We have measured the reaction rate constants of the nitrone spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) with a number of small alkyl and σ parent radicals in dilute aqueous solution using in situ
radiolysis time-resolved electron spin resonance spectroscopy. Unsubstituted alkyl parent radicals (methyl,
ethyl, propyl, and 1-methylethyl (2-propyl)) had rate constants ranging from 5.6 × 106 to 1.6 × 107 M-1 s-1.
Electron-releasing α-hydroxyalkyl radicals (hydroxymethyl, 1-hydroxyethyl, 1-hydroxypropyl, and 1-hydroxy-1-methylethyl (2-hydroxy-2-propyl)) reacted more rapidly than the unsubstituted radicals with rate constants
of (2.2−6.8) × 107 M-1 s-1, while the electron-withdrawing carboxymethyl radical was slower (4.4 × 106
M-1 s-1). The bulky 2-hydroxy-2-methylpropyl radical reacted with DMPO, but with a rate constant smaller
than 106 M-1 s-1. σ radicals such as sulfite anion and carboxyl anion were trapped quickly, with rate constants
of 1.2 × 107 and 6.6 × 107 M-1 s-1, respectively. These results show that the zwitterionic structure of DMPO
results in sensitivity to polar effects in the parent radical-spin trap encounter complex, while steric effects are
also influential in the reaction of DMPO with bulky alkyl radicals. The rate constants for the reaction of
DMPO with the radicals studied herein are, in general, an order of magnitude slower than the same radicals
reacting with the nitroso spin trap 2-methyl-2-nitrosopropane.
The second-order rate constants for the reaction of
5,5-dimethyl-1-pyrroline N-oxide (DMPO) with
radiolytically
produced hydroxyl radicals and hydrated electrons have been measured in
aqueous solution by direct observation
of spin adduct initial yield using time-resolved electron spin
resonance. The rate constants are 2.8 ×
109
mol-1 dm3
s-1 for the DMPO−hydroxyl radical addition
reaction and 3.2 × 109 mol-1
dm3 s-1 for the
reaction
of DMPO and hydrated electron, using sodium formate and chloroacetic
acid as competitive scavengers of
the hydroxyl radical and hydrated electron, respectively. The
hydrated electron−DMPO competition study
determined the fraction of DMPO−H produced directly from
radiolytically produced hydrogen atoms as
0.082 of the total DMPO−H yield, indicating that approximately half
of the hydrogen atoms react with DMPO
to produce non-aminoxyl products. The fraction of the total
hydroxyl radical yield leading to DMPO−OH
spin adduct was determined to be 0.94, using the bleach of
2,2,6,6-tetramethylpiperidone-N-oxyl by carbon
dioxide radical anion as a reference standard.
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