Temperature-Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

The rate coefficients for gas-phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH₂OO + CF₃COOH)=(3.4±0.3)×10⁻¹⁰ cm³ s⁻¹ and k((CH₃)₂COO + CF₃COOH)=(6.1±0.2)×10⁻¹⁰ cm³ s⁻¹ at 294 K exceed estimates for collision-limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre-reactive complex. Fits to a model incorporating this complex formation give k [cm³ s⁻¹]=(3.8±2.6)×10⁻¹⁸ T² exp((1620±180)/T) + 2.5×10⁻¹⁰ and k [cm³ s⁻¹]=(4.9±4.1)×10⁻¹⁸ T² exp((1620±230)/T) + 5.2×10⁻¹⁰ for the CH₂OO + CF₃COOH and (CH₃)₂COO + CF₃COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.