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The radical-pair mechanism explains how a magnetic field can affect reaction kinetics. Most commonly demonstrated in reactions of organic compounds, a magnetic field can speed up a reaction by decreasing the frequency of reverse reactions. A radical, of course, is a molecule with an odd number of electrons. The radical-pair, however, is not simply two radicals. This is because radical-pairs (specifically singlets) are quantum entangled, even as separate molecules. Radical-pair electrons both have spin, short for spin angular momentum, which gives each separate radical a magnetic moment.

The radical-pair is characterized as triplet or singlet by the coupled spin state of the two lone electrons. The spin relationship is such that the two unpaired electrons, one in each radical molecule, may have opposite spin (singlet; anticorrelated), or the same spin (triplet; correlated). The singlet state is called such because there is only one way for the electrons’ spins to anticorrelate (S), whereas the triplet state is called such because the electron’s spin may be parallel in three different fashions (T₊₁, T₀,T_-1).

This relates to chemical and biochemical reaction mechanisms because bonds can only be formed between two electrons of opposite spin (Hund’s Rule). Sometimes when a bond is broken in a particular manner, for example, when struck by photons, each electron in the bond relocates to each respective molecule, and a radical-pair is formed. Furthermore, the spin of each electron previously involved in the bond is conserved, which means that the radical-pair now formed is a singlet (each electron has opposite spin, as in the origin bond). 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 external magnetic field, and shows how a higher occurrence of the triplet state accelerates radical reactions because triplets can only proceed to products, and singlets are in equilibrium with the reactants as well as with the products. 

Zeeman interactions can “flip” one of the radical’s electron’s spin if the radical-pair is anisotropic, thereby converting radical-pairs to triplets.

The Zeeman interaction is an interaction between spin and external magnetic field, and is given by the formula

∆E=hv_L=gµ_BB,

where ∆E is the energy of the Zeeman interaction, in Joules, v_L is the Larmor frequency, in hertz, B is the external magnetic field, in teslas, µ_B is the Bohr magneton constant, h is Planck’s constant, and g is the g-value of a free electron, 2.002319, which is slightly different in different radicals.

Hyperfine interactions, the internal magnetic fields of local magnetic isotopes, play a significant role as well in the spin dynamics of radical-pairs.

And as a function of magnetic field and Larmor frequency, the Zeeman interaction can be obstructed or amplified by altering the external magnetic and/or the Larmor frequency with experimental instruments. This is interesting because migratory birds lose their navigational abilities in similar conditions where the Zeeman interaction is obstructed in radical-pairs.    

The radical-pair mechanism was proposed in 1969 by Closs; Captein and Oosterhoff, as an explanation for anomalous phenomena CIDNIP and CIDEP. Radical-pairs are chemical entities with quantum coherence. The radical-mechanism is primarily concerned with radical-pairs as chemical entities and does not figure in quantum mechanics unless contributing to an understanding of the chemistry.

Besides chlorophyll, which is not found in animals, Cryptochrome is the only protein known to form photoinduced radical-pairs. In vitro, these cryptochrome radical-pairs are affected by very weak magnetic fields, however, these radical-pairs are not necessarily relevant to magnetoreception. Some suggested magnetically sensitive radical-pairs in cryptochrome are FAD-Tryptophan, FAD-superoxide, FAD-ascorbic acid.