 | |
| Names | |
|---|---|
| IUPAC name Triplet oxygen | |
| Systematic IUPAC name | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider | |
| EC Number |
|
| 492 | |
| KEGG |
|
| MeSH | Oxygen |
| RTECS number |
|
| UNII | |
| UN number | 1072 |
| |
| |
| Properties | |
| O2 | |
| Molar mass | 31.998 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 |
| Structure | |
| Linear | |
| 0 D | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 205.152 J K−1 mol−1 |
Std enthalpy of formation(ΔfH⦵298) | 0 kJ mol−1 |
| Pharmacology | |
| V03AN01 (WHO) | |
| Hazards | |
| GHS labelling: | |
 | |
| Danger | |
| H270 | |
| P220,P244,P370+P376,P403 | |
| NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Triplet oxygen,3O2, refers to theS = 1 electronicground state of molecular oxygen (dioxygen). Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountereddiradical:[2] it is more stable as atriplet than asinglet. According tomolecular orbital theory, theelectron configuration of triplet oxygen has two electrons occupying two πmolecular orbitals (MOs) of equal energy (that is,degenerate MOs). In accordance withHund's rules, they remainunpaired and spin-parallel, which accounts for theparamagnetism of molecular oxygen. These half-filled orbitals areantibonding in character, reducing the overall bond order of the molecule to 2 from the maximum value of 3 that would occur when these antibonding orbitals remain fully unoccupied, as indinitrogen. Themolecular term symbol for triplet oxygen is3Σ−
g.[3]
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Thes = 1⁄2spins of the two electrons in degenerate orbitals gives rise to 2 × 2 = 4 independent spin states in total.Exchange interaction splits these into asinglet state (total spinS = 0) and a set of 3 degeneratetriplet states (S = 1). In agreement withHund's rules, the triplet states are energetically more favorable, and correspond to the ground state of the molecule with a total electron spin ofS = 1. Excitation to theS = 0 state results in much more reactive,metastablesinglet oxygen.[4][5]
Because the molecule in its ground state has a non-zero spinmagnetic moment,oxygen isparamagnetic; i.e., it can be attracted to the poles of amagnet. Thus, theLewis 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 determinedbond length of 121pm[6] is much shorter than the single bond inhydrogen peroxide (HO–OH) which has a length of 147.5 pm.[7] 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 π half-bonds, each half-bond accounted for bytwo-center three-electron (2c-3e) bonding, to give a net bond order of two (1+2×1/2), while also accounting for the spin state (S = 1). In the case of triplet dioxygen, each 2c-3e bond consists of two electrons in a πu bonding orbital and one electron in a πg antibonding orbital to give a net bond order contribution of1/2.
The usual rules for constructing Lewis structures must be modified to accommodate molecules like triplet dioxygen ornitric oxide that contain 2c-3e bonds. There is no consensus in this regard;Pauling has suggested the use of three closely spaced collinear dots to represent the three-electron bond (see illustration).[8]
A common experimental way to observe theparamagnetism of dioxygen is to cool it down into the liquid phase. When poured between the poles of strong magnets that are close together the liquid oxygen can be suspended. Or a magnet can pull the stream of liquid oxygen as it is poured. The net magnetic moment of the total electron spin provides an explanation of these observations.
The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in thesinglet state. Triplet oxygen will, however, readily react with molecules in adoublet state to form a new radical.
Conservation of spin quantum number would require a triplettransition state in a reaction of triplet oxygen with aclosed 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 anautoignition temperature at which they will undergo combustion in air without an external flame or spark.
