KINETIC AND THEORETICAL-STUDIES OF THE REACTIONS CH3C(O)O2+NO2+M-REVERSIBLE-CH3C(O)O2NO2+M BETWEEN 248-K AND 393-K AND BETWEEN 30-TORR AND 760-TORR

The kinetics of the thermal decomposition and formation reactions of acetyl peroxynitrate (PAN) CH3C(O)O2 + NO2 + <--> CH3C(O)O2NO2 + M (1, -1) was studied as a function of temperature and pressure by two groups. In one set of experiments, the decomposition of the peroxynitrate formed in the photolysis of CH3CHO/Cl2/O2/N2/NO2 mixtures was followed by in situ FTIR spectrometry. In the second set of experiments, the decay of the acetylperoxy radical formed in the flash photolysis of Cl2/CH3CHO/NO2/air mixtures was monitored by UV absorption. From the two independent kinetic determinations of k1 and k-1, the equilibrium constant K1(T) was calculated to be 0.9 x 10(-28) exp[(14000 +/- 200)/T] cm3 molecule-1. Quantum chemical and RRKM calculations were performed to obtain accurate and predictive representations of the data. In Troe's notation, the RRKM curves corresponding to the experimental results are represented by the following expressions for the limiting low- and high-pressure rate constants, with F(c) = 0.30: k0(-1) = 4.9 x 10(-3) exp[-(12 100 +/- 500)/T] cm3 molecule-1 s-1; k-infinity(-1) = 4.0 x 10(16) exp[-(13 600 +/- 350)/T] s-1; k0(1) = 2.7 x 10(-28)(T/298)-7.1 +/- 1.7 cm6 molecule-2 s-1; k-infinity(1) = (12.1 +/- 0.5) x 10(-12)(T/298)-0.9 +/- 0.15 cm3 molecule-1 s-1. The thermochemistry of reactions 1 and -1 and the atmospheric implications of the thermal stability of PAN are briefly discussed.