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

Hydroxymethyl radical • CH 2 OH and its radical anion • CH 2 O - (p K a 10.7) were generated pulse-radiolytically in N 2 O-saturated aqueous solutions of methanol. The overall decay is by second-order kinetics. At pH ≤ 8 ( • CH 2 OH, 2 k = 1.7 × 10 9 dm 3 mol -1 s -1 ), one observes ethylene glycol and formaldehyde plus methanol in a disproportionation/recombination ratio of 0.17. At pH ≥ 12 ( • CH 2 O - ) the rate constant is 2 k = 0.5 × 10 9 dm 3 mol -1 s -1 . From the p K a and the dependence of the rate of bimolecular decay on pH, a mixed-termination rate constant k ( • CH 2 OH + • CH 2 O - ) = 1.2 × 10 9 dm 3 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 • CH 2 O - possesses considerable spin density also at oxygen (mesomeric structure: - CH 2 O • ). Since in their bimolecular termination reactions the • CH 2 O - 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, CH 3 OCH 2 OH, or that the disproportionation reaction is water-assisted. The • CH 2 O - radical reduces N 2 O ( k = 350 dm 3 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 ).