Diradical

A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO) of the same energy ^{[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.

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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.

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