Spin barriers

In brradicals, where the two unpaired electrons can recombine to form a covalent bond, the resulting species may in turn serve as a potential precursor for the biradical. This only works in cases where the resulting biradical has a triplet ground state, which creates a spin barrier for the re-formation of the original bond. Two typical examples where this strategy succeeded are illustrated in Scheme 17.6. ... 

E Coordinating N2 to Mononuclear Fe(II) Complexes The Role of Spin Barriers... 

Fig. 9. Coordination of N2 to a Fe(II)-S model system with a fluorine ligand in trans-position requires overcoming a spin barrier.

Coordination of N2 to a complex with similar electronic features as the one in Fig. 9 is kinetically not favored because of the spin barrier. However, a single N2 complex would be metastable since it would be protected by the same barrier. 

To overcome this spin barrier, initiators or catalysts are required to start the lipid oxidation process by removing an electron from either the lipid or oxygen or by changing the electron spin of the oxygen. As only trace amounts of catalysts are needed, many situations that appear to be spontaneous or uncatalyzed are actually driven by contaminants or conditions that have gone undetected or unconsidered. Indeed, in most foods, biological systems, and laboratory experiments, it is fair to say that multiple catalysts and initiators are always operative. 

Molecular oxygen is the principal oxidant in the ordinary combustion of fuels. Burning is essentially a high-temperature analog of autooxidation, in which the spin barrier to the initiation step (Equation 4.67) is overcome by use of high temperatures. 

The direct oxidation of unsaturated lipids by triplet oxygen ( O ) is spin forbidden because the lipid ground state of singlet multiplicity has an opposite spin direction from that of oxygen of triplet multiplicity. This spin barrier between lipids and oxygen can be readily overcome in the presence of initiators that can produce radicals by different mechanisms. 

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