Figure 1.

UV–visible studies of TEMPO disproportionation and reactivity with NaNO₂ under acidic conditions in acetonitrile. (A) Spectra of 10 mM TEMPO⁺, TEMPO, and TEMPOH in CH₃CN.²³ (B) Spectra obtained following addition of trifluoroacetic acid (TFAH) to a 10 mM solution of TEMPO, corresponding to TEMPO disproportionation into TEMPO⁺ and TEMPOH eq 6. The linear fit to [TEMPO]⁻¹ (inset) incorporates data from three independent experiments. Conditions: 10 mM TEMPO in CH₃CN, 130 mM TFA added at t = 0. (C) Spectral changes observed upon addition of NaNO₂ (0.09 and 1.4 equiv) to a disproportionated-TEMPO solution in CH₃CN/TFAH under N₂. The changes reflect oxidation of TEMPOH to TEMPO⁺ by nitrite. The gray points represent the expected spectrum for full conversion of NO₂^– to NO or TEMPO to TEMPO⁺ depending on the limiting reagent. Conditions: 10 mM TEMPO in CH₃CN with 130 mM TFA, 0.9 or 14.4 mM NaNO₂, N₂ atmosphere. (D) Aerobic oxidation of disproportionated TEMPO catalyzed by nitrite. The initial spectrum of TEMPO⁺ with colorless TEMPOH shifts to higher absorbance as more TEMPO⁺ is formed (blue → red, 2 min scan interval). The gray dotted spectrum depicts the spectrum expected if all TEMPO-based species are converted to TEMPO⁺. Conditions: 10 mM TEMPO in CH₃CN with 130 mM TFA, 0.9 mM NaNO₂ added at t = 0, 1 atm O₂.