Reaction of hydroxyl radicals with trichloroethylene: Evidence for chlorine elimination channels at elevated temperatures

Abstract

Rate coefficients are reported for the gas-phase reaction of the hydroxyl radical (OH) with C2HCl3 (k1) over an extended temperature range at 740±10 torr in a He bath gas. Rate measurements exhibited complex behavior with a slight negative temperature dependence at temperatures below 650 K and positive temperature dependence at higher temperatures (650–750 K). The three-parameter modified Arrhenius equation adequately describes all of the data and is given by (in units of cm3 molecule−1 s−1) k1(291–750 K)=(3.76±0.36)×10−21 T2.76±0.18 exp(1266.3±41.2)/T Error limits are 2σ values. The activation energy derived from an Arrhenius fit to the data below 650 K is in good agreement with previous studies ( Quantum Rice-Ramsperger-Kassel (QRRK) modeling results are in reasonable agreement with experimental results and indicate the reaction is chemically activated. Although adduct stabilization and chlorine elimination channels are pressure dependent, the overall reaction is pressure independent above 0.1 atm. At temperatures characteristic of postcombustion conditions (i.e. ≈1000 K), Cl elimination is the dominant reaction. H abstraction was found to be significant only at very high temperatures. The lack of a measurable kinetic isotope effect for k1 is consistent with the modeling results. Based on the new measurements reported here and with the assumption that Cl elimination accounts for the entire measured rate at ≈750 K, the extrapolated product-specific rate constant is given by (in units of cm3 molecule−1 s−1) kCl(298–1500 K)=(6.87±0.30)×10−10 T−0.50±0.04 exp(−1530.5±42.4)/T Error limits are 2σ values. This Cl elimination rate constant at 1000–1500 K is a factor of 5–8 times larger than extrapolation of previous low-temperature measurements. At 2000 K, H abstraction is a factor of 2–3 times faster than Cl elimination.