Diradical

A diradical in organic chemistry is a molecular species with two electrons occupying degenerate molecular orbitals (MO).[1][2] The term "diradical" is mainly used to describe organic compounds, where most diradical 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.

Not to be confused with biradical.

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.[3] The singlet state has S = 0 and is diamagnetic. The degeneracy of each state can be found with Hund's rule of maximum multiplicity: 2S + 1.

Examples of diradicals

Stable, isolable, diradicals include singlet oxygen and triplet oxygen. Other important diradicals are certain carbenes and nitrenes. Lesser known diradicals are nitrenium ions, carbon chains[4] and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms.

In inorganic chemistry, the concept of diradical is not applied although many metal complexes have triplet ground states.

Additional reading
  • "Diradicals". Meta-synthesis.com.
  • Pedersen, S.; Herek, J. L.; Zewail, A. H. (1994). "The Validity of the "Diradical" Hypothesis: Direct Femtoscond Studies of the Transition-State Structures". Science. 266 (5189): 1359–1364. Bibcode:1994Sci...266.1359P. doi:10.1126/science.266.5189.1359. PMID 17772843.
  • Zewail, A. H. (2000). "Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond Using Ultrafast Lasers (Nobel Lecture)". Angewandte Chemie International Edition. 39 (15): 2586–2631. doi:10.1002/1521-3773(20000804)39:15<2586::AID-ANIE2586>3.0.CO;2-O. PMID 10934390.
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References
  1. "Diradicals" (pdf). Gold Book. IUPAC.
  2. Manabu Abe (2013). "Diradicals". Chem. Rev. 113: 7011–7088. doi:10.1021/cr400056a.
  3. "Triplet State". Gold Book. IUPAC.
  4. Seenithurai, Sonai; Chai, Jeng-Da (2017-02-09). "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.
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