A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO).[1][2] They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number permitted by the standard rules of valence.[3][4] The electrons can pair up with opposite spin in one MO leaving the other empty. This is called a singlet state. Alternatively each electron can occupy one MO with spins parallel to each other. This is called a triplet state. The related radical has just one free electron. The phrases singlet and triplet are derived from the appearance of diradicals in electron spin resonance: a singlet diradical displays a single peak in its spectrum and a triplet has its peak split into a central peak with two adjacent peaks.
The triplet state has total spin quantum number S = 1 and is paramagnetic.[5] 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.
In molecules the free electrons can reside on one atom or on different atoms. A molecule can have a singlet state or triplet state with different energy and both states can inter-convert by a process called intersystem crossing. Phosphorescence is based on this principle.
Discrete molecules with a diradical nature are singlet oxygen and triplet oxygen. Other important diradicals are carbenes and nitrenes. Lesser known diradicals are nitrenium ions and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms.
External links [link]
- "Diradicals". Meta-synthesis.com. https://www.meta-synthesis.com/webbook/16_diradical/diradical.html.
References [link]
- ^ "Diradicals" (pdf). Gold Book. IUPAC. https://goldbook.iupac.org/D01765.html.
- ^ Turro, N. J. (1969). "The Triplet State". Journal of Chemical Education 46 (1): 2. DOI:10.1021/ed046p2.
- ^ 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. DOI:10.1126/science.266.5189.1359.
- ^ 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.
- ^ "Triplet State". Gold Book. IUPAC. https://goldbook.iupac.org/T06503.html.
https://wn.com/Diradical
Non-Kekulé molecule
A non-Kekulé molecule is a conjugated hydrocarbon that cannot be assigned a classical Kekulé structure.
Since non-Kekulé molecules have two or more formal radical centers, their spin-spin interactions can cause electrical conductivity or ferromagnetism (molecule-based magnets), and applications to functional materials are expected. However, as these molecules are quite reactive and most of them are easily decomposed or polymerized at room temperature, strategies for stabilization are needed for their practical use. Synthesis and observation of these reactive molecules are generally accomplished by matrix-isolation methods.
Biradicals
The simplest non-Kekulé molecules are biradicals. A biradical is an even-electron chemical compound with two free radical centres which act independently of each other. They should not be confused with the more general class of diradicals.
One of the first biradicals was synthesized by Wilhelm Schlenk in 1915 following the same methodology as Moses Gomberg's triphenylmethyl radical. The so-called Schlenk-Brauns hydrocarbons are:
