Diradical Explained

In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate.^[1] 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.^{[2] [3]}

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. 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.^[4]

Examples

Stable, isolable, diradicals include singlet oxygen and triplet oxygen. Other important diradicals are certain carbenes, nitrenes, and their main-group elemental analogues.^[5] Lesser-known diradicals are nitrenium ions, carbon chains,^[6] and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms. Main-group cyclic structures can also exhibit diradicals, such as disulfur dinitride, or diradical character, such as diphosphadiboretanes. 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.^[2]

Further reading

• Web site: Diradicals . Meta-synthesis.com .
• Pedersen S, Herek JL, Zewail AH . The validity of the "diradical" hypothesis: direct femtosecond studies of the transition-state structures . Science . 266 . 5189 . 1359–1364 . November 1994 . 1994Sci...266.1359P . 17772843 . 10.1126/science.266.5189.1359 . 45399626.
• Zewail AH . 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 . August 2000 . 10934390 . 10.1002/1521-3773(20000804)39:15<2586::AID-ANIE2586>3.0.CO;2-O.

Notes and References

1. Abe M . Diradicals . Chemical Reviews . 113 . 9 . 7011–7088 . September 2013 . 23883325 . 10.1021/cr400056a.
2. Aragoni MC, Caltagirone C, Lippolis V, Podda E, Slawin AM, Woollins JD, Pintus A, Arca M . 6 . 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) . Inorganic Chemistry . 59 . 23 . 17385–17401 . December 2020 . 33185438 . 10.1021/acs.inorgchem.0c02696 . 7735710.
3. Ray K, Weyhermüller T, Neese F, Wieghardt K . 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 . Inorganic Chemistry . 44 . 15 . 5345–5360 . July 2005 . 16022533 . 10.1021/ic0507565.
4. Bachler V, Olbrich G, Neese F, Wieghardt K . Theoretical evidence for the singlet diradical character of square planar nickel complexes containing two o-semiquinonato type ligands . Inorganic Chemistry . 41 . 16 . 4179–4193 . August 2002 . 12160406 . 10.1021/ic0113101.
5. Sharma . Mahendra K. . Ebeler . Falk . Glodde . Timo . Neumann . Beate . Stammler . Hans-Georg . Ghadwal . Rajendra S. . 2021-01-13 . Isolation of a Ge(I) Diradicaloid and Dihydrogen Splitting . Journal of the American Chemical Society . en . 143 . 1 . 121–125 . 0002-7863 . 33373236 . 10.1021/jacs.0c11828 . 229719653 .
6. Seenithurai S, Chai JD . Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study . Scientific Reports . 7 . 1 . 4966 . July 2017 . 1702.03055 . 2017NatSR...7.4966S . 28694445 . 10.1038/s41598-017-05202-6 . 5504039.