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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. In spectroscopic notation, the singlet and triplet forms of O2 are labeled 1Δg and 3Σg−, respectively. 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 == 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 1Σg+ singlet state rapidly converts to the more stable, lower energy 1Δg 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.〔 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).〔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.〕 〔 Molecular orbital theory predicts two low-lying excited singlet states, denoted by the molecular term symbols ¹Δg and ¹Σg+. 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 3Σg−. 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 1Δg 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 1Σg+ state.〔 〔 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). The 1Σg+ 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 3Σg−).〔〔〔 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 ← ³Σg−), is reported to be precisely 94.3 kJ/mol (0.98 electron volts, approx. 11340 Kelvin), corresponding to a frequency of 7882 cm−1.〔 The ¹Σg+ 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「singlet oxygen」の詳細全文を読む スポンサード リンク
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