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Concept
Singlet oxygen
Summary
Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as 1[O2] or 1O2), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow.
The lowest excited state of the diatomic oxygen molecule is a 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, 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. It often appears and coexists confounded in environments that also 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 lowest singlet and triplet forms of O2 are labeled 1Δg and 3Σ, respectively.
Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted 1Σ and 1Δg (the preceding superscript "1" indicates a singlet state). 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 singlet state rapidly converts to the more stable, lower energy 1Δg singlet state. This more stable of the two excited states has its two valence electrons spin-paired in one π* orbital while the second π* orbital is empty.
Official source
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