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

Abstract

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