Time-Resolved Experiments on the Chlorine Atom Initiated Oxidation of Tetrachloroethene (Cl2CCCl2)

Abstract

We report time-resolved measurements relevant to the atmospheric oxidation of tetrachloroethene (Cl 2 C= CCl 2 ) and pentachloroethane (CCl 3 CCl 2 H). Cl atoms were produced by photolysis of Cl 2 at 351 nm, using the output from a pulsed excimer laser, in the presence of C 2 Cl 4 and a large excess of O 2 . Experiments were performed with and without NO present. The formation of the reaction products C(O)Cl 2 and CCl 3 C(O)Cl was followed, in real time, via absorption of infrared radiation provided by tunable diode lasers. Contrary to the finding of Hasson and Smith (J. Phys. Chem. A 1999, 103, 2031) but in agreement with the results of Thiiner et al. (J. Phys. Chem. A 1999, 103, 8657), the relative yields of C(O)Cl 2 and CCl 3 C(O)Cl were found to be independent of the concentration of C 2 Cl 4 . However, as found in both previous studies, the relative yields were sensitive to whether the CCl 3 CCl 2 O radical was formed by the reaction between CCl 3 CCl 2 O 2 and NO or by the mutual reaction of two CCl 3 CCl 2 O 2 radicals. From the results it was possible to estimate the branching ratio k 5 a /(k 5 a + k 5 b ) for decomposition of the CCl 3 CCl 2 O radical by the pathways CCl 3 CCl 2 O (+M) → CCl 3 C(O)Cl + Cl (+M) and CCl 3 CCl 2 O (+M) → CCl 3 + C(O)Cl 2 (+M). With CCl 3 CCl 2 O formed from CCl 3 CCl 2 O 2 + NO, (k 5 a /[k 5 a + k 5 b ]) = 0.69 ′ 0.13, whereas with 2CCl 3 CCl 2 O formed from 2CCl 3 CCl 2 O 2 , (k 5 a /[k 5 a + k 5 b ]) = 0.91 ′ 0.21. Attempts to use literature values of the rate constants in modeling, (a) how the concentration of CCl 3 C(O)Cl varied with time following the initiation of reaction and (b) how the yield of CCl 3 C(O)Cl depended on the initial concentration of tetrachloroethene, were not successful. Agreement between the experimental results and the calculations could only be obtained using rate constants for the reactions between CCl 3 CCl 2 O 2 and NO and between CCl 3 CCl 2 O 2 and NO 2 that were appreciably larger than previous values.