Diradical

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