EFFECTS OF WEAK MAGNETIC-FIELDS ON FREE-RADICAL RECOMBINATION REACTIONS

The radical pair mechanism is used to elucidate bow applied magnetic f ields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of fr ee radicals in solution. The so-called low field effect is shown to ar ise from coherent superpositions of degenerate electron-nuclear spin s tates in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, le ading to coherent interconversion of singlet and triplet electronic st ates of the radical pair and hence changes in the yields of recombinat ion products and of the free radicals that escape into solution. For s inglet geminate radical pairs, the low field effect leads to a boost i n the concentration of free radicals, which may be relevant in the con text of in vivo biological effects of electromagnetic fields. Using an alytical approaches in limiting cases, the maximum possible low field effects are calculated fbr a variety of radical pairs. Sizeable change s in reaction yields (similar to 20%) are found for almost any radical pair provided the spin-correlation persists long enough for significa nt evolution of the electron spins under the influence of the weak app lied field. The conditions necessary for observing effects as large as similar to 20% are discussed.