An even-electron
molecular entity with two (possibly delocalized) radical centres which act nearly independently of
each other, e.g.
Species in which the two radical centres interact significantly are often referred
to as '
biradicaloids'. If the two radical centres are located on the same atom, the species are more properly
referred to by their generic names:
carbenes,
nitrenes, etc. The lowest-energy
triplet state of a biradical lies below or at most only a little above its lowest
singlet state (usually judged relative to
, the product of the
Boltzmann constant and the absolute temperature
). The states of those biradicals whose radical centres interact particularly weakly
are most easily understood in terms of a pair of local doublets. Theoretical descriptions
of low-energy states of biradicals display the presence of two unsaturated valences
(biradicals contain one fewer bond than permitted by the rules of
valence): the dominant
valence bond structures have two dots, the low energy
molecular orbitalconfigurations have only two electrons in two approximately nonbonding molecular orbitals, two of
the natural orbitals have occupancies close to one, etc. Although this term has been
recommended in the past for
diradicals, specialists working in the field prefer the latter term.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1089
PAC, 1995, 67, 1307
(Glossary of class names of organic compounds and reactivity intermediates based on
structure (IUPAC Recommendations 1995))
on page 1322