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

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 (O₂NOOH/O₂NOO⁻) 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 O₂NOO⁻ formed upon ONOOH/ONOO⁻ decomposition generates singlet molecular oxygen [O₂ (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 O₂NOOH at neutral to alkaline pH generates O₂ (¹Δ_g) at a yield of ca. 1% and 2–10%, respectively. Characteristic light emission, corresponding to O₂ (¹Δ_g) monomolecular decay was observed for ONOO⁻ and for O₂NOOH prepared by reaction of H₂O₂ with NO₂BF₄ and of H₂O₂ with NO₂⁻ in HClO₄. The generation of O₂ (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 O₂ (¹Δ_g) from peroxynitrate [O₂NOO⁻→ NO₂⁻ + O₂ (¹Δ_g)] generated from peroxynitrite and also from the reactions of H₂O₂ with either NO₂BF₄ or NO₂⁻ in acidic media.