The radical pair mechanism is currently accepted as the most common cause of CIDNP. This theory was proposed by Closs, and, independently, by Kaptein and Oosterhoff. There are, however, exceptions, and the DNP mechanism was found to be operational, for example, in many fluorine-containing radicals.
The chemical bond is a pair of electrons with opposite spins. Photochemical reactions or heat can cause an electron in the bond to change its spin. The electrons are now unpaired, in what is known as a triplet state, and the bond is broken. The orientation of some of the nuclear spins will favour some unpaired electrons changing their spins and so revert to the normal pairs as chemical bonds. This quantum interaction is known as spin–orbit coupling. Other nuclear spins will exert a different influence on the triplet pairs, giving the radical pairs more time to separate and react with other molecules. Consequently, the products of recombination will have different distributions of nuclear spins from the products produced by separated radicals.