Diradical

{{short description|Chemical species with two electrons occupying degenerate molecular orbitals}} {{Distinguish|biradical}}

In chemistry, a '''diradical''' is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate.<ref>{{GoldBookRef|title=Diradicals|file=D01765}}</ref><ref>{{cite journal |vauthors=Abe M |title=Diradicals |journal=Chemical Reviews |volume=113 |issue=9 |pages=7011–7088 |date=September 2013 |pmid=23883325 |doi=10.1021/cr400056a}}</ref> The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and non-Kekulé molecules that are rarely isolated. Diradicals are even-electron molecules but have one fewer bond than the number permitted by the octet rule.

Examples of diradical species can also be found in coordination chemistry, for example among bis(1,2-dithiolene) metal complexes.<ref name=":0">{{cite journal |vauthors=Aragoni MC, Caltagirone C, Lippolis V, Podda E, Slawin AM, Woollins JD, Pintus A, Arca M |display-authors=6 |title=Diradical Character of Neutral Heteroleptic Bis(1,2-dithiolene) Metal Complexes: Case Study of [Pd(Me₂timdt)(mnt)] (Me₂timdt=1,3-Dimethyl-2,4,5-trithioxoimidazolidine; mnt^2-=1,2-Dicyano-1,2-ethylenedithiolate) |journal=Inorganic Chemistry |volume=59 |issue=23 |pages=17385–17401 |date=December 2020 |pmid=33185438 |doi=10.1021/acs.inorgchem.0c02696 |pmc=7735710}}</ref><ref>{{cite journal |vauthors=Ray K, Weyhermüller T, Neese F, Wieghardt K |title=Electronic structure of square planar bis(benzene-1,2-dithiolato)metal complexes [M(L)(2)](z) (z=2-, 1-, 0; M=Ni, Pd, Pt, Cu, Au): an experimental, density functional, and correlated ab initio study |journal=Inorganic Chemistry |volume=44 |issue=15 |pages=5345–5360 |date=July 2005 |pmid=16022533 |doi=10.1021/ic0507565}}</ref>

==Spin states== Diradicals are usually triplets. The phrases ''singlet'' and ''triplet'' are derived from the multiplicity of states of diradicals in electron spin resonance: a singlet diradical has one state (S=0, M_s=2*0+1=1, m_s=0) and exhibits no signal in EPR and a triplet diradical has 3 states (S=1, M_s=2*1+1=3, m_s=-1; 0; 1) and shows in EPR 2 peaks (if no hyperfine splitting). The triplet state has total spin quantum number S=1 and is paramagnetic.<ref>{{GoldBookRef|title=Triplet State|file=T06503}}</ref> 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 be diamagnetic.<ref>{{cite journal |vauthors=Bachler V, Olbrich G, Neese F, Wieghardt K |title=Theoretical evidence for the singlet diradical character of square planar nickel complexes containing two o-semiquinonato type ligands |journal=Inorganic Chemistry |volume=41 |issue=16 |pages=4179–4193 |date=August 2002 |pmid=12160406 |doi=10.1021/ic0113101}}</ref>

==Examples== Stable, isolable, diradicals include singlet oxygen and triplet oxygen. Other important diradicals are certain carbenes, nitrenes, and their main-group elemental analogues.<ref>{{cite journal |last1=Sharma |first1=Mahendra K. |last2=Ebeler |first2=Falk |last3=Glodde |first3=Timo |last4=Neumann |first4=Beate |last5=Stammler |first5=Hans-Georg |last6=Ghadwal |first6=Rajendra S. |date=2021-01-13 |title=Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting |journal=Journal of the American Chemical Society |language=en |volume=143 |issue=1 |pages=121–125 |issn=0002-7863 |pmid=33373236 |doi=10.1021/jacs.0c11828 |s2cid=229719653 |url=https://pubs.acs.org/doi/10.1021/jacs.0c11828|url-access=subscription }}</ref> Lesser-known diradicals are nitrenium ions, carbon chains,<ref>{{cite journal |vauthors=Seenithurai S, Chai JD |title=Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study |journal=Scientific Reports |volume=7 |issue=1 |page=4966 |date=July 2017 |arxiv=1702.03055 |bibcode=2017NatSR...7.4966S |pmid=28694445 |doi=10.1038/s41598-017-05202-6 |pmc=5504039}}</ref> and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms. N-heterocyclic-carbene derived singlet diradicals have been used for solution-phase singlet fission via self-assembly.<ref>{{cite journal |last1=Ullrich |first1=T. |last2=Pinter |first2=P. |last3=Messelberger |first3=J. |last4=Haines |first4=P. |last5=Kaur |first5=R. |last6=Hansmann |first6=M. M. |last7=Munz |first7=D. |last8=Guldi |first8=D. |title=Singlet Fission in Carbene Derived Diradicaloids |journal=Angewandte Chemie International Edition |year=2020 |volume=59 |page=7906 |doi=10.1002/anie.202001286}}</ref><ref>{{cite journal |last1=Messelberger |first1=J. |last2=Grünwald |first2=A. |last3=Pinter |first3=P. |last4=Hansmann |first4=M. |last5=Munz |first5=D. |title=Carbene Derived Diradicaloids – Building Blocks for Singlet Fission? |journal=Chemical Science |year=2018 |volume=9 |page=6107 |doi=10.1039/C8SC01999A}}</ref><ref>{{cite journal |last1=Hansmann |first1=M. M. |last2=Melaimi |first2=M. |last3=Munz |first3=D. |last4=Bertrand |first4=G. |title=Modular Approach to Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes |journal=Journal of the American Chemical Society |year=2018 |volume=140 |page=2546 |doi=10.1021/jacs.7b11183}}</ref> Some isolable Kekulé diradicals are stable on air and room-temperature luminescent with large Stokes-shifts and solvatochromism.<ref>{{cite journal |last1=Bevilacqua |first1=M. |last2=Reato |first2=M. |last3=Cilento |first3=F. |last4=Graiff |first4=C. |last5=Antonello |first5=S. |last6=Schio |first6=L. |last7=Aliprandi |first7=A. |last8=Tubaro |first8=C. |last9=Franco |first9=L. |last10=Dell'Angela |first10=M. |last11=Munz |first11=D. |last12=Baron |first12=M. |title=Hidden Diradical: Conformational Switch for Solvatochromic NIR Emission with Unity Quantum Yield in Thiele's Hydrocarbon |journal=Angewandte Chemie International Edition |year=2026 |article-number=e202524042 |doi=10.1002/anie.202524042|hdl=11577/3586820 |hdl-access=free }}</ref><ref>{{cite journal |last1=Punzi |first1=A. |last2=Ullrich |first2=T. |last3=Orza |first3=M. |last4=Mesto |first4=D. |last5=Moliterni |first5=A. |last6=Olieric |first6=V. |last7=Engilberge |first7=S. |last8=Giannini |first8=C. |last9=others |title=Twist and Shine: The Impact of Halogen Substitution on Thiele Hydrocarbon's Optical Properties |journal=Angewandte Chemie International Edition |year=2026 |article-number=e24043 |doi=10.1002/anie.202524043|pmc=12865251 }}</ref> Main-group cyclic structures can also exhibit diradicals, such as disulfur dinitride, or diradical character, such as diphosphadiboretanes<!--that article title IS plural for now(?)-->. In inorganic chemistry, both homoleptic and heteroleptic 1,2-dithiolene complexes of d⁸ transition metal ions show a large degree of diradical character in the ground state.<ref name=":0"/>

Diradicals in which the unpaired electrons nevertheless interact are sometimes referred to as diradicaloids.

==References== {{Reflist}}

==Further reading== * {{cite web |title=Diradicals |publisher=Meta-synthesis.com |url=https://www.meta-synthesis.com/webbook/16_diradical/diradical.php}} * {{cite journal |vauthors=Pedersen S, Herek JL, Zewail AH |title=The validity of the "diradical" hypothesis: direct femtosecond studies of the transition-state structures |journal=Science |volume=266 |issue=5189 |pages=1359–1364 |date=November 1994 |bibcode=1994Sci...266.1359P |pmid=17772843 |doi=10.1126/science.266.5189.1359 |s2cid=45399626}} * {{cite journal |vauthors=Zewail AH |title=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 |journal=Angewandte Chemie |volume=39 |issue=15 |pages=2586–2631 |date=August 2000 |pmid=10934390 |doi=10.1002/1521-3773(20000804)39:15<2586::AID-ANIE2586>3.0.CO;2-O}}

Category:Inorganic chemistry Category:Magnetism Category:Organic chemistry