Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with two oxygen atoms in a quantum state where all electrons are spin-paired, known as a singlet state. It is the lowest excited state of the diatomic oxygen molecule, which in general has the chemical structure O=O and chemical formula O2. Singlet oxygen can be written more specifically as 1[O2] or 1O2. The more prevalent ground state of O2 is known as triplet oxygen. At room temperature, singlet oxygen will slowly decay into triplet oxygen, releasing the energy of excitation. Singlet oxygen is a gas with physical properties differing only subtly from the ground state. 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 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−g, respectively.
Electronic structure Singlet oxygen refers to one of two singlet electronic excited states. The two singlet states are denoted 1Σ+g 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+g 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. This state is referred to by the title term, singlet oxygen, commonly abbreviated 1O2, to distinguish it from the triplet ground state molecule, 3O2. Molecular orbital theory predicts the electronic ground state denoted by the molecular term symbol 3Σ–g, and two low-lying excited singlet states with 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 (equal in energy). These two orbitals are classified as antibonding and are of higher energy. Following Hund's first rule, in the ground state, these electrons are unpaired and have like (same) spin. This open-shell triplet ground state of molecular oxygen differs from most stable diatomic molecules, which have singlet (1Σ+g) ground states. 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 1Σ+g state. 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 (157.0 kJ/mol or 1.6268 eV) 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 singlet-singlet transition between the two excited states is spin-allowed but parity-forbidden. The lower, O2(1Δg) state is commonly 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 relatively long lived (54-86 milliseconds), although interaction with solvents reduces the lifetime to microseconds or even nanoseconds. In 2021, the lifetime of airborne singlet oxygen at air/solid interfaces was measured to be 550 microseconds. The higher 1Σ+g state is moderately short lived. In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 seconds. However in solvents such as CS2 and CCl4, it relaxes to the lower singlet 1Δg in milliseconds due to radiationless decay channels.
Paramagnetism due to orbital angular momentum 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 (EPR) spectrum. The paramagnetism of the 1Δg state is due to a net orbital (and not spin) electronic angular momentum. In a magnetic field the degeneracy of the M L {\displaystyle M_{L}} levels is split into two levels with z projections of angular momenta +1ħ and −1ħ around the molecular axis. The magnetic transition between these levels gives rise to the g = 1 {\displaystyle g=1} EPR transition.
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