Coupling constant photosynthetic reaction center , electron

The situation is somewhat different in the solid state [50], where stable radical pairs are usually formed by electron transfer over a distance of more than 1 nm and are then localized rather than diffusing. In this case it becomes significant that the To state is a coherent superposition of spin eigenstates of the pair the same is true for the S state [51]. Evolution of this zero-quantum coherence can be observed indirectly, and information on the coupling between the two spins can be obtained from out-of-phase ESEEM experiments [52]. In photosynthetic reaction centers, this technique was used to measure distances between the radicals up to approximately 3 nm [53]. In a cascade of electron transfers, out-of-phase ESEEM spectra depend on the couplings in both the primary and secondary pair and on the time constant for the secondary electron transfer [54]. 

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