US EPA

1190136 

Journal Article 

Termination of (CH2OH)-C-center dot/CH2O center dot- radicals in aqueous solution 

Wang, WF; Schuchmann, MN; Bachler, V; Schuchmann, HP; Vonsonntag, C 

1996 

1 

Journal of Physical Chemistry
ISSN: 0022-3654 

100 

39 

15843-15847 

10.1021/jp962159x 

WOS:A1996VK04600032 

Hydroxymethyl radical (CH2OH)-C-. and its radical anion .CH2O- (pK(a) 10.7) were generated pulse-radiolytically in N2O-saturated aqueous solutions of methanol. The overall decay is by second-order kinetics. At pH less than or equal to 8 ((CH2OH)-C-., 2k = 1.7 x 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 greater than or equal to 12 ((CH2O-)-C-.) the rate constant is 2k = 0.5 x 10(9) dm(3) mol(-1) s(-1). From the pK, and the dependence of the rate of bimolecular decay on pH, a mixed-termination rate constant k((CH2OH)-C-. + .CH2O-) = 1.2 x 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 greater than or equal to 11.3. Quantum-chemical calculations indicate that (CH2O-)-C-. possesses considerable spin density also at oxygen (mesomeric structure: (CH2O.)-C--). Since in their bimolecular termination reactions the (CH2O-)-C-. 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-)-C-. radical reduces N2O (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 gamma-radiolysis (0.02-2 Gy s(-1)).