Radical reactions in aqueous solution: use of the aci-anion of nitromethane as a spin trap

Abstract

The type of radical detected by esr when first-formed short-lived radicals are generated by photolytic, radiolytic, or flow techniques in the presence of nitromethane is shown to depend upon the nature and structure of the transient radical and on the pH. Thus electron-donating radicals may react with CH3NO2 itself under certain conditions to give rise to CH3NO2−• and the alkoxy nitroxides CH3N(OR)O•. At higher pH, a wide variety of radicals (X) react very readily with the aci-anion CH2:NO2− to give adducts XCH2NO2−• (and, from radicals capable of one-electron transfer, O2NCH2CH2NO2−•). Structural characteristics of adducts XCH2NO2−• are described and it is shown how radicals which are normally undetectable in fluid solution can be trapped and recognized in this way. Mechanistic pathways diagnosed with this trap (involving, e.g. alkoxyl radicals, aroyloxyl radicals, aromatic carboxylate cation-radicals) are exemplified.

Photoinduced decarboxylation of 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f]quinoline‐8‐carboxylic acid

“…According to the literature, the radical • CH 2 NO 2 can be generated in strong alkaline media by one‐electron oxidation of the nitromethane aci anion (Eqn (6)) . Therefore, we propose that under the given experimental conditions the nitromethane spin trap is oxidized by the short‐lived intermediates formed in the photodecomposition of selenadiazoloquinolones.…”

“…The spectrum in Fig. (d‐i) indicates the formation of radical anion CH 3 NO 2 • – , generated via electron transfer to nitromethane (Eqn (2)) . This points to an electron transfer from the photoactivated selenadiazoloquinolone (SeQ* in Eqn (3)) to the spin trap (Eqn (4)).…”

“…The basic problem under the given experimental conditions is a very limited choice of traps suitable for experiments in alkaline media. One possibility is the application of nitromethane, which in strong alkaline solutions forms aci anion CH 2 = NO 2 – acting as a spin trapping agent (Eqn (1)) …”

Evolution of Radicals from the Photolysis of High Ionic Strength Alkaline Nitrite Solutions

“…Therefore, to detect • OH, a small amount of DMSO was added. Upon reaction with • OH, it generates a methyl radical ( • CH 3 ), which can be efficiently spin-trapped by aci-nitromethane to produce a distinguishable EPR spectrum Therefore, for each series of experiments, we recorded spectra for otherwise identical samples both with and without DMSO. The spectra from solutions without DMSO were fit with a set of basis spectra for the • NO 2 and • NO adducts.…”

“…However, nitrones are not suitable for use at the very high pH levels associated with our solutions. Previous studies have shown that nitromethane and other nitroalkanes convert to the aci form above ∼0.5 M OH – where they can act as an effective spin trap in alkaline solutions. − …”