Oxidation of aci-nitromethane by singlet oxygen in aqueous solution

Article abstract of DOI:10.1021/ja00101a008

In alkaline solution nitromethane (NM) forms a stable act tautomer (H₂C=NO₂^-) which may be used as a spin trap for radicals, including NO^? and NO₂^?. We have noticed previously that in aerated photochemical systems NM can undergo photosensitized degradation (Bilski et al., J. Am. Chem. Soc. 1992, 114, 549), possibly via singlet oxygen (¹O₂). We have now confirmed that aci-NM does indeed quench ¹O₂ phosphorescence (k_q = 2.4 × 10⁷ M^-1 s^-1), and that the addition of ¹O₂ to aci-NM is solely responsible for this chemical quenching, as confirmed by the high quantum yield of O₂ photoconsumption (φ = 0.65 for [NM] = 35 mM). To obtain "labeled" fragments from the decomposition of the resultant aci-NM-O₂ peroxy species we have applied another spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), in a novel way: we used an EPR silent hydroxylamine adduct (DMPOH/CH=NO₂^-) formed by nucleophilic addition of aci-NM to DMPO. Reaction of DMPOH/CH=NO₂^- with ¹O₂ resulted in the generation of the DMPO/CO₂^?- radical, suggesting that in the absence of DMPO the following mechanism may occur: ¹O₂ + CH₂=NO₂^- → NO₂^- + HCOOH. We have found that nitrite and formate production accounts for most of the oxygen consumed. This suggests that ¹O₂ reacts mainly with the carbon atom of NM producing a biradical transient which decomposes yielding thermodynamically stable products nitrite and formate. A minor product is peroxynitrite (OONO-) which must be produced via an ozonide-type intermediate formed by intramolecular recombination of the biradical. Prolonged irradiation of Rose Bengal and aci-NM resulted in accumulation of nitrite which was then photooxidized to NO₂^?, trapped by unreacted aci-NM and identified as the NM/NO₂^? spin adduct.