Chemistry: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 in fact 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.[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
- ↑ "Diradicals" (pdf). Gold Book (IUPAC). 2014. doi:10.1351/goldbook.D01765. http://goldbook.iupac.org/D01765.html.
- ↑ "Diradicals". Chemical Reviews 113 (9): 7011–7088. September 2013. doi:10.1021/cr400056a. PMID 23883325.
- ↑ 3.0 3.1 "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. December 2020. doi:10.1021/acs.inorgchem.0c02696. PMID 33185438.
- ↑ "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. doi:10.1021/ic0507565. PMID 16022533.
- ↑ "Triplet State". Gold Book (IUPAC). 2014. doi:10.1351/goldbook.T06503. http://goldbook.iupac.org/T06503.html.
- ↑ "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. doi:10.1021/ic0113101. PMID 12160406.
- ↑ 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" (in en). Journal of the American Chemical Society 143 (1): 121–125. doi:10.1021/jacs.0c11828. ISSN 0002-7863. PMID 33373236. https://pubs.acs.org/doi/10.1021/jacs.0c11828.
- ↑ "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. doi:10.1038/s41598-017-05202-6. PMID 28694445. Bibcode: 2017NatSR...7.4966S.
Further reading
- "Diradicals". Meta-synthesis.com. http://www.meta-synthesis.com/webbook/16_diradical/diradical.html.
- "The validity of the "diradical" hypothesis: direct femtoscond studies of the transition-state structures". Science 266 (5189): 1359–1364. November 1994. doi:10.1126/science.266.5189.1359. PMID 17772843. Bibcode: 1994Sci...266.1359P.
- "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. doi:10.1002/1521-3773(20000804)39:15<2586::AID-ANIE2586>3.0.CO;2-O. PMID 10934390.
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