Diradical Chemistry

Diradicals 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.

Introduction to Diradical Reaction Chemistry

Examples of diradicals include:

Oxygen, O₂
Methylene (carbene), CH₂
Dichorocarbene, CCl₂

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.

Some Technical Stuff

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.

An electron 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 principle.

Singlet diradicals 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.

One unpaired electron
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

Triplet diradicals have two "spin-up" electrons in adjacent, degenerate (equal energy) orbitals.

Two electrons 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).

Differential Reactivity

Carbenes undergo addition reactions with alkenes. The singlet and triplet states exhibit subtly different reaction chemistry:

Singlet Diradicals

Singlet diradical 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.
Singlet diradicals undergo 1,1-addition reactions with cis-alkenes with retention of relative stereochemical configuration:

Triplet Diradicals

Triplet Diradicals have two non-spin paired electrons which behave as a pair of radical (SOMO) centres.
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 steps.
The result is that triplet diradicals give stereo mixed addition products:

Forming Carbenes

Carbenes, R₂C, are often called reactive intermediates and they only exist transiently. There are several methods of production:

Triplet Oxygen

Ground state oxygen, O₂, is a triplet diradical, a property which can explain why liquid oxygen is paramagnetic and attracted to the poles of a magnet:

Singlet Oxygen

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:

Radical Chemistry

Photochemistry

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