Preprint number: CP3-Origins-2011-43 and DIAS-2011-38
Kinetic models for quantum systems are quite popular due to their simplicity. It is, however, difficult to justify the use and relevance of such models. We consider a radical pair system with spin selective reactions. The enhancement of the reaction yield predicted by different kinetic models is investigated. The crucial points are how the quantum coherent oscillations can be represented by first order rate constants and how many channels a given interaction corresponds to. We show that the transformation from quantum to kinetic description can be done exactly for the hyperfine interaction of arbitrary nuclear spin.