An in situ radiolysis time-resolved ESR study of the kinetics of spin trapping by 5,5-dimethyl-1-pyrroline-N-oxide

We have measured the reaction rate constants of the nitrone spin trap 5,5-d imethyl-1-pyrroline-N-oxide (DMPO) with a number of small alkyl and sigma p arent radicals in dilute aqueous solution using in situ radiolysis time-res olved electron spin resonance spectroscopy. Unsubstituted alkyl parent radi cals (methyl, ethyl, propyl, and 1-methylethyl (2-propyl)) had rate constan ts ranging from 5.6 x 10(6) to 1.6 x 10(7) M-1 s(-1). Electron-releasing al pha-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) x 10(7) M-1 s( -1), while the electron-withdrawing carboxymethyl radical was slower (4.4 x 10(6) M-1 s(-1)). The bulky 2-hydroxy-2-methylpropyl radical reacted with DMPO, but with a rate constant smaller than 10(6) M-1 s(-1). a radicals suc h as sulfite anion and carboxyl anion were trapped quickly, with rate const ants of 1.2 x 10(7) and 6.6 x 10(7) M-1 s(-1), respectively. These results show that the zwitterionic structure of DMPO results in sensitivity to pola r effects in the parent radical-spin trap encounter complex, while steric e ffects are also influential in the reaction of DMPO with bulky alkyl radica ls. The rate constants for the reaction of DMPO with the radicals studied h erein are, in general, an order of magnitude slower than the same radicals reacting with the nitroso spin trap 2-methyl-2-nitrosopropane.