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 C 2 HCl 3 (k 1 ) over an extended temperature range at 740 ‫ע‬ 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 cm 3 moleculeError limits are 2r values. The activation energy derived from an Arrhenius fit to the data below 650 K is in good agreement with previous studies (Ͻ0.6 kcal/mol deviation). 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., ϳ1000 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 k 1 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 ϳ750 K, the extrapolated product-specific rate constant is given by (in units of cm 3 moleculeError limits are 2r 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.

OH + (E)- and (Z)-1-Chloro-3,3,3-trifluoropropene-1 (CF3CH═CHCl) Reaction Rate Coefficients: Stereoisomer-Dependent Reactivity

“…For example, the difference in the rate coefficients for the reaction of OH radicals with the stereoisomers of 1,2-dichloroethene is relatively small, ∼10 to 15%, , compared with the differences observed in the present study for the CF 3 CHCHCl stereoisomer reactions. There are limited studies available for chlorinated propenes, ,, but more detailed studies are available for the chloroethenes. − ,− In general, the substitution of fluorine (or a CF 3 group in the present case) or chlorine atoms to the double bond in an unsaturated hydrocarbon decreases its overall reactivity with OH. The rate coefficients for reactions and measured in this work are substantially less than the typical range of rate coefficients observed for the OH radical addition reaction for unsaturated hydrocarbons, which are on the order of ∼10 –11 cm 3 molecule –1 s –1 .…”

“…The reported yield for the Cl elimination channel therefore represents a lower-limit because other Cl atom loss processes are expected under the conditions in the Sulbaek Andersen et al study; that is, reaction with the parent and reference compounds. The presence of a Cl atom elimination channel is not unprecedented because several studies have reported or discussed the mechanism for a Cl atom yield in the analogous OH + chloroethene reactions. − ,− In these reaction systems the Cl atom yield was found to be greatest for OH addition to the chlorine-substituted carbon atom. For example, the Cl yield for the OH + 1,2-dichloroethene reaction is near unity, whereas the yield in the OH + 1,1-dichloroethene reaction is much less, <0.1 …”

“…These reactions are expected to proceed predominantly via the addition of OH to the carbon–carbon double bond. Recent studies have reported positive and negative rate coefficient temperature dependences (i.e., not all OH addition reactions exhibit a negative temperature dependence); non-Arrhenius behavior between 200 and 400 K (in some cases); and, in some cases, differences in reaction rate coefficients between stereoisomers. − At present, the database for OH + haloolefin reactions is limited, and additional experimental as well as theoretical studies are needed to provide further insights into the physical properties and mechanisms that most critically influence haloolefin reactivity.…”

Rate Coefficients and Mechanistic Analysis for Reaction of OH with Vinyl Chloride between 293 and 730 K

“…Numerical modeling indicated that the dominant source of OH decay in the absence of the substrate was reaction with HONO (k 298 ) 4.5 × 10 -12 cm 3 molecule -1 s -1 ). 16 A plot of k′ vs substrate concentration at selected temperatures is shown in Figure 2.…”

Rate Coefficients and Mechanistic Analysis for the Reaction of Hydroxyl Radicals with 1,1-Dichloroethylene and trans-1,2-Dichloroethylene over an Extended Temperature Range

“…Spin-projected PMP4/6-311+G-(d,p) theory works well for the other chlorinated ethylenes as seen in this work and elsewhere. [10][11][12] Potential sources of error include deficiencies in the correlation treatment and basis set size, but implementation of sufficiently high levels of theory is nontrivial. Preliminary studies at the QCISD(T)-based G3(MP2) level yielded an even higher barrier of about 3 kcal mol -1 , so presumably, much more demanding correlation treatments and/ or basis sets are needed.…”