Diradical

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

Not to be confused with biradical.

Examples of diradical species can also be found in coordination chemistry, for example among bis(1,2-dithiolene) metal complexes.[3][4]

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, Ms=2*0+1=1, ms=0) and exhibits no signal in EPR 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 total spin quantum number S=1 and is paramagnetic.[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 be diamagnetic.[6]

Examples

Stable, isolable, diradicals include singlet oxygen and triplet oxygen. Other important diradicals are certain carbenes, nitrenes, and their main-group elemental analogues.[7] Lesser-known diradicals are nitrenium ions, carbon chains,[8] 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.[3]

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Diradicals". doi:10.1351/goldbook.D01765
  2. Abe M (September 2013). "Diradicals". Chemical Reviews. 113 (9): 7011–7088. doi:10.1021/cr400056a. PMID 23883325.
  3. Aragoni MC, Caltagirone C, Lippolis V, Podda E, Slawin AM, Woollins JD, et al. (December 2020). "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; mnt2-=1,2-Dicyano-1,2-ethylenedithiolate)". Inorganic Chemistry. 59 (23): 17385–17401. doi:10.1021/acs.inorgchem.0c02696. PMC 7735710. PMID 33185438.
  4. Ray K, Weyhermüller T, Neese F, Wieghardt K (July 2005). "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. doi:10.1021/ic0507565. PMID 16022533.
  5. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Triplet State". doi:10.1351/goldbook.T06503
  6. 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. PMID 12160406.
  7. 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. 143 (1): 121–125. doi:10.1021/jacs.0c11828. ISSN 0002-7863. PMID 33373236. S2CID 229719653.
  8. Seenithurai S, Chai JD (July 2017). "Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study". Scientific Reports. 7 (1): 4966. arXiv:1702.03055. Bibcode:2017NatSR...7.4966S. doi:10.1038/s41598-017-05202-6. PMC 5504039. PMID 28694445.

Further reading
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