Thermal Stability of n-Acyl Peroxynitrates

Peroxynitrates (RO2NO2), in particular acyl peroxynitrates (R = RC(O) with R = alkyl), are prominent constituents of polluted air. In this work, a systematic study on the thermal decomposition rate constants of the first five members of the series of homologous RC(O)O2NO2 with R = CH3 ( =PAN), C2H5, n-C3H7, n-C4H9, and n-C5H11 is undertaken to verify the conclusions from previous laboratory data (Grosjean etal., Environ. Sci. Technol. 1994, 28, 1099-1105; Grosjean etal.,Environ. Sci. Technol. 1996, 30, 1038-1047; Bossmeyer etal., Geophys. Res. Lett. 2006, 33, L18810) that the longer chain peroxynitrates may be considerably more stable than PAN. Experiments are performed in a temperature-controlled, evacuable 200 L-photoreactor made from quartz. n-Acyl peroxynitrates are generated by stationary photolysis of mixtures of molecular bromine, O2, NO2, and the corresponding parent aldehydes, highly diluted in N2. Thermal decomposition of RC(O)O2NO2 is initiated by the addition of an excess of NO. First-order decomposition rate constants k1 of the reactions RC(O)O2NO2 (+M) RC(O)O2 + NO2 (+M) are derived at 298 K and a total pressure of 1 bar from the measured loss rates of RC(O)O2NO2, correcting for wall loss of RC(O)O2NO2 and several percentages of reformation of RC(O)O2NO2 by the reaction of RC(O)O2 radicals with NO2. With increasing chain length of R, k1(298 K) slightly decreases from 4.4 x 10-4 s-1 (R = CH3) to 3.7 x 10-4 s-1 (R = C2H5), leveling off at (3.4 +/- 0.1) x 10-4 s-1 for R = n-C3H7, n-C4H9, and n-C5H11. Temperature dependencies of k1 were measured for CH3C(O)O2NO2 and n-C5H11C(O)O2NO2 in the temperature range 289-308 K, resulting in the same activation energy within the statistical error limits (2 sigma) of 0.9 and 1.5 kJ mol-1, respectively. A few experiments on n-C6H13C(O)O2NO2, n-C7H15C(O)O2NO2, and n-C8H17C(O)O2NO2 were also performed, but the results were considered to be unreliable due to strong wall loss of the peroxynitrate and possible complications caused by radical-sinitiated side reactions.