| Abstract: | Ab initio calculations have been used to characterize the transition states for halogen abstraction by CH3 in reactions with CF4, CF3Cl, CF3Br, 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 were computed via the Gaussian-2 methodology, and the results were employed in transition state theory analyses to obtain the rate constants over 298–2500 K. There is good accord with literature measurements in the approximate temperature range 360–500 K for reactions (2–4), and the computed activation energies are accurate to within ±6 kJ mol−1. Recommended rate constant expressions for use in combustion modeling are k;1=1.6×10−19 (T/K)2.41 exp(−13150 K/T), k2=8.4×10−20(T/K)2.34 exp(−5000 K/T), k3=4.6×10−19 (T/K)2.05 exp(−3990 K/T), and k4=8.3×10−19 (T/K)2.18 exp(−1870 K/T) cm3 molecule−1 s−1. The results are discussed in the context of flame suppression chemistry. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 179–184, 1998. |
