Intermediates in the Autoxidation of Nitrogen Monoxide

We have identified two intermediates in the autoxidation of NO center dot: ONOO center dot, which was detected by EPR spectroscopy at 295 K and atmospheric pressure in the gas phase, and ONOONO, a red substance produced at 113 K in 2-methylbutane. The red compound is diamagnetic and absorbs maximally at 500 nm. The ONOONO intermediate is unstable above the melting point of 2-methylbutane and rapidly converts to O2NNO2. From the semiquantitative determination of mole fractions present in the gas phase by EPR spectroscopy, we estimated the rate constants for the steps that lead to ONOO center dot and ONOONO, from the known overall rate constant of the autoxidation reaction, by assuming that a quasi-stationary mechanism applies. The rate constant for the rate-determining formation of ONOO center dot is about 3.1 x 10(-18) cm(3) molecule(-1) s(-1) (or 80 s(-1) in mole fractions), the dissociation rate constant of ONOO center dot is about 6.5 x 10(3) s(-1), and ONOONO is formed with a rate constant of k = 7.7 x 10(-14) cm(3) molecule(-1) s(-1) (1.9 x 10(6) s(-1) in mole fractions). From these constants, we estimate that the equilibrium constant for the formation of ONOO center dot from NO center dot and O-2 (K-ONOO center dot) is 4.8 x 10(-22) cm(3) molecule(-1) (1.2 x 10(-2)), and. therefore, Delta G = +11.0 kJ mol(-1). In water, the Gibbs energy change is close to zero. The presence of ONOO center dot at steady-state concentrations under dioxygen excess may be important not only for reactions in the atmosphere, but especially for reactions in aerosols and biological environments, because the rate constant for formation in solution is higher than that in the gas phase, and, therefore, the half-life of ONOO center dot is longer.