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(CH2 OO + CF3 COOH)=(3.4±0.3)×10-10  cm3  s-1 and k((CH3 )2 COO + CF3 COOH)=(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 CH2 OO + CF3 COOH and (CH3 )2 COO + CF3 COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.