Modeling of recombination kinetics of radical pairs in magnetic field. Comparison with experimental dependences on the frequency of encounters in biradicals

Abstract

The recombination kinetics of spin-correlated radical pairs (RPs) with three nonequivalent magnetic nuclei were calculated under conditions of enforced encounters between radicals at time-independent frequency ndif. The simplest two-position model of a RP was used, which includes two states (contact state and distance-separated state) of the RP, differing in magnitude of isotropic spin-spin exchange interaction between radicals. The calculated kinetic curves were treated in terms of a three-exponential model. The dependences of corresponding rate constants (k rec) on ndif, external magnetic field strength (B 0), and intensity, A eff, of isotropic hyperfine coupling (HFC) were obtained. The k rec-vs.-ndif or k rec-vs.-viscosity (ndif varies simultaneously with the inverse lifetime of the contact state) plots pass through maxima whose positions are shifted from the ndif region near the A eff value at B 0 = 0.5 G toward high ndif values with an increase in B 0. At ndif ≫ A eff, the k rec-vs.-B 0 plots pass through maxima in the region B 0 = A eff. The calculated dependences are compared with experimental data on recombination of biradicals. The results of calculations show that the experimentally observed maxima on the k rec-vs.-B 0 or k rec-vs.-ndif plots can be due to peculiar features of the spin dynamics induced by the hyperfine coupling rather than the exchange interaction effects, as is commonly accepted.