A time-domain EPR study of the SO−3 combination reaction: Radical pair CIDEP without singlet–triplet mixing

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Abstract
The radical ion ⋅SO−3 was studied by in situ pulsed radiolysis of aqueous alkaline sodium sulfite solutions in a pulsed EPR spectrometer. Initial radical concentrations on the order of 5×10−4 M were generated, and detailed measurements of the decay of ⋅SO−3 magnetization were performed on the microsecond time scale. It was found that the second order decay of both longitudinal and transverse magnetization lags behind the second order decay of radical population when the reaction rate approaches the relaxation rates 1/T1,2. The observations may be explained in terms of the spin‐pairing requirements for formation of a singlet reaction product. The spin pairing gives rise to a ‘‘passive’’ form of radical pair mechanism (RPM) chemically induced dynamic electron polarization (CIDEP) which has previously gone unrecognized. RPM polarization therefore occurs in ⋅SO−3 encounters despite the lack of hyperfine dependent singlet–triplet mixing between radical reencounters, which is necessary for the accepted RPM mechanism. The implications of these observations for the interpretation of time‐resolved EPR experiments are discussed.