  In theories of spin-dependent radical pair reactions, the time evolution of
the radical pair, including the effect of the chemical kinetics, is described
by a master equation in the Liouville formalism. For the description of the
chemical kinetics, a number of possible reaction operators have been formulated
in the literature. In this work, we present a framework that allows for a
unified description of the various proposed mechanisms and the forms of
reaction operators for the spin-selective recombination processes. Based on the
concept that master equations can be derived from a microscopic description of
the spin system interacting with external degrees of freedom, it is possible to
gain insight into the underlying microscopic processes and to develop a
systematic approach towards determining the specific form of reaction operator
in concrete scenarios.
