Term: singlet molecular oxygen (singlet molecular dioxygen)
https://doi.org/10.1351/goldbook.S05695

Definition:
Oxygen molecule (dioxygen), O2, in an excited singlet state. The ground state of O2 is a triplet (3Σg-). The two metastable singlet states derived from the ground-state configuration are 1Δg and 1Σg+ (the latter with the higher energy).

Notes:
0) Use of the term singlet oxygen alone, without mention of the chemical species is discouraged since it can also refer to an oxygen atom in a 1S or 1D @E02257@. The oxygen atom @G02704@ is a triplet 3P state and the 1S and 1D states are also derived from the ground-state configuration.
1) @T06503-1@ @Q05007@ by triplet dioxygen (i.e., a process of @P04652@) is the most common procedure for the production of singlet molecular dioxygen in solution. For many chemical species, the efficiency with which the @T06503-2@ is quenched by triplet dioxygen and, independently, the efficiency with singlet molecular dioxygen is produced, is controlled by the spin-@R05175@.

Related Terms:
1) excited state (http://doi.org/10.1351/goldbook.E02257).
2) ground state (http://doi.org/10.1351/goldbook.G02704).
3) triplet state (http://doi.org/10.1351/goldbook.T06503-1).
4) quenching (http://doi.org/10.1351/goldbook.Q05007).
5) photosensitization (http://doi.org/10.1351/goldbook.P04652).
6) statistical factor (http://doi.org/10.1351/goldbook.R05175).
7) singlet state (http://doi.org/10.1351/goldbook.S05699).
8) metastable (http://doi.org/10.1351/goldbook.M03872).
9) triplet state (http://doi.org/10.1351/goldbook.T06503-2).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 420 (https://doi.org/10.1351/pac200779030293)

Citation: 'singlet molecular oxygen (singlet molecular dioxygen)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.S05695

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