Effect of External Magnetic Field on the Rate of Exchange of Magnetic Dipoles

It is shown theoretically that the quantum‐mechanical exchange rate of magnetic dipoles such as electrons or protons is decreased by applying an external magnetic field, provided that the zero‐field rate constant in frequency units (ωe) is comparable with the ``chemical shift'' (ωs) in the magnetic resonance spectrum of the system subjected to the external field. The decrease of the rate constant depends on the ratio ωe/ωs, and the maximum reduction from ωe to ωe/2 is obtained when the ratio vanishes. The effects of magnetic and electric fields on the rate of transfer of excitons in molecular crystals are also discussed.