Weak exchange interactions in biradicals: A pseudopotential for unpaired electrons and an asymptotic methods for calculating the exchange integral

A theory of weak exchange interactions in biradicals is developed. The basic idea is to describe the motion of two weakly bound electrons on paramagnetic centers by a two-particle Schrobinger equation that takes into account the interaction of these electrons with the closed electron shells of the biradical, described by a model local potential, and their mutual Coulomb repulsion. The model potential can be constructed using the pseudopotential method, based on results of ab initio quantum chemical calculations. The exchange interaction between the unpaired electrons on the paramagnetic centers is calculated by a generalized asymptotic method that takes into account the quasi-classical nature of the subbarrier motion of the electrons in the region between the paramagnetic centers. The developed theory makes it possible to estimate the magnitude of the exchange interaction and to determine how this interaction depends on the distance between the paramagnetic centers and their relative orientation.