A COMPUTATIONAL STUDY OF THE REACTION-KINETICS OF METHYL RADICALS WITH TRIFLUOROHALOMETHANES

Ab initio calculations have been used to characterize the transition s tates for halogen abstraction by CH3 in reactions with CF4, CF3Cl, CF3 Br, and CF3I (1-4). Geometries and frequencies were obtained at the HF /6-31G(d) and MP2 = full/6-31G(d) levels of theory, Energy barriers we re computed via the Gaussian-2 methodology, and the results were emplo yed in transition state theory analyses to obtain the rate constants o ver 298-2500 K. There is good accord with literature measurements in t he approximate temperature range 360-500 K for reactions (2-4), and th e-computed activation energies are accurate to within +/- 6 kJ mol(-1) . Recommended rate constant expressions for use in combustion modeling are k(1) = 1.6 x 10(-19) (T/K)(2.41) exp(-3990 K/T), k(4) = 8.4 x 10( -20) (T/K)(2.34) exp(-5000 K/T), k(3) = 4.6 x 10(-19) (T/K)(2.05) exp( -3990 K/T), and k(4) = 8.3 x 10(-19) (T/K)(2.18) exp(-1870 K/T) cm(3) molecule(-1) s(-1). The results are discussed in the context of flame suppression chemistry. (C) 1998 John Wiley & Sons, Inc.