5-Dehydro-m-xylylene

5-dehydro-m-xylylene, alternatively known as 5-dehydro-1,3-quinodimethane or DMX, is an aromatic organic triradical and the first known organic molecule to violate Hund's Rule. Its electronic ground state is an "open-shell doublet" rather than a quartet; that is, it contains three low-spin coupled unpaired electrons in three singly occupied molecular orbitals. Because there are radical electrons in both spin states, this compound is said to exhibit antiferromagnetism. Though similar ground states are observed in molecules containing transition metal atoms, it is unprecedented in organic molecules.

The 5-dehydro-m-xylylene anion (DMX-) has also been studied extensively. It has a triplet ground state consisting of a phenyl anion and a m-xylylene biradical.