Singlet oxygen

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 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.

In spectroscopic notation, the singlet and triplet forms of O₂ are labeled ¹Δ_g and ³Σ−
g, respectively. The terms 'singlet oxygen' and 'triplet oxygen' refer to the quantum state of the molecules: singlet oxygen exists in the singlet state with a total quantum spin of 0, while triplet oxygen has a total quantum spin of 1.

Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted ¹Σ+
g and ¹Δ_g (the preceding superscripted "1" indicates it as 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+
g singlet state rapidly converts to the more stable, lower energy ¹Δ_g 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.