A mechanism enabling nuclear spin to influence the direction of a chemical reaction. This can occur in certain cases, even though the gap between energy levels of nuclear spin states in a magnetic field is very much smaller than the dissociation energies of chemical bonds. Two radicals, the electrons of which have parallel spins, can only combine if the conversion of a triplet to a singlet can take place. In a magnetic field, a triplet has three nondegenerate states called T₀, T₊, and T_–. For a triplet-to-singlet conversion to take place, one electron must precess faster than the other for a sufficient time to enable a 180° phase difference to develop. This difference in precession can arise when the nuclear spin interacts with the electron on the radical, by means of hyperfine coupling.