Diradicals are usuallytriplets. The phrasessinglet andtriplet are derived from the multiplicity of states of diradicals inelectron spin resonance: a singlet diradical has one state (S=0, Ms=2*0+1=1, ms=0) and exhibits no signal inEPR and a triplet diradical has 3 states (S=1, Ms=2*1+1=3, ms=-1; 0; 1) and shows in EPR 2 peaks (if no hyperfine splitting). The triplet state has totalspin quantum number S=1 and isparamagnetic.[5] Therefore, diradical species display a triplet state when the two electrons are unpaired and display the same spin. When the unpaired electrons with opposite spin are antiferromagnetically coupled, diradical species can display a singlet state (S=0) and bediamagnetic.[6]
Stable, isolable, diradicals includesinglet oxygen andtriplet oxygen. Other important diradicals are certaincarbenes,nitrenes, and their main-group elemental analogues.[7] Lesser-known diradicals arenitrenium ions, carbon chains,[8] and organic so-callednon-Kekulé molecules in which the electrons reside on different carbon atoms. Main-group cyclic structures can also exhibit diradicals, such asdisulfur dinitride, or diradical character, such asdiphosphadiboretanes. In inorganic chemistry, bothhomoleptic andheteroleptic 1,2-dithiolene complexes of d8 transition metal ions show a large degree of diradical character in the ground state.[3]
Diradicals in which the unpaired electrons nevertheless interact are sometimes referred to asdiradicaloids.
^Bachler V, Olbrich G, Neese F, Wieghardt K (August 2002). "Theoretical evidence for the singlet diradical character of square planar nickel complexes containing two o-semiquinonato type ligands".Inorganic Chemistry.41 (16):4179–4193.doi:10.1021/ic0113101.PMID12160406.
Zewail AH (August 2000). "Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond Using Ultrafast Lasers (Nobel Lecture) Copyright((c)) The Nobel Foundation 2000. We thank the Nobel Foundation, Stockholm, for permission to print this lecture".Angewandte Chemie.39 (15):2586–2631.doi:10.1002/1521-3773(20000804)39:15<2586::AID-ANIE2586>3.0.CO;2-O.PMID10934390.
