Editing Triplet state (section)

{{Short description|Quantum state of a system}}
{{more footnotes needed|date=December 2010}}
[[File:Spin multiplicity diagram.svg|thumb|Examples of atoms in [[singlet state|singlet]], [[doublet state|doublet]], and '''triplet''' states.]]
In [[quantum mechanics]], a '''triplet state''', or '''spin triplet''', is the [[quantum state]] of an object such as an electron, atom, or molecule, having a [[Spin (physics)|quantum spin]] ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''<sub>S</sub> = −1, 0, or +1, giving the name "triplet".

[[Spin (physics)|Spin]], in the context of quantum mechanics, is not a mechanical rotation but a more abstract concept that characterizes a particle's intrinsic angular momentum. It is particularly important for systems at atomic length scales, such as individual [[atoms]], [[protons]], or [[electrons]].

A triplet state occurs in cases where the spins of two [[unpaired electron]]s, each having spin ''s'' = {{frac|2}}, align to give ''S'' = 1, in contrast to the more common case of two electrons aligning oppositely to give ''S'' = 0, a [[spin singlet]]. Most molecules encountered in daily life exist in a singlet state because all of their electrons are paired, but [[molecular oxygen]] is an exception.<ref>{{cite journal |last1=Borden |first1=Weston Thatcher |last2=Hoffmann |first2=Roald |last3=Stuyver |first3=Thijs |last4=Chen |first4=Bo |date=2017 |title=Dioxygen: What Makes This Triplet Diradical Kinetically Persistent? |journal=Journal of the American Chemical Society |volume=139|issue=26 |pages=9010–9018 |doi=10.1021/jacs.7b04232 |pmid=28613073 |doi-access=free }}</ref> At [[room temperature]], O<sub>2</sub> exists in a triplet state, which can only undergo a chemical reaction by making the [[forbidden transition]] into a singlet state. This makes it kinetically nonreactive despite being thermodynamically one of the strongest oxidants. [[Photochemistry|Photochemical]] or thermal [[Activation energy|activation]] can bring it into the [[Singlet oxygen|singlet state]], which makes it kinetically as well as thermodynamically a very strong oxidant.