Triplet oxygen

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Triplet oxygen
Preferred IUPAC name
Triplet oxygen[citation needed]
Systematic IUPAC name
Dioxidanediyl[1] (substitutive)
dioxygen(2•)(triplet)[1] (additive)
7782-44-7 YesY
ATC code V03AN01
EC number 231-956-9
Jmol-3D images Image
KEGG D00003 YesY
MeSH Oxygen
RTECS number RS2060000
UN number 1072
Molar mass 32.00 g·mol−1
Appearance Colorless gas
Melting point −218.2 °C; −360.7 °F; 55.0 K
Boiling point −183.2 °C; −297.7 °F; 90.0 K
0 D
205.152 J K−1 mol−1
0 kJ mol−1
EU classification Oxidizing Agent O
R-phrases R8
S-phrases S17
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Triplet oxygen, systematically but less commonly, 1,2-dioxidanediyl, is a term that refers to normal, gaseous oxygen (O2, dioxygen) in its ground state. It is therefore classified as an inorganic chemical, and more specifically as a particular electronic state of one allotrope of the inorganic chemical element, oxygen. In this particular state, according to one type of modern bonding theory, the electron configuration of the oxygen molecule has two electrons occupying two molecular orbitals (MOs) of equal energy (that is, degenerate MOs), therefore remaining unpaired. These orbitals are classified as antibonding and are of higher energy, so the resulting bonding structure between the oxygen atoms is weakened (i.e., is higher in energy)—for instance, it is higher in energy than the bonding in dinitrogen, where bonding MOs are filled instead. The spectroscopic molecular term symbol for triplet (ground state) oxygen is 3Σg.[2][3][better source needed]


The valence orbitals of molecular oxygen (middle); in the ground state, the electrons in the π* orbitals have their spins parallel.

The s=1/2 spins of the two electrons in degenerate orbitals gives rise to 2x2=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 the ground state of the molecule carries an electron spin of S=1. Excitation to the S=0 state results in much more reactive, metastable singlet oxygen.

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. The Lewis structure O=O does not accurately represent the diradical nature of molecular oxygen; molecular orbital theory must be used to adequately account for the unpaired electrons. Triplet oxygen is better described by a bond order of one and two halves instead of two to better reflect its unpaired bonding electrons. This allows for easier reasoning of the bond length.


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[edit]

  • McNaught, A. D. & A. Wilkinson. (1997) "Singlet molecular oxygen (singlet molecular dioxygen)," In IUPAC Compendium of Chemical Terminology, 2nd edn. [the "Gold Book"], Oxford, GBR:Blackwell, ISBN 0967855098, 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 [3] or [4], accessed 11 August 2015.


  1. ^ a b "Triplet Dioxygen (CHEBI:27140)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 
  2. ^ Atkins, Peter; Julio de Paula & Ronald Friedman (2009) Quanta, Matter, and Change: A Molecular Approach to Physical Chemistry, pp. 341-342, Oxford, GBR:Oxford University Press, ISBN 0199206066, see [1]. accessed 11 August 2015.
  3. ^ Christian Hill, 2015, "Molecular Term Symbols," self-published, from Post-Doctoral Research Associate at the Department of Physics and Astronomy, University College London, see [2], accessed 11 August 2015.[better source needed]

External sources[edit]