Temperature-dependent rate coefficient measurements for the reaction of bromine atoms with trichloroethene, ethene, acetylene, and tetrachloroethene in air

In this article, rate coefficients for the reaction of Br atoms with a series of unsaturated species (trichloroethene, k₁; ethene, k₂; acetylene, k₃; and tetrachloroethene, k₄) are reported in 700 Torr air over the temperature range 228-298 K. The data were obtained in a temperature-regulated environmental chamber/FTIR spectrometer system, using standard relative rate techniques, with the reaction of Br with CH₂O and CH₃CHO as the reference reactions. Room-temperature values for k₁ through k₄ were found to be 1.56×10^-13, 1.23×10^-13, 2.86×10^-14, and <1.2×10^-16 cm³ molecule^-1 s^-1, respectively, in reasonable accord with previously available data. At 240 K, values of k₁ = 1.77×10^-12, k₂ = 5.76×10^-13, k₃ = 3.97×10^-14, and k₄ = 3.22×10^-15 cm³ molecule^-1 s^-1 are reported. These k₂ and k₃ values are 2-3 times lower than those currently in use, which are obtained by extrapolation of previous measurements made over the range 298-350 K. An investigation of the mechanism of the Br-atom-initiated oxidation of acetylene, trichloroethene, and tetrachloroethene was also conducted as a function of temperature. Although no temperature dependence to the product yields was observed for any of the systems studied, the product distributions were found to be altered by the presence of NO_x in the case of trichloroethene and tetrachloroethene. An anomalously low rate coefficient for the reaction of Br with glyoxal (HC(O)CHO, a product of acetylene oxidation), k₁₇ = 1.1×10^-14 cm³ molecule^-1 s^-1, is also reported.