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
k
3 = 5.7 × 1015
exp{(−97.7 ± 1.0) kJ
mol-1/RT}
s-1 (2σ).
The temperature and pressure dependencies of
k
3 are well reproduced by the equation
log(k
3/k
3,
∞)
= log{(k
3,0/k
3,
∞)/(1
+
k
3,0/k
3,
∞)}
+ log(F
c){1 +
[log(k
3,0/k
3,
∞)/N
c]2}-1,
N
c = 0.75−1.27
log(F
c) with the parameters
k
3,0/[N2] = 2.4 ×
10-5 exp(−78.4 kJ
mol-1/RT) cm3
molecule-1 s-1,
k
3,
∞ = 1.49 × 1016
exp{(−99.3 ± 1.3)
kJ mol-1/RT}
s-1, F
c = 0.31, and
k
3,0(M=O2) ≈
k
3,0(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
(k
3,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
K
c =
k
3/k
-
3 =
3.80 × 1027 exp{(−12140 ±
240)K/T} molecules cm-3.
Consistency of the data on k
3 (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.
