Spin chemistry

The radical-pair mechanism emerged as an explanation to CIDNP and CIDEP and was proposed in 1969 by Closs; Kaptein and Oosterhoff.

[3] A radical is a molecule with an odd number of electrons, and is induced in a variety of ways, including ultraviolet radiation.

A sun burn is largely due to radical formation from this radiation.

The radical-pair is characterized as triplet or singlet by the spin state of the two lone electrons, paired together.

As such, the reverse reaction, i.e. the reforming of a bond, called recombination, readily occurs.

The radical-pair mechanism explains how external magnetic fields can prevent radical-pair recombination with Zeeman interactions, the interaction between spin and an external magnetic field, and shows how a higher occurrence of the triplet state accelerates radical reactions because triplets can proceed only to products, and singlets are in equilibrium with the reactants as well as with the products.

[1][3][4] Because the Zeeman interaction is a function of magnetic field and Larmor frequency, it can be obstructed or amplified by altering the external magnetic or the Larmor frequency with experimental instruments that generate oscillating fields.

It has been observed that migratory birds lose their navigational abilities in such conditions where the Zeeman interaction is obstructed in radical-pairs.

Example radical: Structure of Hydrocarboxyl radical, lone electron indicated as single black dot
Simple diagram of bonding between electrons with opposite spin
Typical Reaction Scheme of the Radical-pair Mechanism, which shows the effect of alternative product formation from singlet versus triplet radical-pairs. The Zeeman and Hyperfine Interactions take effect in the yellow box, denoted as step 4 in the process