Experimental and computational investigations of the reaction of OH with CF3I and the enthalpy of formation of HOI

The kinetics of the reaction of hydroxyl radicals with trifluoroiodomethane were investigated by the flash photolysis-resonance fluorescence technique. A rate constant of k = 5.8 x 10(-12) exp((-11,3 kJ mol(-1))/RT) cm(3) molecule(-1) s(-1) was measured over the temperature range 280-450 K with accuracy limits of 20% (450 K) to 30% (280 K). Different product channels were investigated by ab initio methods, and the dominant products are CF3 + HOI. The enthalpy of formation of hypoiodous acid was' analyzed with Gaussian-2 theory, in conjunction with G2 energies for INO, ICN, ClCN, and other species. The transition state and reaction coordinate for OH + CF3I was characterized at the G2(MP2) level, and the results suggest a negligible barrier to the reverse reaction of CF3 + HOI, so that the measured forward activation energy can be used to derive Delta(f)H(298)(HOI) = -69.6 +/- 5.4 kJ mol(-1). The implications of the kinetics and thermochemistry for iodine chemistry in flames and the atmosphere are discussed, and for the range 280-2000 K a proposed rate expression is k = 2.9 x 10(-16)(T/K)(1.5) exp(-(960 K)/T) cm(3) molecule(-1) s(-1).