may appear to be rather obscure species, however, the oxygen, O2,
we breath is a triplet diradical. An understanding of diradical structure
and reactivity tells us that the simplistic electron pair covalent bonding
of Lewis octet theory is not the whole story. Liquid oxygen is magnetic,
a fact that can only be explained by understanding the nature of the bonding
in O2, here.
to Diradical Reaction Chemistry
Examples of diradicals include:
Methylene or carbene, CH2
Diradicals are species with
a pair of degenerate (equal energy) molecular orbitals and two electrons.
There are three possible diradical arrangements: the singlet state, the
triplet state and an unstable intermediate state.
The terms "singlet",
"doublet" and "triplet" concern the degeneracy of
the electronic state.
Degeneracy = 2S+1,
where S is the total electron spin angular momentum.
has a spin of +1/2 or 1/2, and an orbital can contain up to
two electrons but they must be of opposite spin: the Pauli exclusion
have a pair of electrons, one spin-up and one spin-down [+1/2 and 1/2],
in one orbital with the second, equal energy orbital, empty.
The two electron
spins are +1/2 and 1/2
Total electron spin angular momentum, S = 0
Degeneracy = (2*0) + 1 = 0+1 = 1 = singlet
A singlet electronic state does not give a signal in the
electron spin resonance (ESR) spectrum
A simple free
radical, R, is a doublet.
Total electron spin angular momentum, S = +1/2
Degeneracy = (2*1/2) + 1 = 1+1 = 2 = doublet
A doublet electronic state gives a single ESR line, provided
there is no nuclear hyperfine splitting
have two "spin-up" electrons in adjacent, degenerate (equal
of the same spin, +1/2 and +1/2
Total electron spin angular momentum, S = 1
Degeneracy = (2*1) + 1 = 2+1 = 3 = triplet
A triplet electronic state gives a pair of ESR lines (provided
there is no nuclear hyperfine splitting)
Thanks to Prof.
Paul Percival of the Simon Fraser University
and TRIUMF for help with this section (personal communication).
Carbenes undergo addition reactions
with alkenes. The singlet and triplet states exhibit subtly different
species behave as if they have both a Lewis base (HOMO) centre and a
Lewis acid (LUMO) centre. For carbenes, nitrenes and oxenes these two
centres occur at the same atom.
Singlet carbene, CH2,
has two electrons in a Lobe-HOMO Lewis base centre and a vacant p-orbital
LUMO. The two centres react with alkenes in a concerted, single-step
manner and so give rise to stereospecific products.
undergo 1,1-addition reactions with cis-alkenes with retention
of relative stereochemical configuration:
have two non-spin paired electrons which behave as a pair of radical
These two centres
react with an alkene in a stepwise fashion. This means that molecular
rotation can occur around the "single" bond between the reactions
The result is that
triplet diradicals give stereo mixed addition products:
are often called reactive intermediates and they only exist transiently.
There are several methods of production:
Ground state oxygen, O2,
triplet diradical, a property which can explain why liquid oxygen
is paramagnetic and attracted to the poles of a magnet:
Triplet oxygen, the formal form, can be converted into singlet oxygen with UV light and a dye such as rose bengal, discussed here.
The singlet oxygen can undergo
cycloaddition reactions, for example with 1,4-dimethyl naphthalene: