THE ORIGIN OF MAGNETIC-FIELD DEPENDENT RECOMBINATION IN ALKYLCOBALAMIN RADICAL PAIRS

Magnetic field effect studies of alkylcobalamin photolysis provide evi dence for the formation of a reactive radical pair that is born in the singlet spin state, The radical pair recombination process that is re sponsible for the magnetic field dependence of the continuous-wave (CW ) quantum yield is limited to the diffusive radical pair, Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnet ic held dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem, Soc. 115, 12152-12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is onl y observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime o f the spin-correlated diffusive radical pair. This enables magnetic fi eld dependent recombination among spin-correlated diffusive radical pa irs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropa nol, thereby indicating the importance of the increased microviscosity of the medium, Paramagnetic radical scavengers that trap alkyl radica ls that escape the solvent cage do not diminish the magnetic field eff ect on the CW quantum yield, thereby ruling out radical pair recombina tion among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals, Magnetic field dependent reco mbination among alkylcobalamin radical pairs has been simulated by a s emiclassical model of radical pair dynamics and recombination, These c alculations support the existence of a singlet radical pair precursor.