Kinetic and Mechanistic Studies of the Reactions of CF3O Radicals with NO and NO2

Abstract

The reactions of CF₃O radicals with (1) NO and (2) NO₂ were studied using two different experimental techniques. A laser photolysis/LIF detection method was applied for measuring the rate constants as a function of temperature (T = 222–302 K) and total pressure (p_tot = 7–107 mbar). Whereas the reaction with (1) NO was found to be independent of temperature and pressure with k₁ = (4.5±1.2)×10⁻¹¹ cm³ s⁻¹, the reaction with (2) NO₂ was found to be dependent on both of these variables. The temperature dependence of k₂ in the high pressure limit can be given by the expression k_2,∝ (T) = (8±5)×10⁻¹³ exp ((863±194) K/T) cm³ s⁻¹. The product distributions of the two reactions were determined in separate experiments using steady-state photolysis combined with FTIR spectroscopy. For reaction (1) only CF₂O was found as a reaction product with a yield of 0.93±0.10, independent of temperature. For reaction (2) several products (CF₃ONO₂, CF₂O, FNO₂) were identified, the overall yield, however, is dominated (≥90%) by the recombination product CF₃ONO₂. A theoretical analysis of the detailed mechanisms of both reactions was made by performing ab initio energy and geometry predictions in combination with RRKM calculations. Both reactions were found to proceed via an initial addition mechanism involving the CF₃ONO_x (x = 1, 2) intermediate and a four-center transition state. A direct abstraction of an F atom by NO or NO₂ can be excluded.