Thermal Decomposition of CF₃O₂NO₂

Abstract

The unimolecular decomposition rate constant of CF3O2NO2 has been measured in detail as a function of temperature, pressure, and collision partner (M = N2, O2, NO). Temperatures were between 264 and 297 K, and total pressures ranged from 3 to 1013 mbar. The first-order decay of CF3O2NO2 in the presence of excess NO was followed in a temperature-controlled DURAN glass chamber by long-path IR absorption, using the absorption bands at 1768 and 1303 cm-1. At 1013 mbar, the first-order decomposition rate constants are best represented by the Arrhenius expression k3 = 5.7 × 1015 exp{(−97.7 ± 1.0) kJ mol-1/RT} s-1 (2σ). The temperature and pressure dependencies of k3 are well reproduced by the equation log(k3/k3,∞) = log{(k3,0/k3,∞)/(1 + k3,0/k3,∞)} + log(Fc){1 + [log(k3,0/k3,∞)/Nc]2}-1, Nc = 0.75−1.27 log(Fc) with the parameters k3,0/[N2] = 2.4 × 10-5 exp(−78.4 kJ mol-1/RT) cm3 molecule-1 s-1, k3,∞ = 1.49 × 1016 exp{(−99.3 ± 1.3) kJ mol-1/RT} s-1, Fc = 0.31, and k3,0(M=O2) ≈ k3,0(M=N2). By combining the present decomposition rate constants with recombination rate constants k-3 from Caralp et al., the following thermochemical data for the equilibrium CF3O2NO2 ⇔ CF3O2 + NO2 (k3,k-3) are derived from second- and third-law evaluations: ΔH°r,298 = 102.7 ± 2.0 kJ mol-1, ΔS°r,298 = 163 ± 7 J mol-1 K-1. The temperature dependence of the equilibrium constant between 200 and 300 K is described by the expression Kc = k3/k-3 = 3.80 × 1027 exp{(−12140 ± 240)K/T} molecules cm-3. Consistency of the data on k3 (this work) and k-3 is shown by comparing experimental and theoretical limiting low-pressure rate constants, which lead to the reasonable value βc = 0.17 for the collision efficiency of N2. The present data confirm that CF3O2NO2 is thermally quite stable in the upper troposphere and lower stratosphere and that its lifetime is probably limited by photolysis in these regions of the atmosphere.