Year 2005, Month 5 (Section Full Articles, Page 1131)
Modeling of recombination kinetics of radical pairs in magnetic field.
Comparison with experimental dependences on the frequency
of encounters in biradicals
V. F. Tarasov(a) and P. P. Levin(b)*
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 magni-
tude 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 (krec) on ndif, external magnetic field strength (B0), and intensity, Àeff, of isotropic
hyperfine coupling (HFC) were obtained. The krec-vs.-ndif or krec-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 Àeff value at B0 = 0.5 G toward high ndif values
with an increase in B0. At ndif >> Àeff, the krec-vs.-B0 plots pass through maxima in the region
B0 = Àeff. The calculated dependences are compared with experimental data on recombination
of biradicals. The results of calculations show that the experimentally observed maxima on the
krec-vs.-B0 or krec-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.
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