Thermal Stability of n‐Acyl Peroxynitrates

Peroxynitrates (RO₂NO₂), in particular acyl peroxynitrates (R = R′C(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 R′C(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 et al., Environ. Sci. Technol. 1994, 28, 1099–1105; Grosjean et al., Environ. Sci. Technol. 1996, 30, 1038–1047; Bossmeyer et al., 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 R′C(O)O₂NO₂ is initiated by the addition of an excess of NO. First‐order decomposition rate constants k₁ of the reactions R′C(O)O₂NO₂ (+M) → R′C(O)O₂ + NO₂ (+M) are derived at 298 K and a total pressure of 1 bar from the measured loss rates of R′C(O)O₂NO₂, correcting for wall loss of R′C(O)O₂NO₂ and several percentages of reformation of R′C(O)O₂NO₂ by the reaction of R′C(O)O₂ radicals with NO₂. With increasing chain length of R′, k₁(298 K) slightly decreases from 4.4 × 10^?4 s^?1 (R′ = CH₃) to 3.7 × 10^?4 s^?1 (R′ = C₂H₅), leveling off at (3.4 ± 0.1) × 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 (2σ) 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.