Study of slow molecular motions by stable-radical EPR

A review is given of theoretical papers devoted to the calculation of EPR spectra of stable radicals in the range of slow molecular motions not accessible to analysis by perturbation theory. Modulation of anisotropic magnetic interactions of the spin of an unpaired electron by random rotations of its carrier is assumed to be the principal relaxation mechanism in viscous media and at low concentrations of the radicals. Molecular reorientations are modeled by various random processes of the Markov type. A method is described for obtaining the basic relaxation equation for the spin density matrix which is known as the random path method or the method of the stochastic Liouville equation. A method is described for solving relaxation equations which is common for different models of rotation. Approximate methods for solving such equations are discussed. Results are given of the comparison of calculations with observed spectra which permit conclusions to be drawn concerning the nature and intensity of molecular motions.