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

Rate coefficients are reported for the gas-phase reaction of the hydroxyl r adical (OH) with 1,1-dichloroethylene (k(1)) and trans- 1,2-dichloroethylen e (k(2)) over an extended temperature range at 740 +/- 10 Torr in a He bath gas. Absolute rate measurements were obtained using a laser photolysis/las er-induced fluorescence (LP/LIF) technique under slow flow conditions. Rate measurements for k(1) exhibited complex behavior with negative temperature dependence at temperatures below 640 K, a rapid falloff in rate between 65 0 and 700 K, and positive temperature dependence from 700 to 750 K. The sim ple Arrhenius equation adequately describes the data below 640 K and above 700 K and is given (in units of cm(3) molecule(-1) s(-1)) by k(1)(291-640 K ) = (1.81 +/- 0.36) x 10(-12) exp(511 +/- 71)/T and k(1)(700-750 K) = 3.13 x 10(-10) exp(-5176/T). Rate measurements for k2 also exhibited complex beh avior with a near-zero or slightly negative temperature dependence below 50 0 K and a near-zero or slightly positive temperature dependence above 500 K . The modified Arrhenius equation adequately describes all of the data and is given (in units of cm(3) molecule(-1) s(-1)) by k(2)(293-720 K) = (9.75 +/- 1.14) x 10(-18) T1.73 +/- 0.05 exp(727 +/- 46)/T. Error limits are 2 si gma values. The room-temperature values for k(1) and k(2) are within +/-2 s igma of previous data using different techniques. The rate measurements wer e modeled using, QRRK theory. OH addition to the unsubstituted carbon follo wed by adduct stabilization describes the low-temperature measurements for k(1). Analysis of equilibration in this system yields a C-O bond dissociati on enthalpy of 32.8 +/- 1.5 kcal mol(-1) at 298 K, a value confirmed by ab initio calculations. OH addition followed by Cl elimination described the e xperimental data for k(2). Ab initio based transition state calculations fo r the H atom abstraction channel indicated that this mechanism is consisten t with the rate measurements for k(1) above 700 K. The H abstraction channe l for k(2) could not be observed because of the presence of a more rapid Cl elimination channel at elevated temperatures. H abstraction is predicted t o be the dominant reaction channel for both k(1) and k(2) at flame temperat ures.