Kinetics of OH radical reactions with dibenzo- p-dioxin and selected chlorinated dibenzo- p-dioxins

The pulsed laser photolysis/pulsed laser-induced fluorescence (PLP/PLIF) technique has been applied to obtain rate coefficients for OH + dioxin (DD) ( k 1), OH + 2-chlorodibenzo- p-dioxin (2-CDD) ( k 2), OH + 2,3-dichlorodibenzo- p-dioxin (2,3-DCDD) ( k 3), OH + 2,7-dichlorodibenzo- p-dioxin (2,7-DCDD) ( k 4), OH + 2,8-dichlorodibenzo- p-dioxin (2,8-DCDD) ( k 5), OH + 1,2,3,4-tetrachlorodibenzo- p-dioxin (1,2,3,4-TCDD) ( k 6), and OH + octachlorodibenzo- p-dioxin (OCDD) ( k 7) over an extended range of temperature. The atmospheric pressure (740 ± 10 Torr) rate measurements are characterized by the following Arrhenius parameters (in units of cm 3 molecule −1 s −1, error limits are 1 σ): k 1 ( 326 – 907 K ) = ( 1.70 ± 0.22 ) × 10 - 12 exp ( 979 ± 55 ) / T , k 2 ( 346 – 905 K ) = ( 2.79 ± 0.27 ) × 10 - 12 exp ( 784 ± 54 ) / T , k 3 ( 400 – 927 K ) = ( 1.83 ± 0.19 ) × 10 - 12 exp ( 742 ± 67 ) / T , k 4 ( 390 – 769 K ) = ( 1.10 ± 0.10 ) × 10 - 12 exp ( 569 ± 53 ) / T , k 5 ( 379 – 931 K ) = ( 1.02 ± 0.10 ) × 10 - 12 exp ( 580 ± 68 ) / T , k 6 ( 409 – 936 K ) = ( 1.66 ± 0.38 ) × 10 - 12 exp ( 713 ± 114 ) / T , k 7 ( 514 – 928 K ) = ( 3.18 ± 0.54 ) × 10 - 11 exp ( - 667 ± 115 ) / T . The overall uncertainty in the measurements, taking into account systematic errors dominated by uncertainty in the substrate reactor concentration, range from a factor of 2 for DD, 2-CDD, 2,3-DCDD, 2,7-DCDD, and 2,8-DCDD to ± a factor of 4 for 1,2,3,4-TCDD and OCDD. Negative activation energies characteristic of an OH addition mechanism were observed for k 1– k 6. k 7 exhibited a positive activation energy. Cl substitution was found to reduce OH reactivity, as observed in prior studies at lower temperatures. At elevated temperatures (500 K < T < 500 K), there was no experimental evidence for a change in reaction mechanism from OH addition to H abstraction. Theoretical calculations suggest that H abstraction will dominate OH reactivity for most if not all dioxins (excluding OCDD) at combustion temperatures (>1000 K). For OCDD, the dominant reaction mechanism at all temperatures is OH addition followed by Cl elimination.