Direct evidence of singlet molecular oxygen generation from peroxynitrate, a decomposition product of peroxynitrite

摘要

The decomposition of peroxynitrite to nitrite and dioxygen at neutral pH follows complex kinetics, compared to its isomerization to nitrate at low pH. Decomposition may involve radicals or proceed by way of the classical peracid decomposition mechanism. Peroxynitrite (ONOOH/ONOO~-) decomposition has been proposed to involve formation of peroxynitrate (O_2NOOH/O_2NOO~-) at neutral pH (D. Gupta, B. Harish, R. Kissner and W. H. Koppenol, Dalton Trans., 2009, DOI: 10.1039/b905535e, see accompanying paper in this issue). Peroxynitrate is unstable and decomposes to nitrite and dioxygen. This study aimed to investigate whether 0_2NOO~- formed upon ONOOH/ONOO_decomposition generates singlet molecular oxygen [O_2(~1Δ_g)]. As unequivocally revealed by the measurement of monomol light emission in the near infrared region at 1270 nm and by chemical trapping experiments, the decomposition of ONOO~- or O_2NOOH at neutral to alkaline pH generates O_2(~1Δ_g) at a yield of ca. 1% and 2-10%, respectively. Characteristic light emission, corresponding to O_2 (~1Δg) monomolecular decay was observed for ONOO~- and for O_2NOOH prepared by reaction of H_2O, with NO_2BF_4 and of H,O_2 with NO_2~- in HClO_4. The generation of O_2 (~1Δ_g) from ONOO~- increased in a concentration-dependent manner in the range of 0.1-2.5 mM and was dependent on pH, giving a sigmoid profile with an apparent pK_a around pD 8.1 (pH 7.7). Taken together, our results clearly identify the generation of O_2 (~1Δ_g) from peroxynitrate [O_2NOO~- → NO_2~- + O_2 (~1Δ_g)] generated from peroxynitrite and also from the reactions of H_2O_2 with either NO_2BF_4 or NO_2~- in acidic media.