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Triplet oxygen

|Section2={{Chembox Properties | O=2 | Appearance = Colorless gas | MeltingPtK = 55 | BoilingPtK = 90 }} |Section3={{Chembox Structure | MolShape = Linear | Dipole = 0 D }} |Section5={{Chembox Thermochemistry | DeltaHf = 0 kJ mol−1 | Entropy = 205.152 J K−1 mol−1 }} |Section6={{Chembox Pharmacology | ATCCode_prefix = V03 | ATCCode_suffix = AN01 }} |Section7={{Chembox Hazards | EUClass = | RPhrases = | SPhrases = | NFPA-F = 0 | NFPA-H = 0 | NFPA-R = 1 | NFPA-S = Ox }} }} Triplet oxygen, 3O2, refers to the S = 1 electronic ground state of molecular oxygen (dioxygen). It is the most stable and common allotrope of oxygen. Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradical. According to molecular orbital theory, the electron configuration of triplet oxygen has two electrons occupying two πg molecular orbitals (MOs) of equal energy (that is, degenerate MOs). In accordance with Hund's rules, they remain unpaired and spin-parallel and account for the paramagnetism of molecular oxygen. These half-filled orbitals are antibonding in character, reducing the overall bond order of the molecule to 2 from a maximum value of 3 (e.g., dinitrogen), which occurs when these antibonding orbitals remain fully unoccupied. The molecular term symbol for triplet oxygen is 3Σ.Atkins, Peter; De Paula, Julio; Friedman, Ronald (2009) Quanta, Matter, and Change: A Molecular Approach to Physical Chemistry, pp. 341–342, Oxford: Oxford University Press, , see linkhttps://books.google.com/books?isbn=0199206066. accessed 11 August 2015.


The s =  spins of the two electrons in degenerate orbitals gives rise to 2 × 2 = 4 independent spin states in total. Exchange interaction splits these into a singlet state (total spin S = 0) and a set of 3 degenerate triplet states (S = 1). In agreement with Hund's rules, the triplet states are energetically more favorable, and correspond to the ground state of the molecule with a total electron spin of S = 1. Excitation to the S = 0 state results in much more reactive, metastable singlet oxygen.

Lewis structure

Because the molecule in its ground state has a non-zero spin magnetic moment, oxygen is paramagnetic; i.e., it can be attracted to the poles of a magnet. Thus, the Lewis structure O=O with all electrons in pairs does not accurately represent the nature of the bonding in molecular oxygen. However, the alternative structure •O–O• is also inadequate, since it implies single bond character, while the experimentally determined bond length of 121 pm is much shorter than the single bond in hydrogen peroxide (HO–OH) which has a length of 147.5 pm.Housecroft and Sharpe p.443 This indicates that triplet oxygen has a higher bond order. Molecular orbital theory must be used to correctly account for the observed paramagnetism and short bond length simultaneously. Under a molecular orbital theory framework, the oxygen-oxygen bond in triplet dioxygen is better described as one full σ bond plus two half π bonds, the two half-bonds accounted for by two-center three-electron (2c3e) bonding, to give a net bond order of two (1+2×½), while also accounting for the spin state (S = 1). While no satisfactory Lewis structure is universally accepted, Pauling and Linnett have proposed Lewis structures that attempt to depict the 2c3e bond using three dots.


The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet state. Triplet oxygen will, however, readily react with molecules in a doublet state, such as radicals, to form a new radical. Conservation of spin quantum number would require a triplet transition state in a reaction of triplet oxygen with a closed shell (a molecule in a singlet state). The extra energy required is sufficient to prevent direct reaction at ambient temperatures with all but the most reactive substrates, e.g. white phosphorus. At higher temperatures or in the presence of suitable catalysts the reaction proceeds more readily. For instance, most flammable substances are characterised by an autoignition temperature at which they will undergo combustion in air without an external flame or spark.

Further reading

  • McNaught, A. D.; Wilkinson, A. (1997) "Singlet molecular oxygen (singlet molecular dioxygen)," In IUPAC Compendium of Chemical Terminology, 2nd edn. "Gold Book", Oxford, GBR: Blackwell, , DOI 10.1351/goldbook and 10.1351/goldbook.S05695; XML on-line corrected version created by M. Nic, J. Jirat, & B. Kosata, with updates compiled by A. Jenkins, see linkhttps://goldbook.iupac.org/S05695.html or http://goldbook.iupac.org, accessed 11 August 2015.


External sources

  • http://meta-synthesis.com/webbook/16_diradical/diradical.html
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