Statistical derivation of a kinetic equation for a subsystem in a “viscous medium”

Kinetic equations for the subsystem interacting weakly with the thermostat including “fast” as well as “slow” motions, are derived by meang of Zubarev's method. Slowly changing coordinates of the thermostat become parameters of the subsystem. Correlation times of the velocities corresponding to these parameters is supposed to be small. The high-temperature approximation is used. The equations obtained are similar to the known equations of the theory of magnetic resonance in viscous media, which are based on the phenomenological assumption that the changing of interaction parameters can be considered as a markovian random process. A criterion of applicability of this approach is given. As an illustration the shape of the electron paramagnetic resonance sygnal is considered for the case of a rigid macromolecule labelled with the axial-symmetric $g$-tensor radical.