Spin states relate to chemical and biochemical reaction mechanisms because bonds can be formed only between two electrons of opposite spin (Hund's rules). 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 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.
Zeeman interactions can “flip” only one of the radicals electrons spin if the radical-pair is anisotropic, thereby converting singlet radical-pairs to triplets.
The Zeeman interaction is an interaction between spin and external magnetic field, and is given by the equation
where  is the energy of the Zeeman interaction,  is the Larmor frequency,  is the external magnetic field,  is the Bohr magneton,  is Planck's constant, and  is the g-factor of a free electron, 2.002319, which is slightly different in different radicals.
It is common to see the Zeeman interaction formulated in other ways.