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)  cm(3)  s(-1) and k((CH3 )2 COO + CF3 COOH)=(6.1±0.2)×10(-10)  cm(3)  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 [cm(3)  s(-1) ]=(3.8±2.6)×10(-18)  T(2) exp((1620±180)/T) + 2.5×10(-10) and k [cm(3)  s(-1) ]=(4.9±4.1)×10(-18)  T(2) 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.