Temperature dependence of the rate coefficients for the reaction of chlorine atoms with chloromethanes

Rate coefficients for the reaction of Cl atoms with CH₃Cl (k₁), CH₂Cl₂ (k₂), and CHCl₃ (k₃) have been determined over the temperature range 222–298 K using standard relative rate techniques. These data, when combined with evaluated data from previous studies, lead to the following Arrhenius expressions (all in units of cm³ molecule^?1 s^?1): k₁ = (2.8 ± 0.3) × 10^?11 exp(?1200 ± 150/T); k₂ = (1.5 ± 0.2) × 10^?11 exp(?1100 ± 150/T); k₃ = (0.48 ± 0.05) × 10^?11 exp(?1050 ± 150/T). Values for k₁ are in substantial agreement with previous measurements. However, while the room temperature values for k₂ and k₃ agree with most previous data, the activation energies for these rate coefficients are substantially lower than previously recommended values. In addition, the mechanism of the oxidation of CH₂Cl₂ has been studied. The dominant fate of the CHCl₂O radical is decomposition via Cl‐atom elimination, even at the lowest temperatures studied in this work (218 K). However, a small fraction of the CHCl₂O radicals are shown to react with O₂ at low temperatures. Using an estimated value for the rate coefficient of the reaction of CHCl₂O with O₂ (1 × 10^?14 cm³ molecule^?1 s^?1), the decomposition rate coefficient for CHCl₂O is found to be about 4 × 10⁶ s^?1 at 218 K, with the barrier to its decomposition estimated at 6 kcal/mole. As part of this work, the rate coefficient for Cl atoms with HCOCl was also been determined, k₇ = 1.4 × 10^?11 exp(?885/T) cm³ molecule^?1 s^?1, in agreement with previous determinations. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 515–524, 1999