Termination of•CH2OH/CH2O•-Radicals in Aqueous Solution

Hydroxymethyl radical •CH2OH and its radical anion •CH2O- (pKa 10.7) were generated pulse-radiolytically in N2O-saturated aqueous solutions of methanol. The overall decay is by second-order kinetics. At pH ≤ 8 (•CH2OH, 2k = 1.7 × 109 dm3 mol-1 s-1), one observes ethylene glycol and formaldehyde plus methanol in a disproportionation/recombination ratio of 0.17. At pH ≥ 12 (•CH2O-) the rate constant is 2k = 0.5 × 109 dm3 mol-1 s-1. From the pKa and the dependence of the rate of bimolecular decay on pH, a mixed-termination rate constant k(•CH2OH + •CH2O-) = 1.2 × 109 dm3 mol-1 s-1 is calculated. With increasing pH, the yield of ethylene glycol decreases while that of formaldehyde (plus methanol) shows a corresponding increase. Ethylene glycol is no longer formed at pH ≥ 11.3. Quantum-chemical calculations indicate that •CH2O- possesses considerable spin density also at oxygen (mesomeric structure: -CH2O•). Since in their bimolecular termination reactions the •CH2O- radicals can no longer disproportionate by a straightforward H atom transfer, it is concluded either that termination must occur by a C−O type recombination, giving rise to the (unstable) hemiacetal, CH3OCH2OH, or that the disproportionation reaction is water-assisted. The •CH2O- radical reduces N2O (k = 350 dm3 mol-1 s-1); this gives rise to a chain reaction at the low (compared to pulse radiolysis) dose rates of γ-radiolysis (0.02−2 Gy s-1).