OXIDATION OF ACI-NITROMETHANE BY SINGLET OXYGEN IN AQUEOUS-SOLUTION

In alkaline solution nitromethane (NM) forms a stable aci tautomer (H2 C double bond NO2-) which may be used as a spin trap for radicals, inc luding NO. and NO2.. We have noticed previously that in aerated photoc hemical systems NM can undergo photosensitized degradation (Bilski et al., J. Am. Chem. Soc. 1992, 114, 549), possibly via singlet oxygen (O -1(2)). We have now confirmed that aci-NM does indeed quench O-1(2) ph osphorescence (k(q) 2.4 X 10(7) M(-1) s(-1)), and that the addition of O-1(2) to aci-NM is solely responsible for this chemical quenching, a s confirmed by the high quantum yield of O-2 photoconsumption (phi = 0 .65 for [NM] = 35 mM). To obtain ''labeled'' fragments from the decomp osition of the resultant aci-NM-O-2 peroxy species we have applied ano ther spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), in a novel wa y: we used an EPR silent hydroxylamine adduct (DMPOH/CH double bond NO 2-) formed by nucleophilic addition of aci-NM to DMPO. Reaction of DMP OH/CH double bond NO2- with O-1(2) resulted in the generation of the D MPO/CO2.- radical, suggesting that in the absence of DMPO the followin g mechanism may occur: O-1(2) + CH2 double bond NO2- --> NO2- + HCOOH. We have found that nitrite and formate production accounts for most o f the oxygen consumed. This suggests that O-1(2) reacts mainly with th e carbon atom of NM producing a biradical transient which decomposes y ielding thermodynamically stable products nitrite and formate. A minor product is peroxynitrite (OONO-) which must be produced via an ozonid e-type intermediate formed by intramolecular recombination of the bira dical. Prolonged irradiation of Rose Bengal and aci-NM resulted in acc umulation of nitrite which was then photooxidized to NO2., trapped by unreacted aci-NM and identified as the NM/NO2. spin adduct.