The effect of intramolecular paramagnetic-diamagnetic exchange on the magnetic field effect of radical pair recombination

Diamagnetic-paramagnetic exchange has a profound influence on the magnetic field dependence of the geminate recombination between the partners of a radical pair generated in a pure spin state. We consider the situation that the exchange takes place between two groups which are part of one molecule. We outline the proper theory of the spin dynamics, discuss the limits of slow and of fast exchange, and for these two situations provide an estimate of the B _1/2 value which characterizes the magnetic field dependence at intermediate fields. The theory provides the basis for a novel method of measuring the rates of electron transfer between two molecular sites with a defined geometry.