Flash Photolysis Study for Substituent and Solvent Effects on Spin-trapping Rates of Phenylthiyl Radicals with Nitrones

The spin-trapping rates for the reactions of para-substituted phenylthiyl radicals with nitrones have been determined by flash photolysis. For the phenylthiyl radical, the rate constant (in mol−1 dm3s−1) in benzene for a cyclic nitrone, 5,5-dimethyl-1-pyrroline N-oxide, is 5.2×108, which is greater than that for acyclic one, N-t-butyl-α-toluenimine N-oxide (1.1×107). The rates decrease with the solvent polarity and with the hydrogen-bonding ability; the maximum ratio of ca. 30 is observed for the rates between cyclohexane and methanol. Combined with the observed substituent effects, the mechanisms of the reactions and properties of the transition states are discussed.