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