Kinetics of spin systems with “fast” and “slow” degrees of freedom

To describe the relaxation of spin systems interacting with a dissipative subsystem described by “fast” and “slow” variables a method of secondary kinetic equations is developed. The approximations used for these equations are of the same nature as the approximations needed to derive the Wangsness–Bloch–Redfield master equation. A detailed study is made of the relaxation of paramagnetic ions with anisotropic $g$-factor attached to macromolecules in a solution.