyanates, with expulsion of carbon monoxide; this method is analogous to the formation of carbenes from ketenes. Nitrene reactions include: Nitrene C–H insertion. A nitrene can insert into a carbon to hydrogen covalent bond yielding an amine or amide. A singlet nitrene reacts with retention of configuration. In one study a nitrene, formed by oxidation of a carbamate with potassium persulfate, gives an insertion reaction into the palladium to nitrogen bond of the reaction product of palladium acetate with 2-phenylpyridine to methyl N-carbamate in a cascade reaction:A nitrene intermediate is suspected in this C–H insertion involving an oxime, acetic anhydride leading to an isoindole:Nitrene cycloaddition.
With alkenes, nitrenes react to form aziridines often with nitrenoid precursors such as nosyl- or tosyl-substituted phenyliodinane ) but the reaction is known to work directly with the sulfonamide in presence of a transition metal based catalyst such as copper, palladium, or gold:In most cases, phenyliodinane is prepared separately as follows:Nitrene transfer takes place next:In this particular reaction both the cis-stilbene illustrated and the trans form result in the same trans-aziridine product, suggesting a two-step reaction mechanism. The energy difference between triplet and singlet nitrenes can be small in some cases, allowing interconversion at room temperature. Triplet nitrenes are thermodynamically more stable but react stepwise allowing free rotation and thus producing a mixture of stereochemistry. Arylnitrene ring-expansion and ring-contraction. Aryl nitrenes show ring expansion to 7-membered ring cumulenes, ring opening reactions and nitrile formations many times in complex reaction paths.
For instance the azide 2 in the scheme below trapped in an argon matrix at 20 K on photolysis expels nitrogen to the triplet nitrene 4, in equilibrium with the ring-expansion product 6. The nitrene converts to the ring-opened nitrile 5 through the diradical intermediate 7. In a high-temperature reaction, FVT at 500–600 °C yields the nitrile 5 in 65% yield. For several compounds containing both a nitrene group and a free radical group an ESR high-spin quartet has been recorded. One of these has an amine oxide radical group incorporated, another system has a carbon radical group. In this system one of the nitrogen unpaired electrons is delocalized in the aromatic ring making the compound a σ–σ–π triradical. A carbene nitrogen radical resonance structure makes a contribution to the total electronic picture
Paramagnetism is a form of magnetism whereby certain materials are weakly attracted by an externally applied magnetic field, form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, diamagnetic materials are repelled by magnetic fields and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Paramagnetic materials include some compounds; the magnetic moment induced by the applied field is rather weak. It requires a sensitive analytical balance to detect the effect and modern measurements on paramagnetic materials are conducted with a SQUID magnetometer. Paramagnetism is due to the presence of unpaired electrons in the material, so all atoms with incompletely filled atomic orbitals are paramagnetic. Due to their spin, unpaired electrons have a magnetic dipole act like tiny magnets. An external magnetic field causes the electrons' spins to align parallel to the field, causing a net attraction.
Paramagnetic materials include aluminium, oxygen and iron oxide. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field because thermal motion randomizes the spin orientations, thus the total magnetization drops to zero. In the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field; this fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnetic materials is non-linear and much stronger, so that it is observed, for instance, in the attraction between a refrigerator magnet and the iron of the refrigerator itself. Constituent atoms or molecules of paramagnetic materials have permanent magnetic moments in the absence of an applied field; the permanent moment is due to the spin of unpaired electrons in atomic or molecular electron orbitals. In pure paramagnetism, the dipoles do not interact with one another and are randomly oriented in the absence of an external field due to thermal agitation, resulting in zero net magnetic moment.
When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field. In the classical description, this alignment can be understood to occur due to a torque being provided on the magnetic moments by an applied field, which tries to align the dipoles parallel to the applied field. However, the true origins of the alignment can only be understood via the quantum-mechanical properties of spin and angular momentum. If there is sufficient energy exchange between neighbouring dipoles, they will interact, may spontaneously align or anti-align and form magnetic domains, resulting in ferromagnetism or antiferromagnetism, respectively. Paramagnetic behavior can be observed in ferromagnetic materials that are above their Curie temperature, in antiferromagnets above their Néel temperature. At these temperatures, the available thermal energy overcomes the interaction energy between the spins. In general, paramagnetic effects are quite small: the magnetic susceptibility is of the order of 10−3 to 10−5 for most paramagnets, but may be as high as 10−1 for synthetic paramagnets such as ferrofluids.
In conductive materials, the electrons are delocalized, that is, they travel through the solid more or less as free electrons. Conductivity can be understood in a band structure picture as arising from the incomplete filling of energy bands. In an ordinary nonmagnetic conductor the conduction band is identical for both spin-up and spin-down electrons; when a magnetic field is applied, the conduction band splits apart into a spin-up and a spin-down band due to the difference in magnetic potential energy for spin-up and spin-down electrons. Since the Fermi level must be identical for both bands, this means that there will be a small surplus of the type of spin in the band that moved downwards; this effect is a weak form of paramagnetism known as Pauli paramagnetism. The effect always competes with a diamagnetic response of opposite sign due to all the core electrons of the atoms. Stronger forms of magnetism require localized rather than itinerant electrons. However, in some cases a band structure can result in which there are two delocalized sub-bands with states of opposite spins that have different energies.
If one subband is preferentially filled over the other, one can have itinerant ferromagnetic order. This situation only occurs in narrow bands, which are poorly delocalized. Strong delocalization in a solid due to large overlap with neighboring wave functions means that there will be a large Fermi velocity; this is why s- and p-type metals are either Pauli-paramagnetic or as in the case of gold diamagnetic. In the latter case the diamagnetic contribution from the closed shell inner electrons wins over the weak paramagnetic term of the free electrons. Stronger magnetic effects are only observed when d or f electrons are involved; the latter are strongly localized. Moreover, the size of the magnetic
Organic chemistry is a subdiscipline of chemistry that studies the structure and reactions of organic compounds, which contain carbon in covalent bonding. Study of structure determines their chemical formula. Study of properties includes physical and chemical properties, evaluation of chemical reactivity to understand their behavior; the study of organic reactions includes the chemical synthesis of natural products and polymers, study of individual organic molecules in the laboratory and via theoretical study. The range of chemicals studied in organic chemistry includes hydrocarbons as well as compounds based on carbon, but containing other elements oxygen, sulfur and the halogens. Organometallic chemistry is the study of compounds containing carbon–metal bonds. In addition, contemporary research focuses on organic chemistry involving other organometallics including the lanthanides, but the transition metals zinc, palladium, cobalt and chromium. Organic compounds constitute the majority of known chemicals.
The bonding patterns of carbon, with its valence of four—formal single and triple bonds, plus structures with delocalized electrons—make the array of organic compounds structurally diverse, their range of applications enormous. They form the basis of, or are constituents of, many commercial products including pharmaceuticals; the study of organic chemistry overlaps organometallic chemistry and biochemistry, but with medicinal chemistry, polymer chemistry, materials science. Before the nineteenth century, chemists believed that compounds obtained from living organisms were endowed with a vital force that distinguished them from inorganic compounds. According to the concept of vitalism, organic matter was endowed with a "vital force". During the first half of the nineteenth century, some of the first systematic studies of organic compounds were reported. Around 1816 Michel Chevreul started a study of soaps made from various alkalis, he separated the different acids. Since these were all individual compounds, he demonstrated that it was possible to make a chemical change in various fats, producing new compounds, without "vital force".
In 1828 Friedrich Wöhler produced the organic chemical urea, a constituent of urine, from inorganic starting materials, in what is now called the Wöhler synthesis. Although Wöhler himself was cautious about claiming he had disproved vitalism, this was the first time a substance thought to be organic was synthesized in the laboratory without biological starting materials; the event is now accepted as indeed disproving the doctrine of vitalism. In 1856 William Henry Perkin, while trying to manufacture quinine accidentally produced the organic dye now known as Perkin's mauve, his discovery, made known through its financial success increased interest in organic chemistry. A crucial breakthrough for organic chemistry was the concept of chemical structure, developed independently in 1858 by both Friedrich August Kekulé and Archibald Scott Couper. Both researchers suggested that tetravalent carbon atoms could link to each other to form a carbon lattice, that the detailed patterns of atomic bonding could be discerned by skillful interpretations of appropriate chemical reactions.
The era of the pharmaceutical industry began in the last decade of the 19th century when the manufacturing of acetylsalicylic acid—more referred to as aspirin—in Germany was started by Bayer. By 1910 Paul Ehrlich and his laboratory group began developing arsenic-based arsphenamine, as the first effective medicinal treatment of syphilis, thereby initiated the medical practice of chemotherapy. Ehrlich popularized the concepts of "magic bullet" drugs and of systematically improving drug therapies, his laboratory made decisive contributions to developing antiserum for diphtheria and standardizing therapeutic serums. Early examples of organic reactions and applications were found because of a combination of luck and preparation for unexpected observations; the latter half of the 19th century however witnessed systematic studies of organic compounds. The development of synthetic indigo is illustrative; the production of indigo from plant sources dropped from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the synthetic methods developed by Adolf von Baeyer.
In 2002, 17,000 tons of synthetic indigo were produced from petrochemicals. In the early part of the 20th century and enzymes were shown to be large organic molecules, petroleum was shown to be of biological origin; the multiple-step synthesis of complex organic compounds is called total synthesis. Total synthesis of complex natural compounds increased in complexity to terpineol. For example, cholesterol-related compounds have opened ways to synthesize complex human hormones and their modified derivatives. Since the start of the 20th century, complexity of total syntheses has been increased to include molecules of high complexity such as lysergic acid and vitamin B12; the discovery of petroleum and the development of the petrochemical industry spurred the development of organic chemistry. Converting individual petroleum compounds into different types of compounds by various chemical processes led to organic reactions enabling a broad range of
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, biochemistry, the term molecule is used less also being applied to polyatomic ions. In the kinetic theory of gases, the term molecule is used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are monatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with oxygen. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances, they make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust and core of the Earth, contain many chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for ionic crystals and covalent crystals, although these are composed of repeating unit cells that extend either in a plane or three-dimensionally. The theme of repeated unit-cellular-structure holds for most condensed phases with metallic bonding, which means that solid metals are not made of molecules. In glasses, atoms may be held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating units that characterizes crystals; the science of molecules is called molecular chemistry or molecular physics, depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, this distinction is vague. In molecular sciences, a molecule consists of a stable system composed of two or more atoms.
Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for reactive species, i.e. short-lived assemblies of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate. According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. Molecule – "extremely minute particle", from French molécule, from New Latin molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the definition of the molecule has evolved. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties; this definition breaks down since many substances in ordinary experience, such as rocks and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
Molecules are held together by ionic bonding. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements. A covalent bond is a chemical bond; these electron pairs are termed shared pairs or bonding pairs, the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding. Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, is the primary interaction occurring in ionic compounds; the ions are atoms that have lost one or more electrons and atoms that have gained one or more electrons. This transfer of electrons is termed electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. An ionic bond is the transfer of electrons from a metal to a non-metal for both atoms to obtain a full valence shell.
Most molecules are far too small to be seen with the naked eye. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. Molecules used as building blocks for organic synthesis have a dimension of a few angstroms to several dozen Å, or around one billionth of a meter. Single molecules cannot be observed by light, but small molecules and the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope; some of the largest molecules are supermolecules. The smallest molecule is the diatomic hydrogen, with a bond length of 0.74 Å. Effective molecular radius is the size; the table of permselectivity for different substances contains examples. The chemical formula for a molecule uses one line of chemical element symbols and sometimes al
Diamagnetic materials are repelled by a magnetic field. In contrast and ferromagnetic materials are attracted by a magnetic field. Diamagnetism is a quantum mechanical effect. In paramagnetic and ferromagnetic substances the weak diamagnetic force is overcome by the attractive force of magnetic dipoles in the material; the magnetic permeability of diamagnetic materials is the permeability of vacuum. In most materials diamagnetism is a weak effect which can only be detected by sensitive laboratory instruments, but a superconductor acts as a strong diamagnet because it repels a magnetic field from its interior. Diamagnetism was first discovered when Sebald Justinus Brugmans observed in 1778 that bismuth and antimony were repelled by magnetic fields. In 1845, Michael Faraday demonstrated that it was a property of matter and concluded that every material responded to an applied magnetic field. On a suggestion by William Whewell, Faraday first referred to the phenomenon as diamagnetic later changed it to diamagnetism.
Diamagnetism is a property of all materials, always makes a weak contribution to the material's response to a magnetic field. However, other forms of magnetism are so much stronger that when multiple different forms of magnetism are present in a material, the diamagnetic contribution is negligible. Substances where the diamagnetic behaviour is the strongest effect are termed diamagnetic materials, or diamagnets. Diamagnetic materials are those that laypeople think of as non-magnetic, include water, most organic compounds such as petroleum and some plastics, many metals including copper the heavy ones with many core electrons, such as mercury and bismuth; the magnetic susceptibility values of various molecular fragments are called Pascal's constants. Diamagnetic materials, like water, or water-based materials, have a relative magnetic permeability, less than or equal to 1, therefore a magnetic susceptibility less than or equal to 0, since susceptibility is defined as χv = μv − 1; this means. However, since diamagnetism is such a weak property, its effects are not observable in everyday life.
For example, the magnetic susceptibility of diamagnets such as water is χv = −9.05×10−6. The most diamagnetic material is bismuth, χv = −1.66×10−4, although pyrolytic carbon may have a susceptibility of χv = −4.00×10−4 in one plane. These values are orders of magnitude smaller than the magnetism exhibited by paramagnets and ferromagnets. Note that because χv is derived from the ratio of the internal magnetic field to the applied field, it is a dimensionless value. In rare cases, the diamagnetic contribution can be stronger than paramagnetic contribution; as is the case for gold, which has a magnetic susceptibility less than 0, so is by definition a diamagnetic material, but when measured with X-ray magnetic circular dichroism, shows an weak paramagnetic contribution, overcome by a stronger diamagnetic contribution. Superconductors may be considered perfect diamagnets, because they expel all magnetic fields due to the Meissner effect. If a powerful magnet is covered with a layer of water the field of the magnet repels the water.
This causes a slight dimple in the water's surface. Diamagnets may be levitated in stable equilibrium with no power consumption. Earnshaw's theorem seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theorem applies only to objects with positive susceptibilities, such as ferromagnets and paramagnets; these are attracted to field maxima. Diamagnets are attracted to field minima, there can be a field minimum in free space. A thin slice of pyrolytic graphite, an unusually strong diamagnetic material, can be stably floated in a magnetic field, such as that from rare earth permanent magnets; this can be done with all components at room temperature, making a visually effective demonstration of diamagnetism. The Radboud University Nijmegen, the Netherlands, has conducted experiments where water and other substances were levitated. Most spectacularly, a live frog was levitated. In September 2009, NASA's Jet Propulsion Laboratory in Pasadena, California announced it had levitated mice using a superconducting magnet, an important step forward since mice are closer biologically to humans than frogs.
JPL said it hopes to perform experiments regarding the effects of microgravity on bone and muscle mass. Recent experiments studying the growth of protein crystals have led to a technique using powerful magnets to allow growth in ways that counteract Earth's gravity. A simple homemade device for demonstration can be constructed out of bismuth plates and a few permanent magnets that levitate a permanent magnet; the electrons in a material settle in orbitals, with zero resistance and act like current loops. Thus it might be imagined that diamagnetism effects in general would be common, since any applied magnetic field would generate currents in these loops that would
The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas. The rule is applicable to carbon, nitrogen and the halogens, but to metals such as sodium or magnesium; the valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted once for each atom. In carbon dioxide each oxygen shares four electrons with the central carbon, two from the oxygen itself and two from the carbon. All four of these electrons are counted in both the oxygen octet. Ionic bonding is common between pairs of atoms, where one of the pair is a metal of low electronegativity and the second a nonmetal of high electronegativity. A chlorine atom has seven electrons in its outer electron shell, the first and second shells being filled with two and eight electrons respectively.
The first electron affinity of chlorine is -328.8 kJ per mole of chlorine atoms. Adding a second electron to chlorine requires energy, energy that cannot be recovered by the formation of a chemical bond; the result is that chlorine will often form a compound in which it has eight electrons in its outer shell. A sodium atom has a single electron in its outermost electron shell, the first and second shells again being full with two and eight electrons respectively. To remove this outer electron requires only the first ionization energy, +495.8 kJ per mole of sodium atoms, a small amount of energy. By contrast, the second electron resides in the deeper second electron shell, the second ionization energy required for its removal is much larger: +4562.4 kJ per mole. Thus sodium will, in most cases, form a compound in which it has lost a single electron and have a full outer shell of eight electrons, or octet; the energy required to transfer an electron from a sodium atom to a chlorine atom is small: +495.8 − 328.8 = +167 kJ mol−1.
This energy is offset by the lattice energy of sodium chloride: −787.3 kJ mol−1. This completes the explanation of the octet rule in this case. In the late 19th century it was known that coordination compounds were formed by the combination of atoms or molecules in such a manner that the valencies of the atoms involved became satisfied. In 1893, Alfred Werner showed that the number of atoms or groups associated with a central atom is 4 or 6. In 1904 Richard Abegg was one of the first to extend the concept of coordination number to a concept of valence in which he distinguished atoms as electron donors or acceptors, leading to positive and negative valence states that resemble the modern concept of oxidation states. Abegg noted that the difference between the maximum positive and negative valences of an element under his model is eight. In 1916, Gilbert N. Lewis referred to this insight as Abegg's rule and used it to help formulate his cubical atom model and the "rule of eight", which began to distinguish between valence and valence electrons.
In 1919 Irving Langmuir refined these concepts further and renamed them the "cubical octet atom" and "octet theory". The "octet theory" evolved into what is now known as the "octet rule". Walther Kossel and Gilbert N. Lewis saw that noble gases did not have the tendency of taking part in chemical reactions under ordinary conditions. On the basis of this observation they concluded that atoms of noble gases are stable and on the basis of this conclusion they proposed a theory of valency known as "Electronic Theory of valency" in 1916: During the formation of a chemical bond, atoms combine together by gaining, losing or sharing electrons in such a way that they acquire nearest noble gas configuration; the quantum theory of the atom explains the eight electrons as a closed shell with an s2p6 electron configuration. A closed-shell configuration is one in which low-lying energy levels are full and higher energy levels are empty. For example, the neon atom ground state has an empty n = 3 shell. According to the octet rule, the atoms before and after neon in the periodic table, tend to attain a similar configuration by gaining, losing, or sharing electrons.
The argon atom has an analogous 3s2 3p6 configuration. There is an empty 3d level, but it is at higher energy than 3s and 3p, so that 3s2 3p6 is still considered a closed shell for chemical purposes; the atoms before and after argon tend to attain this configuration in compounds. There are, some hypervalent molecules in which the 3d level may play a part in the bonding, although this is controversial. For helium there is no 1p level according to the quantum theory, so that 1s2 is a closed shell with no p electrons; the atoms before and after helium follow a duet rule and tend to have the same 1s2 configuration as helium. The octet rule is only applicable to main group elements and there are many molecules that do not obey the octet rule; these molecules can be divided into two types: unstable intermediates that react so as to attain stability, stable molecules that follow other electron counting rules. Although stable odd-electron molecules and hypervalent molecules are taught as v
Singlet oxygen, systematically named dioxygen and dioxidene, is a gaseous inorganic chemical with the formula O=O, in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, however the rate of decay is slow; the lowest excited state of the diatomic oxygen molecule is the singlet state. It is a gas with physical properties differing only subtly from those of the more prevalent triplet ground state of O2. In terms of its chemical reactivity, 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 in polluted urban atmospheres where it contributes to the formation of lung-damaging nitrogen dioxide, it appears and coexists confounded in environments that generate ozone, such as pine forests with photodegradation of turpentine.
The terms'singlet oxygen' and'triplet oxygen' derive from each form's number of electron spins. The singlet has only one possible arrangement of electron spins with a total quantum spin of 0, while the triplet has three possible arrangements of electron spins with a total quantum spin of 1, corresponding to three degenerate states. In spectroscopic notation, the singlet and triplet forms of O2 are labeled 1Δg and 3Σ−g, respectively. Singlet oxygen refers to one of two singlet electronic excited states; the two singlet states are denoted 1Σ + 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 converts to the more stable, lower energy 1Δg singlet state. Molecular orbital theory predicts the electronic ground state denoted by the molecular term symbol 3Σ–g and two low-lying excited singlet states, with molecular term symbols 1Δg and 1Σ+g; these three electronic states differ only in the spin and the occupancy of oxygen's two antibonding πg-orbitals, which are degenerate.
These two orbitals are of higher energy. Following Hund's first rule, in the ground state, these electrons have like spin; this open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet ground states. Two less stable, higher energy excited states are accessible from this ground state, again in accordance with Hund's first rule. 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; the ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below. The 1Δg 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 above the ground state. Radiative transitions between the three low-lying electronic states of oxygen are formally forbidden as electric dipole processes; the two singlet-triplet transitions are forbidden both because of the spin selection rule ΔS = 0 and because of the parity rule that g-g transitions are forbidden.
The lower, O2 state is referred to as singlet oxygen. The energy difference of 94.3 kJ/mol between ground state and singlet oxygen corresponds to a forbidden singlet-triplet transition in the near-infrared at ~1270 nm. As a consequence, singlet oxygen in the gas phase is long lived, although interaction with solvents reduces the lifetime to microseconds or nanoseconds; the higher 1Σ+g state is short lived. In the gas phase, it relaxes to the ground state triplet with a mean lifetime of 11.8 s. However in solvents such as CS2 and CCl4, it relaxes to the lower singlet 1Δg in milliseconds due to nonradiative decay channels. Both singlet oxygen states have no unpaired electrons and therefore no net electron spin; the 1Δg is however paramagnetic as shown by the observation of an electron paramagnetic resonance spectrum. The paramagnetism is due to a net orbital 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. 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 be in non-photochemical, preparative chemical procedures. One chemical method involves the decomposition of triet