Diradical

A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO). 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.